REV. 0
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
a
AD9755*
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 2001
14-Bit, 300 MSPS
High-Speed TxDAC+
®
D/A Converter
FUNCTIONAL BLOCK DIAGRAM
AVDD
ACOM
REFIO
FSADJ
PORT1
I
OUTA
I
OUTB
AD9755
DVDD
DCOM
LATCH
LATCH
CLK+
CLK
CLKVDD
PLLVDD
CLKCOM
RESET LPF DIV0 DIV1 PLLLOCK
PLL
CLOCK
MULTIPLIER
DAC LATCH
DAC
REFERENCE
MUX
PORT2
PRODUCT DESCRIPTION
The AD9755 is a dual, muxed port, ultrahigh-speed, single-
channel, 14-bit CMOS DAC. It integrates a high-quality 14-bit
TxDAC+
core, a voltage reference, and digital interface circuitry
into a small 48-lead LQFP package. The AD9755 offers excep-
tional ac and dc performance while supporting update rates up
to 300 MSPS.
The AD9755 has been optimized for ultrahigh-speed applica-
tions up to 300 MSPS where data rates exceed those possible on
a single data interface port DAC. The digital interface consists
of two buffered latches as well as control logic. These latches
can be time multiplexed to the high-speed DAC in several ways.
This PLL drives the DAC latch at twice the speed of the exter-
nally applied clock and is able to interleave the data from the
two input channels. The resulting output data rate is twice that
of the two input channels. With the PLL disabled, an external
2
× clock may be supplied and divided by two internally.
The CLK inputs (CLK+/CLK) can be driven either differen-
tially or single-endedly, with a signal swing as low as 1 V p-p.
FEATURES
14-Bit Dual Muxed Port DAC
300 MSPS Output Update Rate
Excellent SFDR and IMD Performance
SFDR to Nyquist @ 25 MHz Output: 71 dB
Internal Clock Doubling PLL
Differential or Single-Ended Clock Input
On-Chip 1.2 V Reference
Single 3.3 V Supply Operation
Power Dissipation: 155 mW @ 3.3 V
48-Lead LQFP
APPLICATIONS
Communications: LMDS, LMCS, MMDS
Base Stations
Digital Synthesis
QAM and OFDM
The DAC utilizes a segmented current source architecture com-
bined with a proprietary switching technique to reduce glitch
energy and to maximize dynamic accuracy. Differential current
outputs support single-ended or differential applications. The
differential outputs each provide a nominal full-scale current
from 2 mA to 20 mA.
The AD9755 is manufactured on an advanced low cost 0.35
µm
CMOS process. It operates from a single supply of 3.1 V to 3.5 V
and consumes 155 mW of power.
PRODUCT HIGHLIGHTS
1. The AD9755 is a member of a pin-compatible family of high-
speed TxDAC+s providing 10-, 12-, and 14-bit resolution.
2. Ultrahigh-Speed 300 MSPS Conversion Rate.
3. Dual 14-Bit Latched, Multiplexed Input Ports. The AD9755
features a flexible digital interface allowing high-speed data
conversion through either a single or dual port input.
4. Low Power. Complete CMOS DAC function operates on
155 mW from a 3.1 V to 3.5 V single supply. The DAC full-
scale current can be reduced for lower power operation.
5. On-Chip Voltage Reference. The AD9755 includes a 1.20 V
temperature-compensated bandgap voltage reference.
TxDAC+ is a registered trademark of Analog Devices, Inc.
*Protected by U.S. Patent numbers 5450084, 5568145, 5689257 and 5703519.
Other patents pending.
REV. 0
2
AD9755SPECIFICATIONS
DC SPECIFICATIONS
Parameter
Min
Typ
Max
Unit
RESOLUTION
14
Bits
DC ACCURACY
1
Integral Linearity Error (INL)
5
±2.5
+5
LSB
Differential Nonlinearity (DNL)
3
±1.5
+3
LSB
ANALOG OUTPUT
Offset Error
0.025
±0.01
+0.025
% of FSR
Gain Error (Without Internal Reference)
2
±0.5
+2
% of FSR
Gain Error (With Internal Reference)
2
±0.25
+2
% of FSR
Full-Scale Output Current
2
2.0
20.0
mA
Output Compliance Range
1.0
+1.25
V
Output Resistance
100
k
Output Capacitance
5
pF
REFERENCE OUTPUT
Reference Voltage
1.14
1.20
1.26
V
Reference Output Current
3
100
nA
REFERENCE INPUT
Input Compliance Range
0.1
1.25
V
Reference Input Resistance
1
M
TEMPERATURE COEFFICIENTS
Offset Drift
0
ppm of FSR/
°C
Gain Drift (Without Internal Reference)
±50
ppm of FSR/
°C
Gain Drift (With Internal Reference)
±100
ppm of FSR/
°C
Reference Voltage Drift
±50
ppm/
°C
POWER SUPPLY
Supply Voltages
AVDD
3.0
3.3
3.6
V
DVDD
3.0
3.3
3.6
V
PLLVDD
3.0
3.3
3.6
V
CLKVDD
3.0
3.3
3.6
V
Analog Supply Current (I
AVDD
)
4
33
36
mA
Digital Supply Current (I
DVDD
)
4
3.5
4.5
mA
PLL Supply Current (I
PLLVDD
)
4
4.5
5.1
mA
Clock Supply Current (I
CLKVDD
)
4
10.0
11.5
mA
Power Dissipation
4
(3 V, I
OUTFS
= 20 mA)
155
165
mW
Power Dissipation
5
(3 V, I
OUTFS
= 20 mA)
216
mW
Power Supply Rejection Ratio
6
--AVDD
1
+1
% of FSR/V
Power Supply Rejection Ratio
6
--DVDD
0.04
+0.04
% of FSR/V
OPERATING RANGE
40
+85
°C
NOTES
1
Measured at I
OUTA
, driving a virtual ground.
2
Nominal full-scale current, I
OUTFS
, is 32
× the I
REF
current.
3
An external buffer amplifier is recommended to drive any external load.
4
100 MSPS f
DAC
with PLL on, f
OUT
= 100 MHz, all supplies = 3.0 V.
5
300 MSPS f
DAC
.
6
± 5% power supply variation.
Specifications subject to change without notice.
(T
MIN
to T
MAX
, AVDD = 3.3 V, DVDD = 3.3 V, PLLVDD = 3.3 V, CLKVDD = 3.3 V, I
OUTFS
= 20 mA, unless
otherwise noted.)
REV. 0
3
AD9755
DYNAMIC SPECIFICATIONS
Parameter
Min
Typ
Max
Unit
DYNAMIC PERFORMANCE
Maximum Output Update Rate (f
DAC
)
300
MSPS
Output Settling Time (t
ST
) (to 0.1%)
1
11
ns
Output Propagation Delay (t
PD
)
1
1
ns
Glitch Impulse
1
5
pV-s
Output Rise Time (10% to 90%)
1
2.5
ns
Output Fall Time (90% to 10%)
1
2.5
ns
Output Noise (I
OUTFS
= 20 mA)
50
pA/
Hz
Output Noise (I
OUTFS
= 2 mA)
30
pA/
Hz
AC LINEARITY
Spurious-Free Dynamic Range to Nyquist
f
DAC
= 100 MSPS; f
OUT
= 1.00 MHz
0 dBFS Output
74
84
dBc
6 dBFS Output
77
dBc
12 dBFS Output
79
dBc
f
DATA
= 65 MSPS; f
OUT
= 1.1 MHz
2
79
dBc
f
DATA
= 65 MSPS; f
OUT
= 5.1 MHz
2
79
dBc
f
DATA
= 65 MSPS; f
OUT
= 10.1 MHz
2
78
dBc
f
DATA
= 65 MSPS; f
OUT
= 20.1 MHz
2
74
dBc
f
DATA
= 65 MSPS; f
OUT
= 30.1 MHz
2
70
dBc
f
DAC
= 160 MSPS; f
OUT
= 1.1 MHz
80
dBc
f
DAC
= 160 MSPS; f
OUT
= 11.1 MHz
77
dBc
f
DAC
= 160 MSPS; f
OUT
= 31.1 MHz
72
dBc
f
DAC
= 160 MSPS; f
OUT
= 51.1 MHz
72
dBc
f
DAC
= 160 MSPS; f
OUT
= 71.1 MHz
69
dBc
f
DAC
= 300 MSPS; f
OUT
= 1.1 MHz
80
dBc
f
DAC
= 300 MSPS; f
OUT
= 26.1 MHz
71
dBc
f
DAC
= 300 MSPS; f
OUT
= 51.1 MHz
67
dBc
f
DAC
= 300 MSPS; f
OUT
= 101.1 MHz
61
dBc
f
DAC
= 300 MSPS; f
OUT
= 141.1 MHz
60
dBc
Spurious-Free Dynamic Range within a Window
f
DAC
= 100 MSPS; f
OUT
= 1 MHz; 2 MHz Span
0 dBFS Output
83.5
93
dBc
f
DAC
= 65 MSPS; f
OUT
= 5.02 MHz; 2 MHz Span
85
dBc
f
DAC
= 150 MSPS; f
OUT
= 5.04 MHz; 4 MHz Span
85
dBc
Total Harmonic Distortion
f
DAC
= 100 MSPS; f
OUT
= 1.00 MHz
T
A
= 25
°C
83
73
dBc
T
MIN
to T
MAX
83
71
dBc
f
DAC
= 65 MHz; f
OUT
= 2.00 MHz
78
dBc
f
DAC
= 160 MHz; f
OUT
= 2.00 MHz
78
dBc
Multitone Power Ratio (Eight Tones at 110 kHz Spacing)
f
DAC
= 65 MSPS; f
OUT
= 2.00 MHz to 2.77 MHz
0 dBFS Output
75
dBc
6 dBFS Output
73
dBc
12 dBFS Output
70
dBc
NOTES
1
Measured single-ended into 50
load.
2
Single-Port Mode (PLL disabled, DIV0 = 1, DIV1 = 0, data on Port 1).
Specifications subject to change without notice.
(T
MIN
to T
MAX
, AVDD = 3.3 V, DVDD = 3.3 V, PLLVDD = 0 V, CLKVDD = 3.3 V, I
OUTFS
= 20 mA,
Differential Transformer Coupled Output, 50
Doubly Terminated, unless otherwise noted.)
REV. 0
4
AD9755SPECIFICATIONS
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection. Although
the AD9755 features proprietary ESD protection circuitry, permanent damage may occur on
devices subjected to high-energy electrostatic discharges. Therefore, proper ESD precautions are
recommended to avoid performance degradation or loss of functionality.
WARNING!
ESD SENSITIVE DEVICE
(T
MIN
to T
MAX
, AVDD = 3.3 V, DVDD = 3.3 V, PLLVDD = 3.3 V, CLKVDD = 3.3 V, I
OUTFS
= 20 mA, unless
otherwise noted.)
DIGITAL SPECIFICATIONS
Parameter
Min
Typ
Max
Unit
DIGITAL INPUTS
Logic "1"
2.1
3.3
V
Logic "0"
0
0.9
V
Logic "1" Current
10
+10
µA
Logic "0" Current
10
+10
µA
Input Capacitance
5
pF
Input Setup Time (t
S
)
0.5
1.0
ns
Input Hold Time (t
H
)
0.5
1.0
ns
Input Setup Time (t
S,
PLLVDD = 0 V)
1.5
1.0
ns
Input Hold Time (t
H,
PLLVDD = 0 V)
1.7
2.5
ns
Min CLK Frequency
1
6.25
MHz
NOTE
1
Min CLK freq only applies when using internal PLL. When PLL is disabled, there is no minimum CLK frequency.
ABSOLUTE MAXIMUM RATINGS
*
Parameter
With Respect to
Min
Max
Unit
AVDD, DVDD, CLKVDD, PLLVDD
ACOM, DCOM, CLKCOM, PLLCOM
0.3
+3.9
V
AVDD, DVDD, CLKVDD, PLLVDD
ACOM, DCOM, CLKCOM, PLLCOM
3.9
+3.9
V
ACOM, DCOM, CLKCOM, PLLCOM
ACOM, DCOM, CLKCOM, PLLCOM
0.3
+3.9
V
REFIO, REFLO, FSADJ
ACOM
0.3
AVDD + 0.3
V
I
OUTA
, I
OUTB
ACOM
1.0
AVDD + 0.3
V
Digital Data Inputs (DB13 to DB0)
DCOM
0.3
DVDD + 0.3
V
CLK+/CLK, PLLLOCK
CLKCOM
0.3
CLKVDD + 0.3
V
DIV0, DIV1, RESET
CLKCOM
0.3
CLKVDD + 0.3
V
LPF
PLLCOM
0.3
PLLVDD + 0.3
V
Junction Temperature
150
°C
Storage Temperature
65
+150
°C
Lead Temperature (10 sec)
300
°C
*Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device
at these or any other conditions above those indicated in the operational sections of this specification is not implied. Exposure to absolute maximum ratings for extended
periods may affect device reliability.
ORDERING GUIDE
Temperature
Package
Package
Model
Range
Description
Option
AD9755AST
40
°C to +85°C
48-Lead LQFP
ST-48
AD9755-EB
Evaluation Board
THERMAL CHARACTERISTIC
Thermal Resistance
48-Lead LQFP
JA
= 91
°C/W
PORT 1
DATA X
DATA Y
t
H
t
S
t
LPW
t
PD
DATA X
DATA Y
t
PD
PORT 2
I
OUTA
OR I
OUTB
INPUT CLK
(PLL ENABLED)
1 CLOCK @ PLLLOCK
(PLL DISABLED)
DATA IN
Figure 1. I/O Timing
REV. 0
AD9755
5
PIN FUNCTION DESCRIPTIONS
Pin No.
Mnemonic
Description
1
RESET
Internal Clock Divider Reset
2
CLK+
Differential Clock Input
3
CLK
Differential Clock Input
4, 22
DCOM
Digital Common
5, 21
DVDD
Digital Supply Voltage
6
PLLLOCK
PLL Lock Indicator Output
720
P1B13P1B0
Data Bits P1B13 to P1B0, Port 1
2336
P2B13P2B0
Data Bits P2B13 to P2B0, Port 2
37, 38
DIV0, DIV1
Control inputs for PLL and input port selector mode, see Tables I and II for details.
39
REFIO
Reference Input/Output
40
FSADJ
Full-Scale Current Output Adjust
41
AVDD
Analog Supply Voltage
42
I
OUTB
Differential DAC Current Output
43
I
OUTA
Differential DAC Current Output
44
ACOM
Analog Common
45
CLKCOM
Clock and Phase-Locked Loop Common
46
LPF
PLL Loop Filter
47
PLLVDD
Phase-Locked Loop Supply Voltage
48
CLKVDD
Clock Supply Voltage
PIN CONFIGURATION
13 14 15 16 17 18 19 20 21 22 23 24
1
2
3
4
5
6
7
8
9
10
11
12
48 47 46 45 44
39 38 37
43 42 41 40
PIN 1
IDENTIFIER
TOP VIEW
(Not to Scale)
36
35
34
33
32
31
30
29
28
27
26
25
AD9755
P2B0LSB
P2B1
P2B2
P2B3
P2B4
P2B5
P2B6
P2B7
P2B8
P2B9
P2B10
P2B11
RESET
CLK+
CLK
DCOM
DVDD
PLLLOCK
MSBP1B13
P1B12
P1B11
P1B10
P1B9
P1B8
CLKVDD
PLLVDD
LPF
CLKCOM
ACOM
I
OUTA
I
OUTB
AVDD
FSADJ
REFIO
DIV1
DIV0
P1B7
P1B6
P1B5
P1B4
P1B3
P1B2
P1B1
LSB
P1B0
DVDD
DCOM
MSB
P2B13
P2B12
RESERVED = NO
USER CONNECTIONS
REV. 0
AD9755
6
DEFINITIONS OF SPECIFICATIONS
Linearity Error (Also Called Integral Nonlinearity or INL)
Linearity error is defined as the maximum deviation of the actual
analog output from the ideal output, determined by a straight
line drawn from zero to full scale.
Differential Nonlinearity (or DNL)
DNL is the measure of the variation in analog value, normalized
to full scale, associated with a 1 LSB change in digital input code.
Monotonicity
A D/A converter is monotonic if the output either increases or
remains constant as the digital input increases.
Offset Error
The deviation of the output current from the ideal of zero is
called offset error. For I
OUTA
, 0 mA output is expected when the
inputs are all 0s. For I
OUTB
, 0 mA output is expected when all
inputs are set to 1s.
Gain Error
The difference between the actual and ideal output span. The
actual span is determined by the output when all inputs are set
to 1s, minus the output when all inputs are set to 0s.
Output Compliance Range
The range of allowable voltage at the output of a current-output
DAC. Operation beyond the maximum compliance limits may
cause either output stage saturation or breakdown, resulting in
nonlinear performance.
Temperature Drift
Specified as the maximum change from the ambient (25
°C)
value to the value at either T
MIN
or T
MAX
. For offset and gain
drift, the drift is reported in ppm of full-scale range (FSR) per
degree C. For reference drift, the drift is reported in ppm per
degree C.
Power Supply Rejection
The maximum change in the full-scale output as the supplies
are varied from minimum to maximum specified voltages.
Settling Time
The time required for the output to reach and remain within a
specified error band about its final value, measured from the
start of the output transition.
Glitch Impulse
Asymmetrical switching times in a DAC give rise to undesired
output transients that are quantified by a glitch impulse. It is
specified as the net area of the glitch in pV-s.
Spurious-Free Dynamic Range
The difference, in dB, between the rms amplitude of the output
signal and the peak spurious signal over the specified bandwidth.
Total Harmonic Distortion
THD is the ratio of the rms sum of the first six harmonic com-
ponents to the rms value of the measured fundamental. It is
expressed as a percentage or in decibels (dB).
Signal-to-Noise Ratio (SNR)
S/N is the ratio of the rms value of the measured output signal
to the rms sum of all other spectral components below the
Nyquist frequency, excluding the first six harmonics and dc.
The value for SNR is expressed in decibels.
Adjacent Channel Power Ratio (or ACPR)
A ratio in dBc between the measured power within a channel
relative to its adjacent channel.
AD9755
I
OUTA
I
OUTB
SEGMENTED
SWITCHES FOR
DB0 TO DB13
DAC
FSADJ
REFIO
1.2V REF
CLK+
PLLLOCK
DIGITAL DATA INPUTS
0.1 F
R
SET
2k
1k
50
MINI
CIRCUITS
T1-1T
TO ROHDE
AND
SCHWARZ
FSEA30
SPECTRUM
ANALYZER
DB0 DB13
TEKTRONIX DG2020
OR
AWG2021 w/OPTION 4
LECROY 9210
PULSE GENERATOR
(FOR DATA RETIMING)
DCOM
PMOS CURRENT
SOURCE ARRAY
AVDD
3.1V TO 3.5V
DVDD
2 -1 MUX
PORT 1 LATCH
DAC LATCH
ACOM
PORT 2 LATCH
CLK
PLL
CIRCUITRY
PLLVDD
CLKVDD
RESET
LPF
CLKCOM
DIV0
DIV1
50
DB0 DB13
MINI
CIRCUITS
T1-1T
1k
3.1V TO 3.5V
HP8644
SIGNAL
GENERATOR
Figure 2. Basic AC Characterization Test Setup
REV. 0
7
Typical Performance CharacteristicsAD9755
f
OUT
MHz
90
70
40
100
10
0
SFDR
dBc
80
60
50
20
30
40
50
60
70
80
90
0dBmFS
6dBmFS
12dBmFS
TPC 2. Single-Tone SFDR vs. f
OUT
@
f
DAC
= 200 MSPS
f
OUT
MHz
90
70
40
100
10
0
SFDR
dBc
80
60
50
20
30
40
50
60
70
80
90
SFDR NEAR CARRIERS
(2F1-F2, 2F2-F1)
SFDR OVER
NYQUIST BAND
TPC 5. Two-Tone IMD vs. f
OUT
@
f
DAC
= 200 MSPS, 1 MHz Spacing
between Tones, 0 dBFS
A
OUT
dBm
90
70
40
6
16
SFDR
dBc
80
60
50
14
12
10
8
4
2
0
26MHz @ 130MSPS
40MHz @ 200MSPS
60MHz @ 300MSPS
TPC 8. Single-Tone SFDR vs. A
OUT
@ f
OUT
= f
DAC
/5
f
OUT
MHz
90
70
40
100
0
SFDR
dBc
80
60
50
20
40
60
80
120
140
160
0dBmFS
6dBmFS
12dBmFS
TPC 3. Single-Tone SFDR vs. f
OUT
@
f
DAC
= 300 MSPS
f
OUT
MHz
90
70
40
100
0
SFDR
dBc
80
60
50
20
40
60
80
120
140
160
SFDR CLOSE TO CARRIERS
(2F1-F2, 2F2-F1)
SFDR OVER NYQUIST BAND
TPC 6. Two-Tone IMD vs. f
OUT
@
f
DAC
= 300 MSPS, 1 MHz Spacing
between Tones, 0 dBFS
A
OUT
dBm
90
70
40
6
16
SFDR
dBc
80
60
50
14 12 10 8
4
2
0
18
20
18.18/19.18MHz
@ 200MSPS
11.82/12.82MHz
@ 130MSPS
27.27/28.27MHz
@ 300MSPS
TPC 9. Two-Tone IMD (Third Order
Products) vs. A
OUT
@ f
OUT
= f
DAC
/11
f
OUT
MHz
90
70
40
35
5
0
SFDR
dBc
80
60
50
10
15
20
25
30
0dBmFS
6dBmFS
12dBmFS
TPC 1. Single-Tone SFDR vs. f
OUT
@
f
DAC
= 65 MSPS; Single Port Mode
f
OUT
MHz
90
70
40
100
0
SFDR
dBc
80
60
50
20
40
60
80
120
140
200MSPS
300MSPS
65MSPS
TPC 4. SFDR vs. f
OUT
@ 0 dBFS
A
OUT
dB
90
70
40
6
16
SFDR
dBc
80
60
50
14
12
10
8
4
2
0
11.82MHz @ 130MSPS
18.18MHz @ 200MSPS
27.27MHz @ 300MSPS
TPC 7. Single-Tone SFDR vs. A
OUT
@ f
OUT
= f
DAC
/11
REV. 0
AD9755
8
A
OUT
dBm
90
70
40
6
16
SFDR
dBc
80
60
50
14 12 10 8
4
2
0
18
20
60/61MHz
@ 300MSPS
26/27MHz
@ 130MSPS
40/41MHz
@ 200MSPS
TPC 11. Two-Tone IMD (Third Order
Products) vs. A
OUT
@ f
OUT
= f
DAC
/5
f
OUT
MHz
75
65
50
160
20
0
SFDR
dBc
70
60
55
40
60
80
100
120
45
40
140
I
OUTFS
= 20mA
I
OUTFS
= 10mA
I
OUTFS
= 5mA
TPC 14. SFDR vs. I
OUTFS
, f
DAC
=
300 MSPS @ 0 dBFS
CODE
2048
0
DNL
LSB
1
0
1
2
4096
6144
8192
10240
14336
12288
TPC 17. Typical DNL
A
OUT
dBm
90
70
40
6
16
SFDR
dBc
80
60
50
14 12 10 8
4
2
0
18
20
60/61MHz
@ 300MSPS
26/27MHz
@ 130MSPS
40/41MHz
@ 200MSPS
TPC 12. Two-Tone IMD (to Nyquist)
vs. A
OUT
@ f
OUT
= f
DAC
/5
TEMPERATURE C
75
65
50
90
30
50
SFDR
dBc
70
60
55
10
10
30
50
45
40
70
80
10MHz
40MHz
120MHz
80MHz
TPC 15. SFDR vs. Temperature,
f
DAC
= 300 MSPS @ 0 dBFS
FREQUENCY MHz
10
30
80
20
0
AMPLITUDE
dBm
20
40
70
40
60
100
120
90
100
140
0
50
60
80
f
DAC
= 300MSPS
f
OUT1
= 24MHz
f
OUT2
= 25MHz
f
OUT3
= 26MHz
f
OUT4
= 27MHz
f
OUT5
= 28MHz
f
OUT6
= 29MHz
f
OUT7
= 30MHz
f
OUT8
= 31MHz
SFDR = 58dBc
MAGNITUDE = 0dBFS
TPC 18. Eight-Tone SFDR @ f
OUT
f
DAC
/11, f
DAC
= 300 MSPS
A
OUT
dBm
90
70
40
6
16
SFDR
dBc
80
60
50
14 12 10 8
4
2
0
18
20
18.18/19.18MHz
@ 200MSPS
11.82/12.82MHz
@ 130MSPS
27.27/28.27MHz
@ 300MSPS
TPC 10. Two-Tone IMD (to Nyquist)
vs. A
OUT
@ f
OUT
= f
DAC
/11
f
DAC
MHz
90
70
85
300
50
SINAD
dBm
80
60
50
100
150
200
250
75
65
55
TPC 13. SINAD vs. f
DAC
@ f
OUT
=
10 MHz, 0 dBFS
CODE
0
0
INL
LSB
0.5
2048
2.0
2.0
1.0
1.5
1.0
0.5
1.5
4096
8192
6144
12288
10240
14336
16383
TPC 16. Typical INL
REV. 0
AD9755
9
FUNCTIONAL DESCRIPTION
Figure 3 shows a simplified block diagram of the AD9755. The
AD9755 consists of a PMOS current source array capable of
providing up to 20 mA of full-scale current, I
OUTFS
. The array is
divided into 31 equal sources that make up the five most signifi-
cant bits (MSBs). The next four bits, or middle bits, consist of
15 equal current sources whose value is 1/16th of an MSB cur-
rent source. The remaining LSBs are a binary weighted fraction
of the middle bit current sources. Implementing the middle
and lower bits with current sources, instead of an R-2R ladder,
enhances dynamic performance for multitone or low-amplitude
signals and helps maintain the DAC's high output impedance
(i.e., >100 k
).
All of the current sources are switched to one or the other of the
two outputs (i.e., I
OUTA
or I
OUTB
) via PMOS differential current
switches. The switches are based on a new architecture that
drastically improves distortion performance. This new switch
architecture reduces various timing errors and provides match-
ing complementary drive signals to the inputs of the differential
current switches.
The analog and digital sections of the AD9755 have separate
power supply inputs (i.e., AVDD and DVDD) that can operate
independently over a 3.1 V to 3.5 V range. The digital section,
which is capable of operating at a 300 MSPS clock rate, consists
of edge-triggered latches and segment decoding logic circuitry.
The analog section includes the PMOS current sources, the
associated differential switches, a 1.20 V bandgap voltage refer-
ence and a reference control amplifier.
The full-scale output current is regulated by the reference
control amplifier and can be set from 2 mA to 20 mA via an
external resistor, R
SET
. The external resistor, in combination
with both the reference control amplifier and voltage reference
V
REFIO
, sets the reference current I
REF
, which is replicated to the
segmented current sources with the proper scaling factor. The
full-scale current, I
OUTFS
, is 32 times the value of I
REF
.
REFERENCE OPERATION
The AD9755 contains an internal 1.20 V bandgap reference.
This can easily be overdriven by an external reference with no
effect on performance. REFIO serves as either an input or out-
put, depending on whether the internal or an external reference
is used. To use the internal reference, simply decouple the
REFIO pin to ACOM with a 0.1
µF capacitor. The internal
reference voltage will be present at REFIO. If the voltage at
REFIO is to be used elsewhere in the circuit, an external buffer
amplifier with an input bias current less than 100 nA should be
used. An example of the use of the internal reference is given in
Figure 4.
A low impedance external reference can be applied to REFIO
as shown in Figure 5. The external reference may provide either
a fixed reference voltage to enhance accuracy and drift perfor-
mance or a varying reference voltage for gain control. Note that
the 0.1
µF compensation capacitor is not required since the
internal reference is overdriven, and the relatively high input
impedance of REFIO minimizes any loading of the external
reference.
REFERENCE CONTROL AMPLIFIER
The AD9755 also contains an internal control amplifier that is
used to regulate the DAC's full-scale output current, I
OUTFS
.
The control amplifier is configured as a voltage-to-current con-
verter as shown in Figure 4, so that its current output, I
REF
, is
determined by the ratio of V
REFIO
and an external resistor, R
SET
,
as stated in Equation 4. I
REF
is applied to the segmented current
sources with the proper scaling factor to set I
OUTFS
as stated in
Equation 3.
The control amplifier allows a wide (10:1) adjustment span of
I
OUTFS
over a 2 mA to 20 mA range by setting I
REF
between
62.5
µA and 625 µA. The wide adjustment span of I
OUTFS
provides
several application benefits. The first benefit relates directly to
the power dissipation of the AD9755, which is proportional to
I
OUTFS
(refer to the Power Dissipation section). The second
benefit relates to the 20 dB adjustment, which is useful for sys-
tem gain control purposes.
The small signal bandwidth of the reference control amplifier is
approximately 500 kHz and can be used for low frequency, small
signal multiplying applications.
AD9755
I
OUTA
I
OUTB
SEGMENTED
SWITCHES FOR
DB0 TO DB13
DAC
FSADJ
REFIO
1.2V REF
DIV0
PLLLOCK
DIGITAL DATA INPUTS
0.1 F
R
SET
2k
R
LOAD
50
DB0 DB13
DCOM
PMOS CURRENT
SOURCE ARRAY
AVDD
3.1V TO 3.5V
DVDD
2 -1 MUX
PORT 1 LATCH
DAC LATCH
ACOM
DIV1
PLL
CIRCUITRY
PLLVDD
CLKVDD
CLK+
CLK
CLKCOM
RESET
LPF
DB0 DB13
V
OUT
B
R
LOAD
50
V
OUT
A
V
DIFF
= V
OUT
A V
OUT
B
PORT 2 LATCH
Figure 3. Simplified Block Diagram
REV. 0
AD9755
10
1.2V REF
AVDD
I
REF
CURRENT
SOURCE
ARRAY
REFIO
FSADJ
2k
0.1 F
AD9755
REFERENCE
SECTION
ADDITIONAL
EXTERNAL
LOAD
OPTIONAL
EXTERNAL
REFERENCE
BUFFER
Figure 4. Internal Reference Configuration
1.2V REF
AVDD
I
REF
CURRENT
SOURCE
ARRAY
REFIO
FSADJ
2k
AD9755
REFERENCE
SECTION
EXTERNAL
REFERENCE
AVDD
Figure 5. External Reference Configuration
PLL CLOCK MULTIPLIER OPERATION
The Phase Locked Loop (PLL) is intrinsic to the operation of the
AD9755 in that it produces the necessary internally synchronized
2
× clock for the edge-triggered latches, multiplexer, and DAC.
With PLLVDD connected to its supply voltage, the AD9755 is
in PLL ACTIVE mode. Figure 6 shows a functional block dia-
gram of the AD9755 clock control circuitry with PLL active.
The circuitry consists of a phase detector, charge pump, voltage
controlled oscillator (VCO), input data rate range control, clock
logic circuitry and control input/outputs. The
÷ 2 logic in the
feedback loop allows the PLL to generate the 2
× clock needed
for the DAC output latch.
Figure 7 defines the input and output timing for the AD9755
with the PLL active. CLK in Figure 7 represents the clock that
is generated external to the AD9755. The input data at both
ports 1 and 2 is latched on the same CLK rising edge. CLK may
be applied as a single ended signal by tying CLK to mid supply
and applying CLK to CLK+, or as a differential signal applied
to CLK+ and CLK.
RESET has no purpose when using the internal PLL and should
be grounded. When the AD9755 is in PLL ACTIVE mode,
PLLLOCK is the output of the internal phase detector. When
locked, the lock output in this mode will be a Logic "1."
CLKCOM
TO INPUT
LATCHES
CLKVDD
(3.1V TO 3.5V)
PLLLOCK
CHARGE
PUMP
PHASE
DETECTOR
LPF
PLLVDD
VCO
392
1.0 F
3.1V TO
3.5V
RANGE
CONTROL
(
1, 2, 4, 8)
DIV0
DIV1
DIFFERENTIAL-
TO-
SINGLE-ENDED
AMP
2
TO DAC
LATCH
CLK+
CLK
AD9755
Figure 6. Clock Circuitry with PLL Active
PORT 1
DATA X
DATA Y
t
H
t
S
t
LPW
t
PD
DATA X
DATA Y
1/2 CYCLE +
t
PD
PORT 2
I
OUTA
OR I
OUTB
CLK
DATA IN
7a.
PORT 1
DATA X
DATA Z
DATA X
DATA Y
PORT 2
I
OUTA
OR I
OUTB
CLK
DATA IN
DATA Z
DATA W
XXX
DATA W
DATA Y
7b.
Figure 7. DAC Input Timing Requirements with PLL Active
Typically, the VCO can generate outputs of 100 MHz to 400 MHz.
The range control is used to keep the VCO operating within its
designed range, while allowing input clocks as low as 6.25 MHz.
With the PLL active, logic levels at DIV0 and DIV1 determine
the divide (prescaler) ratio of the range controller. Table I gives
the frequency range of the input clock for the different states of
DIV0 and DIV1.
Table I. CLK Rates for DIV0, DIV1 Levels With PLL Active
CLK Frequency
DIV1
DIV0
Range Controller
50 MHz150 MHz
0
0
÷1
25 MHz100 MHz
0
1
÷2
12.5 MHz50 MHz
1
0
÷4
6.25 MHz25 MHz
1
1
÷8
A 392
resistor and 1.0 µF capacitor connected in series from
LPF to PLLVDD are required to optimize the phase noise vs.
settling/acquisition time characteristics of the PLL. To obtain
optimum noise and distortion performance, PLLVDD should
be set to a voltage level similar to DVDD and CLKVDD.
In general, the best phase noise performance for any PLL range
control setting is achieved with the VCO operating near its
maximum output frequency of 400 MHz.
As stated earlier, applications requiring input data rates below
6.25 MSPS must disable the PLL clock multiplier and provide
an external 2
× reference clock. At higher data rates however,
applications already containing a low phase noise (i.e., jitter)
reference clock that is twice the input data rate should consider
disabling the PLL clock multiplier to achieve the best SNR
performance from the AD9755. Note, the SFDR performance
of the AD9755 remains unaffected with or without the PLL
clock multiplier enabled.
REV. 0
AD9755
11
The effects of phase noise on the AD9755's SNR performance
become more noticeable at higher reconstructed output frequen-
cies and signal levels. Figure 8 compares the phase noise of a
full-scale sine wave at exactly f
DATA
/4 at different data rates
(hence carrier frequency) with the optimum DIV1, DIV0 setting.
FREQUENCY OFFSET MHz
0
20
110
5
1
0
NOISE DENSITY
dBm/Hz
10
30
40
50
60
70
80
90
100
2
3
4
Figure 8. Phase Noise of PLL Clock Multiplier at f
OUT
=
f
DATA
/4 at Different f
DATA
Settings with DIV0/DIV1
Optimized, Using R & S FSEA30 Spectrum Analyzer
SNR is partly a function of the jitter generated by the clock
circuitry. As a result, any noise on PLLVDD or CLKVDD may
decrease the SNR at the output of the DAC. To minimize this
potential problem, PLLVDD and CLKVDD can be connected
to DVDD using an LC filter network similar to that shown in
Figure 9.
100 F
ELECT.
1022 F
TANT.
0.1 F
CER.
TTL/CMOS
LOGIC
CIRCUITS
3.1V OR 3.5V
POWER SUPPLY
FERRITE
BEADS
CLKVDD
PLLVDD
CLKCOM
Figure 9. LC Network for Power Filtering
DAC TIMING WITH PLL ACTIVE
As described previously in Figure 7, in PLL ACTIVE mode,
Port 1 and Port 2 input latches are updated on the rising edge of
CLK. On the same rising edge, data previously present in the
input Port 2 latch is written to the DAC output latch. The DAC
output will update after a short propagation delay (t
PD
).
Following the rising edge of CLK, at a time equal to half of its
period, the data in the Port 1 latch will be written to the DAC
output latch, again with a corresponding change in the DAC
output. Due to the internal PLL, the time at which the data in
the Port 1 and Port 2 input latches is written to the DAC latch
is independent of the duty cycle of CLK.
When using the PLL,
the external clock can be operated at any duty cycle that
meets the specified input pulsewidth.
On the next rising edge of CLK, the cycle begins again with the
two input port latches being updated, and the DAC output latch
being updated with the current data in the Port 2 input latch.
PLL DISABLED MODE
When PLLVDD is grounded, the PLL is disabled. An external
clock must now drive the CLK inputs at the desired DAC out-
put update rate. The speed and timing of the data present at
input Ports 1 and 2 are now dependent on whether or not the
AD9755 is interleaving the digital input data, or only responding
to data on a single port. Figure 10 is a functional block diagram
of the AD9755 clock control circuitry with the PLL disabled.
PLLVDD
TO DAC
LATCH
PLLLOCK
CLOCK
LOGIC
( 1 OR 2)
DIFFERENTIAL-
TO-
SINGLE-ENDED
AMP
TO
INTERNAL
MUX
CLKIN+
CLKIN
AD9755
RESET DIV0 DIV1
TO INPUT
LATCHES
Figure 10. Clock Circuitry with PLL Disabled
DIV0 and DIV1 no longer control the PLL, but are used to set
the control on the input mux for either interleaving or non-
interleaving the input data. The different modes for states of
DIV0 and DIV1 are given in Table II.
Table II. Input Mode for DIV0,
DIV1 Levels with PLL Disabled
Input Mode
DIV1
DIV0
Interleaved (2
×)
0
0
Noninterleaved
Port 1 Selected
0
1
Port 2 Selected
1
0
Not Allowed
1
1
REV. 0
AD9755
12
INTERLEAVED (2 ) MODE WITH PLL DISABLED
The relationship between the internal and external clocks in this
mode is shown in Figure 11. A clock at the output update data
rate (2
× the input data rate) must be applied to the CLK inputs.
Internal dividers then create the internal 1
× clock necessary for
the input latches. Although the input latches are updated on the
rising edge of the delayed internal 1
× clock, the setup-and-hold
times given in the Digital Specifications table are with respect to
the rising edge of the external 2
× clock. With the PLL disabled,
a load-dependent delayed version of the 1
× clock is present at
the PLLLOCK pin. This signal can be used to synchronize the
external data.
PORT 1
DATA X
DATA Y
t
H
t
S
t
LPW
t
PD
DATA X
DATA Y
PORT 2
I
OUTA
OR I
OUTB
DELAYED
INTERNAL
1 CLK
DATA IN
t
PD
t
D
DATA ENTERS
INPUT LATCHES
ON THIS EDGE
INTERNAL
2 CLK
EXTERNAL
1 CLK
@ PLLLOCK
Figure 11. Timing Requirements, Interleaved (2
×) Mode
With PLL Disabled
Updates to the data at input Ports 1 and 2 should be synchro-
nized to the specific rising edge of the external 2
× clock which
corresponds to the rising edge of the 1
× internal clock as shown
in Figure 11. To ensure synchronization, a Logic "1" must be
momentarily applied to the RESET pin. Doing this and return-
ing RESET to Logic "0" brings the 1
× clock at PLLLOCK to a
Logic "1." On the next rising edge of the 2
× clock, the 1× clock
will go to Logic "0." On the second rising edge of the 2
× clock,
the 1
× clock (PLLLOCK) will again go to Logic "1," as well as
update the data in both of the input latches. The details of this
are given in Figure 12.
RESET
PLLLOCK
EXTERNAL
2 CLOCK
t
RH
= 1.2ns
t
RS
= 0.2ns
DATA ENTERS
INPUT LATCHES
ON THESE EDGES
Figure 12. Reset Function Timing with PLL Disabled
For proper synchronization, sufficient delay must be present
between the time RESET goes low and the rising edge of the 2
×
clock. RESET going low must occur either at least t
RS
ns before
the rising edge of the 2
× clock, or t
RH
ns afterwards. In the former
case, the immediately occurring CLK rising edge will cause
PLLLOCK to go low. In the latter case, the next CLK rising
edge will toggle PLLLOCK.
NONINTERLEAVED MODE WITH PLL DISABLED
If the data at only one port is required, the AD9755 interface
can operate as a simple double buffered latch with no interleav-
ing. On the rising edge of the 1
× clock, input latch 1 or 2 is
updated with the present input data (depending on the state of
DIV0/DIV1). On the next rising edge, the DAC latch is updated
and a time t
PD
later, the DAC output reflects this change. Fig-
ure 13 represents the AD9755 timing in this mode.
t
H
t
S
t
LPW
t
PD
DATA OUT
PORT 1 OR
PORT 2
1 CLOCK
I
OUTA
OR I
OUTB
XX
DATA IN
PORT 1 OR
PORT 2
Figure 13. Timing Requirements, Noninterleaved Mode
with PLL Disabled
DAC TRANSFER FUNCTION
The AD9755 provides complementary current outputs, I
OUTA
and I
OUTB
. I
OUTA
will provide a near full-scale current output,
I
OUTFS
, when all bits are high (i.e., DAC CODE = 16383) while
I
OUTB
, the complementary output, provides no current. The
current output appearing at I
OUTA
and I
OUTB
is a function of
both the input code and I
OUTFS
, and can be expressed as:
I
OUTA
= (DAC CODE/16383)
× I
OUTFS
(1)
I
OUTB
= (16383 DAC CODE)/16384
× I
OUTFS
(2)
where DAC CODE = 0 to 16383 (i.e., Decimal Representation).
As mentioned previously, I
OUTFS
is a function of the reference
current I
REF
, which is nominally set by a reference voltage,
V
REFIO
, and external resistor R
SET
. It can be expressed as:
I
OUTFS
= 32
× I
REF
(3)
where I
REF
= V
REFIO
/R
SET
(4)
The two current outputs will typically drive a resistive load
directly or via a transformer. If dc-coupling is required, I
OUTA
and I
OUTB
should be directly connected to matching resistive
loads, R
LOAD
, that are tied to analog common, ACOM. Note,
R
LOAD
may represent the equivalent load resistance seen by
I
OUTA
or I
OUTB
as would be the case in a doubly terminated
50
or 75 cable. The single-ended voltage output appearing
at the I
OUTA
and I
OUTB
nodes is simply :
V
OUTA
= I
OUTA
× R
LOAD
(5)
V
OUTB
= I
OUTB
× R
LOAD
(6)
Note the full-scale value of V
OUTA
and V
OUTB
should not exceed
the specified output compliance range to maintain specified
distortion and linearity performance.
V
DIFF
= (I
OUTA
I
OUTB
)
× R
LOAD
(7)
Substituting the values of I
OUTA
, I
OUTB
and I
REF
; V
DIFF
can be
expressed as:
V
DIFF
= {(2 DAC CODE 16383)/16384}
×
(32 R
LOAD
/R
SET
)
× V
REFIO
(8)
REV. 0
AD9755
13
These last two equations highlight some of the advantages of
operating the AD9755 differentially. First, the differential opera-
tion will help cancel common-mode error sources associated
with I
OUTA
and I
OUTB
such as noise, distortion, and dc offsets.
Second, the differential code-dependent current and subsequent
voltage, V
DIFF
, is twice the value of the single-ended voltage
output (i.e., V
OUTA
or V
OUTB
), thus providing twice the signal
power to the load.
Note that the gain drift temperature performance for a single-
ended (V
OUTA
and V
OUTB
) or differential output (V
DIFF
) of the
AD9755 can be enhanced by selecting temperature tracking
resistors for R
LOAD
and R
SET
due to their ratiometric relation-
ship as shown in Equation 8.
ANALOG OUTPUTS
The AD9755 produces two complementary current outputs,
I
OUTA
and I
OUTB
, which may be configured for single-ended or
differential operation. I
OUTA
and I
OUTB
can be converted into
complementary single-ended voltage outputs, V
OUTA
and V
OUTB
,
via a load resistor, R
LOAD
, as described by Equations 5 through 8
in the DAC Transfer Function section. The differential voltage,
V
DIFF
, existing between V
OUTA
and V
OUTB
can also be converted
to a single-ended voltage via a transformer or differential ampli-
fier configuration. The ac performance of the AD9755 is optimum
and specified using a differential transformer coupled output in
which the voltage swing at I
OUTA
and I
OUTB
is limited to
±0.5 V.
If a single-ended unipolar output is desirable, I
OUTA
should be
selected as the output, with I
OUTB
grounded.
The distortion and noise performance of the AD9755 can be
enhanced when it is configured for differential operation. The
common-mode error sources of both I
OUTA
and I
OUTB
can be
significantly reduced by the common-mode rejection of a trans-
former or differential amplifier. These common-mode error
sources include even-order distortion products and noise. The
enhancement in distortion performance becomes more signifi-
cant as the frequency content of the reconstructed waveform
increases. This is due to the first order cancellation of various
dynamic common-mode distortion mechanisms, digital feed-
through and noise.
Performing a differential-to-single-ended conversion via a
transformer also provides the ability to deliver twice the recon-
structed signal power to the load (i.e., assuming no source
termination). Since the output currents of I
OUTA
and I
OUTB
are
complementary, they become additive when processed differen-
tially. A properly selected transformer will allow the AD9755 to
provide the required power and voltage levels to different loads.
Refer to Applying the AD9755 section for examples of various
output configurations.
The output impedance of I
OUTA
and I
OUTB
is determined by the
equivalent parallel combination of the PMOS switches associ-
ated with the current sources and is typically 100 k
in parallel
with 5 pF. It is also slightly dependent on the output voltage
(i.e., V
OUTA
and V
OUTB
) due to the nature of a PMOS device.
As a result, maintaining I
OUTA
and/or I
OUTB
at a virtual ground
via an I-V op amp configuration will result in the optimum dc
linearity. Note that the INL/DNL specifications for the AD9755
are measured with I
OUTA
and I
OUTB
maintained at virtual ground
via an op amp.
I
OUTA
and I
OUTB
also have a negative and positive voltage
compliance range that must be adhered to in order to achieve
optimum performance. The negative output compliance range
of 1.0 V is set by the breakdown limits of the CMOS process.
Operation beyond this maximum limit may result in a break-
down of the output stage and affect the reliability of the AD9755.
The positive output compliance range is slightly dependent on
the full-scale output current, I
OUTFS
. It degrades slightly from its
nominal 1.25 V for an I
OUTFS
= 20 mA to 1.00 V for an I
OUTFS
= 2 mA. The optimum distortion performance for a single-
ended or differential output is achieved when the maximum
full-scale signal at I
OUTA
and I
OUTB
does not exceed 0.5 V.
Applications requiring the AD9755's output (i.e., V
OUTA
and/or
V
OUTB
) to extend its output compliance range should size R
LOAD
accordingly. Operation beyond this compliance range will adversely
affect the AD9755's linearity performance and subsequently
degrade its distortion performance.
DIGITAL INPUTS
The AD9755's digital inputs consist of two channels of 14 data
input pins each and a pair of differential clock input pins. The
14-bit parallel data inputs follow standard straight binary coding
where DB13 is the most significant bit (MSB) and DB0 is the
least significant bit (LSB). I
OUTA
produces a full-scale output
current when all data bits are at Logic 1. I
OUTB
produces a comple-
mentary output with the full-scale current split between the two
outputs as a function of the input code.
The digital interface is implemented using an edge-triggered
master slave latch. With the PLL active or disabled, the DAC
output is updated twice for every input latch rising edge, as
shown in Figures 7 and 11. The AD9755 is designed to support
an input data rate as high as 150 MSPS giving a DAC output
update rate of 300 MSPS. The setup-and-hold times can also be
varied within the clock cycle as long as the specified minimum
times are met. Best performance is typically achieved when the
input data transitions on the falling edge of a 50% duty cycle clock.
The digital inputs are CMOS-compatible with logic thresholds,
VTHRESHOLD, set to approximately half the digital positive
supply (DVDD) or
VTHRESHOLD = DVDD/2 (
±20%)
The internal digital circuitry of the AD9755 is capable of oper-
ating over a digital supply range of 3.1 V to 3.5 V. As a result,
the digital inputs can also accommodate TTL levels when DVDD
is set to accommodate the maximum high level voltage of the
TTL drivers V
OH
(MAX). A DVDD of 3.1 V to 3.3 V will typi-
cally ensure proper compatibility with most TTL logic families.
Figure 14 shows the equivalent digital input circuit for the data
and clock inputs.
DVDD
DIGITAL
INPUT
Figure 14. Equivalent Digital Input
REV. 0
AD9755
14
The AD9755 features a flexible differential clock input operating
from separate supplies (i.e., CLKVDD, CLKCOM) to achieve
optimum jitter performance. The two clock inputs, CLK+ and
CLK, can be driven from a single-ended or differential clock
source. For single-ended operation, CLK+ should be driven by
a logic source while CLK should be set to the threshold voltage
of the logic source. This can be done via a resistor divider/
capacitor network as shown in Figure 15a. For differential opera-
tion, both CLK+ and CLK should be biased to CLKVDD/2
via a resistor divider network as shown in Figure 15b.
Because the output of the AD9755 is capable of being updated
at up to 300 MSPS, the quality of the clock and data input signals
are important in achieving the optimum performance. The drivers
of the digital data interface circuitry should be specified to meet
the minimum setup-and-hold times of the AD9755 as well as its
required min/max input logic level thresholds.
Digital signal paths should be kept short and run lengths matched
to avoid propagation delay mismatch. The insertion of a low
value resistor network (i.e., 20
to 100 ) between the AD9755
digital inputs and driver outputs may be helpful in reducing any
overshooting and ringing at the digital inputs that contribute to
data feedthrough. For longer run lengths and high data update
rates, strip line techniques with proper termination resistors
should be considered to maintain "clean" digital inputs.
The external clock driver circuitry should provide the AD9755
with a low jitter clock input meeting the min/max logic levels
while providing fast edges. Fast clock edges will help minimize
any jitter that will manifest itself as phase noise on a reconstructed
waveform. Thus, the clock input should be driven by the fastest
logic family suitable for the application.
Note that the clock input could also be driven via a sine wave,
which is centered around the digital threshold (i.e., DVDD/2)
and meets the min/max logic threshold. This will typically result
in a slight degradation in the phase noise, which becomes more
noticeable at higher sampling rates and output frequencies. Also,
at higher sampling rates, the 20% tolerance of the digital logic
threshold should be considered since it will affect the effective
clock duty cycle and, subsequently, cut into the required data
setup and hold times.
R
SERIES
0.1 F
V
THRESHOLD
CLK+
CLKVDD
CLK
CLKCOM
AD9755
Figure 15a. Single-Ended Clock Interface
0.1 F
CLK+
CLKVDD
CLK
CLKCOM
AD9755
0.1 F
0.1 F
Figure 15b. Differential Clock Interface
INPUT CLOCK AND DATA TIMING RELATIONSHIP
SNR in a DAC is dependent on the relationship between the
position of the clock edges and the point in time at which the
input data changes. The AD9755 is rising edge triggered, and so
exhibits SNR sensitivity when the data transition is close to this
edge. In general, the goal when applying the AD9755 is to make
the data transition close to the falling clock edge. This becomes
more important as the sample rate increases. Figure 16 shows
the relationship of SNR to clock placement with different sample
rates. Note that the setup and hold times implied in Figure 16
appear to violate the maximums stated in the Digital Specifica-
tions of this data sheet. The variation in Figure 16 is due to the
skew present between data bits inherent in the digital data gen-
erator used to perform these tests. Figure 16 is presented to
show the effects of violating setup and hold times, and to show
the insensitivity of the AD9755 to clock placement when data
transitions fall outside of the so-called "bad window." The setup
and hold times stated in the Digital Specifications were measured
on a bit-by-bit basis, therefore eliminating the skew present in
the digital data generator. At higher data rates, it becomes very
important to account for the skew in the input digital data when
defining timing specifications.
TIME OF DATA TRANSITION RELATIVE TO PLACEMENT OF
CLK RISING EDGE (ns), f
OUT
= 10MHz, f
DAC
= 300MHz
80
40
0
3
0
3
SNR
dBc
60
20
70
30
50
10
2
1
1
2
Figure 16. SNR vs. Time of Data Transition Relative to
Clock Rising Edge
POWER DISSIPATION
The power dissipation, P
D
, of the AD9755 is dependent on sev-
eral factors that include: (1) The power supply voltages (AVDD
and DVDD), (2) the full-scale current output I
OUTFS
, (3) the
update rate f
CLOCK
, and (4) the reconstructed digital input wave-
form. The power dissipation is directly proportional to the analog
supply current, I
AVDD
, and the digital supply current, I
DVDD
.
I
AVDD
is directly proportional to I
OUTFS
as shown in Figure 17,
and is insensitive to f
CLOCK
. Conversely, I
DVDD
is dependent on
both the digital input waveform, f
CLOCK
, and digital supply DVDD.
Figure 18 shows I
DVDD
as a function of the ratio (f
OUT
/f
DAC
) for
various update rates. In addition, Figure 19 shows the effect
that the speed of f
DAC
has on the PLLVDD current, given the
PLL divider ratio.
REV. 0
AD9755
15
I
OUTFS
mA
40
20
0
20
10
0
I
AVDD
mA
35
10
30
25
15
5
2.5
5
7.5
12.5
15
17.5
Figure 17. I
AVDD
vs. I
OUTFS
RATIO f
OUT
/f
DAC
20
16
0
1
0.01
0.001
I
DVDD
mA
18
14
12
10
8
6
4
2
0.1
300MSPS
200MSPS
100MSPS
50MSPS
25MSPS
Figure 18. I
DVDD
vs. f
OUT
/f
DAC
Ratio
f
DAC
MHz
10
0
300
150
0
PLL_V
DD
mA
9
8
7
6
5
4
3
2
50
100
200
250
1
175
25
75
125
225
275
DIV SETTING 00
DIV SETTING 11
DIV SETTING 10
DIV SETTING 01
Figure 19. PLLVDD vs. f
DAC
APPLYING THE AD9755
OUTPUT CONFIGURATIONS
The following sections illustrate some typical output configura-
tions for the AD9755. Unless otherwise noted, it is assumed
that I
OUTFS
is set to a nominal 20 mA. For applications requir-
ing the optimum dynamic performance, a differential output
configuration is suggested. A differential output configuration
may consist of either an RF transformer or a differential op amp
configuration. The transformer configuration provides the opti-
mum high-frequency performance and is recommended for any
application allowing for ac-coupling. The differential op amp
configuration is suitable for applications requiring dc-coupling,
a bipolar output, signal gain, and/or level-shifting, within the
bandwidth of the chosen op amp.
A single-ended output is suitable for applications requiring a
unipolar voltage output. A positive unipolar output voltage will
result if I
OUTA
and/or I
OUTB
are connected to an appropriately
sized load resistor, R
LOAD
, referred to ACOM. This configura-
tion may be more suitable for a single-supply system requiring a
dc-coupled, ground referred output voltage. Alternatively, an
amplifier could be configured as an I-V converter, thus converting
I
OUTA
or I
OUTB
into a negative unipolar voltage. This configuration
provides the best dc linearity since I
OUTA
or I
OUTB
is maintained
at a virtual ground. Note that I
OUTA
provides slightly better per-
formance than I
OUTB
.
DIFFERENTIAL COUPLING USING A TRANSFORMER
An RF transformer can be used to perform a differential-to-
single-ended signal conversion as shown in Figure 20. A
differentially coupled transformer output provides the optimum
distortion performance for output signals whose spectral content
lies within the transformer's passband. An RF transformer such
as the Mini-Circuits T11T provides excellent rejection of
common-mode distortion (i.e., even-order harmonics) and
noise over a wide frequency range. When I
OUTA
and I
OUTB
are
terminated to ground with 50
, this configuration provides
0 dBm power to a 50
load on the secondary with a DAC full-
scale current of 20 mA. A 2:1 transformer such as the Coilcraft
WB2040-PC can also be used in a configuration in which I
OUTA
and I
OUTB
are terminated to ground with 75
. This configura-
tion improves load matching and increases power to 2 dBm into
a 50
load on the secondary. Transformers with different
impedance ratios may also be used for impedance matching
purposes. Note that the transformer provides ac-coupling only.
The center tap on the primary side of the transformer must
be connected to ACOM to provide the necessary dc current
path for both I
OUTA
and I
OUTB
. The complementary voltages
appearing at I
OUTA
and I
OUTB
(i.e., V
OUTA
and V
OUTB
) swing
symmetrically around ACOM and should be maintained
with the specified output compliance range of the AD9755.
A differential resistor, R
DIFF
, may be inserted in applications
where the output of the transformer is connected to the load,
R
LOAD
, via a passive reconstruction filter or cable. R
DIFF
is
determined by the transformer's impedance ratio and provides
the proper source termination that results in a low VSWR.
REV. 0
AD9755
16
R
LOAD
AD9755
MINI-CIRCUITS
T1-1T
I
OUTA
I
OUTB
Figure 20. Differential Output Using a Transformer
DIFFERENTIAL COUPLING USING AN OP AMP
An op amp can also be used to perform a differential-to-single-
ended conversion as shown in Figure 21. The AD9755 is configured
with two equal load resistors, R
LOAD
, of 25
. The differential
voltage developed across I
OUTA
and I
OUTB
is converted to a
single-ended signal via the differential op amp configuration.
An optional capacitor can be installed across I
OUTA
and I
OUTB
,
forming a real pole in a low-pass filter. The addition of this
capacitor also enhances the op amp's distortion performance by
preventing the DAC's high slewing output from overloading the
op amp's input.
AD9755
I
OUTA
I
OUTB
C
OPT
500
225
225
500
25
25
AD8047
Figure 21. DC Differential Coupling Using an Op Amp
The common-mode rejection of this configuration is typically
determined by the resistor matching. In this circuit, the dif-
ferential op amp circuit using the AD8047 is configured to
provide some additional signal gain. The op amp must oper-
ate from a dual supply since its output is approximately
±1.0 V.
A high-speed amplifier capable of preserving the differential
performance of the AD9755, while meeting other system
level objectives (i.e., cost, power), should be selected. The
op amp's differential gain, its gain setting resistor values, and
full-scale output swing capabilities should all be considered
when optimizing this circuit.
The differential circuit shown in Figure 22 provides the nec-
essary level-shifting required in a single supply system. In this
case, AVDD, which is the positive analog supply for both the
AD9755 and the op amp, is also used to level-shift the differ-
ential output of the AD9755 to midsupply (i.e., AVDD/2).
The AD8041 is a suitable op amp for this application.
AD9755
I
OUTA
I
OUTB
C
OPT
500
225
225
500
25
25
AD8041
1k
AVDD
Figure 22. Single Supply DC Differential Coupled Circuit
SINGLE-ENDED UNBUFFERED VOLTAGE OUTPUT
Figure 23 shows the AD9755 configured to provide a unipolar
output range of approximately 0 V to 0.5 V for a doubly termi-
nated 50
cable since the nominal full-scale current, I
OUTFS
, of
20 mA flows through the equivalent R
LOAD
of 25
. In this case,
R
LOAD
represents the equivalent load resistance seen by I
OUTA
or
I
OUTB
. The unused output (I
OUTA
or I
OUTB
) can be connected to
ACOM directly or via a matching R
LOAD
. Different values of
I
OUTFS
and R
LOAD
can be selected as long as the positive com-
pliance range is adhered to. One additional consideration in
this mode is the integral nonlinearity (INL) as discussed in the
Analog Output section of this data sheet. For optimum INL
performance, the single-ended, buffered voltage output configu-
ration is suggested.
AD9755
I
OUTA
I
OUTB
50
25
50
V
OUTA
= 0V TO 0.5V
I
OUTFS
= 20mA
Figure 23. 0 V to 0.5 V Unbuffered Voltage Output
SINGLE-ENDED, BUFFERED VOLTAGE OUTPUT
Figure 24 shows a buffered single-ended output configuration in
which the op amp performs an I-V conversion on the AD9755
output current. The op amp maintains I
OUTA
(or I
OUTB
) at a
virtual ground, thus minimizing the nonlinear output impedance
effect on the DAC's INL performance as discussed in the
Analog Output section. Although this single-ended configura-
tion typically provides the best dc linearity performance, its ac
distortion performance at higher DAC update rates may be
limited by the op amp's slewing capabilities. The op amp pro-
vides a negative unipolar output voltage and its full-scale output
voltage is simply the product of R
FB
and I
OUTFS
. The full-scale
output should be set within the op amp's voltage output swing
capabilities by scaling I
OUTFS
and/or R
FB
. An improvement in ac
distortion performance may result with a reduced I
OUTFS
, since
the signal current the op amp will be required to sink will be
subsequently reduced.
REV. 0
AD9755
17
AD9755
I
OUTA
I
OUTB
C
OPT
200
V
OUT
= I
OUTFS
R
FB
R
FB
200
Figure 24. Unipolar Buffered Voltage Output
POWER AND GROUNDING CONSIDERATIONS, POWER
SUPPLY REJECTION
Many applications seek high speed and high performance under
less than ideal operating conditions. In these applications, the
implementation and construction of the printed circuit board is
as important as the circuit design. Proper RF techniques must
be used for device selection, placement, and routing, as well as
power supply bypassing and grounding to ensure optimum per-
formance. Figures 34 to 41 illustrate the recommended printed
circuit board ground, power and signal plane layouts which are
implemented on the AD9755 evaluation board.
One factor that can measurably affect system performance is
the ability of the DAC output to reject dc variations or ac
noise superimposed on the analog or digital dc power distri-
bution. This is referred to as the Power Supply Rejection
Ratio. For dc variations of the power supply, the resulting
performance of the DAC directly corresponds to a gain error
associated with the DAC's full-scale current, I
OUTFS
. AC
noise on the dc supplies is common in applications where the
power distribution is generated by a switching power supply.
Typically, switching power supply noise will occur over the
spectrum from tens of kHz to several MHz. The PSRR vs.
frequency of the AD9755 AVDD supply over this frequency
range is shown in Figure 25.
FREQUENCY MHz
85
40
12
6
0
PSRR
dB
80
75
70
65
60
55
50
2
4
8
10
45
Figure 25. Power Supply Rejection Ratio
Note that the units in Figure 25 are given in units of (amps out/
volts in). Noise on the analog power supply has the effect of
modulating the internal switches, and therefore the output current.
The voltage noise on AVDD will thus be added in a nonlinear
manner to the desired I
OUT
. Due to the relative different size of
these switches, PSRR is very code-dependent. This can produce
a mixing effect that can modulate low frequency power supply
noise to higher frequencies. Worst-case PSRR for either one of
the differential DAC outputs will occur when the full-scale current
is directed toward that output. As a result, the PSRR measure-
ment in Figure 25 represents a worst-case condition in which
the digital inputs remain static and the full-scale output cur-
rent of 20 mA is directed to the DAC output being measured.
An example serves to illustrate the effect of supply noise on the
analog supply. Suppose a switching regulator with a switching
frequency of 250 kHz produces 10 mV rms of noise and, for
simplicity sake (i.e., ignore harmonics), all of this noise is con-
centrated at 250 kHz. To calculate how much of this undesired
noise will appear as current noise superimposed on the DAC's
full-scale current, I
OUTFS
, one must determine the PSRR in dB
using Figure 25 at 250 kHz. To calculate the PSRR for a given
R
LOAD
, such that the units of PSRR are converted from A/V to
V/V, adjust the curve in Figure 25 by the scaling factor 20
× Log
(R
LOAD
). For instance, if R
LOAD
is 50
, the PSRR is reduced by
34 dB (i.e., PSRR of the DAC at 250 kHz, which is 85 dB in
Figure 25, becomes 51 dB V
OUT
/V
IN
).
Proper grounding and decoupling should be a primary objective
in any high-speed, high-resolution system. The AD9755 features
separate analog and digital supply and ground pins to optimize
the management of analog and digital ground currents in a sys-
tem. In general, AVDD, the analog supply, should be decoupled
to ACOM, the analog common, as close to the chip as physi-
cally possible. Similarly, DVDD, the digital supply, should be
decoupled to DCOM as close to the chip as physically possible.
For those applications that require a single 3.3 V supply for both
the analog and digital supplies, a clean analog supply may be
generated using the circuit shown in Figure 26. The circuit
consists of a differential LC filter with separate power supply
and return lines. Lower noise can be attained by using low ESR
type electrolytic and tantalum capacitors.
AVDD
ACOM
100 F
ELECT.
10-22 F
TANT.
0.1 F
CER.
TTL/CMOS
LOGIC
CIRCUITS
3.3V
POWER SUPPLY
FERRITE
BEADS
Figure 26. Differential LC Filter for a Single 3.3 V Application
REV. 0
AD9755
18
APPLICATIONS
QAM/PSK Synthesis
Quadrature modulation (QAM or PSK) consists of two baseband
PAM (Pulse Amplitude Modulated) data channels. Both chan-
nels are modulated by a common frequency carrier. However,
the carriers for each channel are phase-shifted 90
° from each
other. This orthogonality allows twice the spectral efficiency
(data for a given bandwidth) of digital data transmitted via AM.
Receivers can be designed which selectively choose the `in phase'
and `quadrature' carriers, and then recombine the data. The
recombination of the QAM data can be mapped as points repre-
senting digital words in a two dimensional constellation as shown
in Figure 27. Each point, or symbol, represents the transmission
of multiple bits in one symbol period.
0100
0101
0001
0000
0110
0111
0011
0010
1110
1111
1011
1010
1100
1101
1001
1000
Figure 27. 16 QAM Constellation, Gray Coded (Two 4-Level
PAM Signals with Orthogonal Carriers)
Typically, the I and Q data channels are quadrature-modulated
in the digital domain. The high data rate of the AD9755 allows
extremely wide band (>10 MHz) quadrature carriers to be syn-
thesized at IFs of over 100 MHz. Figure 28 shows an example
of a 25 MSymbol/S QAM signal, raised cosine-like pulse,
oversampled by eight at a data rate of 200 MSPS modulated
onto a 25 MHz carrier and reconstructed using the AD9755.
30
START 100kHz
40
50
60
70
80
90
100
110
120
130
12.49MHz/
STOP 125MHz
FREQUENCY MHz
REFERENCE LEVEL
dBm
COMMENT A: 25 MSYMBOL, 64 QAM, CARRIER = 25MHz
A
1 [T1]
CH PWR
ACP UP
ACP LOW
74.25dBm
9.71442886MHz
77.42dBm
78.88dBm
11.83dBm
1
C11
CU1
C0
C0
74.25dBm
9.71442886MHz
VBW 50kHz
SWT 12.5s UNIT dBm
1RM
C11
Figure 28. Reconstructed 64-QAM Signal at 25 MHz IF
A figure of merit for wideband signal synthesis is the ratio of signal
power in the transmitted band to the power in an adjacent chan-
nel. In Figure 28, the adjacent channel power ratio (ACPR) at
the output of the AD9755 is measured to be 65 dB. The limita-
tion on making a measurement of this type is often not the DAC
but the noise inherent in creating the digital data record using
computer tools. To find how much this is limiting the perceived
DAC performance, the signal amplitude can be reduced, as is
shown in Figure 29. The noise contributed by the DAC will
remain constant as the signal amplitude is reduced. When the
signal amplitude is reduced to the level where the noise floor
drops below that of the spectrum analyzer, ACPR will fall off at
the same rate that the signal level is being reduced. Under the
conditions measured in Figure 28, this point occurs in Figure 29
at 10 dBFS. This shows that the data record is actually degrad-
ing the measured ACPR by up to 10 dB.
AMPLITUDE dBFS
40
0
ACPR
dB
50
20
5
60
70
80
15
10
Figure 29. ACPR vs. Amplitude for QAM Carrier
A single-channel active mixer such as the Analog Devices AD8343
can then be used for the hop to the transmit frequency. Figure 30
shows an applications circuit using the AD9755 and the AD8343.
The AD8343 is capable of mixing carriers from dc to 2.5 GHz.
Figure 31 shows the result of mixing the signal in Figure 28 up
to a carrier frequency of 800 MHz. ACPR measured at the
output of the AD8343 is shown in Figure 31 to be 60 dB.
REV. 0
AD9755
19
30
CENTER 860MHz
40
50
60
70
80
90
110
120
11MHz/
SPAN 110MHz
FREQUENCY MHz
REFERENCE LEVEL
dBm
COMMENT A: 25 MSYMBOL, 64 QAM, CARRIER = 825MHz
A
1 [T1]
CH PWR
ACP UP
ACP LOW
1 [T2]
2 [T2]
100.55dBm
859.91983968MHz
66.52dBm
60.16dBm
6.86dBm
33.62dBm
49.91983968MHz
33.62dBm
49.91983968MHz
2
CU1
CL1
C0
C0
100.55dBm
859.91983968MHz
VBW 10kHz
SWT 2.8s UNIT dBm
20
CU1
100
CL1
2mA
Figure 31. Signal of Figure 28 Mixed to Carrier
Frequency of 800 MHz
Effects of Noise and Distortion on Bit Error Rate (BER)
Textbook analysis of Bit Error Rate (BER) performance are
generally stated in terms of E (energy in watts-per-symbol or
watts-per-bit) and N
O
(spectral noise density in watts/Hz). For
QAM signals, this performance is shown graphically in Figure
32. M represents the number of levels in each quadrature PAM
signal (i.e., M = 8 for 64 QAM, M = 16 for 256 QAM). Figure
32 implies grey coding in the QAM constellation, as well as the
use of matched filters at the receiver, which is typical. The
horizontal axis of Figure 32 can be converted to units of energy/
symbol by adding to the horizontal axis 10 log of the number of
bits in the desired curve. For instance, to achieve a BER of 1e-6
with 64 QAM, an energy per bit of 20 dB is necessary. To
calculate energy per symbol, we add 10 log(6), or 7.8 dB. 64
QAM with a BER of 1e-6 (assuming no source or channel cod-
ing) can therefore theoretically be achieved with an energy/
symbol-to-noise (E/N
O
) ratio of 27.8 dB. Due to the loss and
interferers inherent in the wireless path, this signal-to-noise ratio
must be realized at the receiver to achieve the given bit error rate.
Distortion effects on BER are much more difficult to determine
accurately. Most often in simulation, the energies of the strongest
distortion components are root-sum-squared with the noise, and
the result is treated as if it were all noise. That being said, if the
example above of 64 QAM with the BER of 1e-6, using the E/N
O
ratio is much greater than the worst-case SFDR, the noise will
dominate the BER calculation.
The AD9755 has a worst-case in-band SFDR of 47 dB at the
upper end of its frequency spectrum (see TPCs 2, 3). When
used to synthesize high-level QAM signals as described above,
noise, as opposed to distortion, will dominate its performance in
these applications.
SNR/BIT dB
1E0
1E+3
1E+6
20
5
0
SYMBOL ERROR PROBABILITY
1E+2
1E+5
1E+1
1E+4
10
15
16 QAM
64 QAM
4 QAM
Figure 32. Probability of a Symbol Error for QAM
DAC
LA
TCHES
DAC
INPUT
LATCHES
INPUT
LATCHES
PLL/DIVIDER
CLK+ CLK PLLLOCK
DV
DD
AV
DD
I
OUTA
I
OUTB
PORT 1
DATA
INPUT
PORT 2
DATA
INPUT
RSET2
1.9k
FSADJ
0.1 F
REFIO ACOM1 ACOM
DCOM
AD9755
50
50
0.1 F
0.1 F
68
68
INPM
INPP
LOIM
LOIP
OUTP
OUTM
AD8343 ACTIVE MIXER
0.1 F
0.1 F
LOINPUT
M/A-COM ETC-1-1-13 WIDEBAND BALUM
Figure 30. QAM Transmitter Architecture Using AD9755 and AD8343 Active Mixer
REV. 0
AD9755
20
Pseudo Zero Stuffing/IF Mode
The excellent dynamic range of the AD9755 allows its use in
applications where synthesis of multiple carriers is desired. In
addition, the AD9755 can be used in a pseudo zero-stuffing
mode which improves dynamic range at IF frequencies. In this
mode, data from the two input channels is interleaved to the
DAC, which is running at twice the speed of either of the input
ports. However, the data at Port 2 is held constant at midscale.
The effect of this is shown in Figure 33. The IF signal is the
image, with respect to the input data rate, of the fundamental.
Normally, the sinx/x response of the DAC will attenuate this
image. Zero stuffing improves the passband flatness so that the
image amplitude is closer to that of the fundamental signal.
Zero stuffing can be an especially useful technique in the syn-
thesis of IF signals.
FREQUENCY Normalized To Input Data Rate
0
30
2
0.5
0
EFFECT OF SINX/X ROLL-OFF
20
50
10
40
1
1.5
AMPLITUDE
OF IMAGE
WITHOUT
ZERO STUFFING
AMPLITUDE
OF IMAGE
USING
ZERO STUFFING
Figure 33. Effects of Pseudo Zero Stuffing on Spectrum
of AD9755
EVALUATION BOARD
The AD9755-EB is an evaluation board for the AD9755 TxDAC.
Careful attention to layout and circuit design, combined with
prototyping area, allows the user to easily and effectively evalu-
ate the AD9755 in different modes of operation.
Referring to Figures 34 and 35, the AD9755's performance
can be evaluated differentially or single-endedly using either a
transformer, or directly coupling the output. To evaluate the
output differentially using the transformer, it is recommended
that either the Mini-Circuits T1-1T (through-hole) or the
Coilcraft TTWB-1-B (SMT) be placed in the position of T1
on the evaluation board. To evaluate the output either single-
ended or direct-coupled, remove the transformer and bridge
either BL1 or BL2.
The digital data to the AD9755 comes from two ribbon cables
that interface to the 40-lead IDC connectors P1 and P2. Proper
termination or voltage scaling can be accomplished by installing
the resistor pack networks RN1RN12. RN1, 4, 7, 10 are 22
DIP resistor packs and should be installed as they help reduce the
digital edge rates and therefore peak current on the inputs.
A single-ended clock can be applied via J3. By setting the SE/
DIFF labeled jumpers J2, 3, 4, 6, the input clock can be directed
to the CLK+/CLK inputs of the AD9755 in either a single-
ended or differential manner. If a differentially applied clock is
desired, a Mini-Circuits T1-1T transformer should be used in
the position of T2. Note that with a single-ended square wave
clock input, T2 must be removed. A clock can also be applied
via the ribbon cable on Port 1 (P1), Pin 33. By inserting the
EDGE jumper (JP1), this clock will be applied to the CLK+
input of the AD9755. JP3 should be set in its SE position in this
application to bias CLK to 1/2 the supply voltage.
The AD9755's PLL clock multiplier can be enabled by inserting
JP7 in the IN position. As described in the Typical Performance
Characteristics and Functional Description sections, with the
PLL enabled, a clock at 1/2 the output data rate should be
applied as described in the last paragraph. The PLL takes care
of the internal 2
× frequency multiplication and all internal timing
requirements. In this application, the PLLLOCK output indi-
cates when lock is achieved on the PLL. With the PLL enabled,
the DIV0 and DIV1 jumpers (JP8 and JP9) provide the PLL
divider ratio as described in Table I.
The PLL is disabled when JP7 is in the EX setting. In this mode,
a clock at the speed of the output data rate must be applied to the
clock inputs. Internally, the clock is divided by 2. For data syn-
chronization, a 1
× clock is provided on the PLLLOCK pin
in this application. Care should be taken to read the timing
requirements described earlier in the data sheet for optimum perfor-
mance. With the PLL disabled, the DIV0 and DIV1 jumpers
define the mode (interleaved, noninterleaved) as described in
Table II.
REV. 0
AD9755
21
1
16
2B13
2
4
6
8
12
10
16
14
P2
P2
P2
P2
P2
P2
P2
P2
P2B13
1
2
15
2B12
P2B12
2
3
14
2B11
P2B11
3
4
13
2B10
P2B10
4
5
12
2B09
P2B09
5
6
11
2B08
P2B08
6
7
10
2B07
P2B07
7
8
9
2B06
P2B06
8
9
10
RN8
VALUE
RN7
VALUE
P2
P2
P2
P2
P2
P2
P2
P2
1
3
5
7
9
11
13
15
1
2
3
4
5
6
7
8
9
10
2B13
2B12
2B11
2B10
2B09
2B08
2B07
2B06
RN9
VALUE
1
16
1B13
2
4
6
8
12
10
16
14
P1
P1
P1
P1
P1
P1
P1
P1
P1B13
1
2
15
1B12
P1B12
2
3
14
1B11
P1B11
3
4
13
1B10
P1B10
4
5
12
1B09
P1B09
5
6
11
1B08
P1B08
6
7
10
1B07
P1B07
7
8
9
1B06
P1B06
8
9
10
RN2
VALUE
RN1
VALUE
P1
P1
P1
P1
P1
P1
P1
P1
11
13
15
1
3
5
7
9
1
16
1B05
18
20
22
24
28
26
32
30
P1
P1
P1
P1
P1
P1
P1
P1
17
19
21
23
25
27
29
31
P1
P1
P1
P1
P1
P1
P1
P1
P1B05
1
2
15
1B04
P1B04
2
3
14
1B03
P1B03
3
4
13
1B02
P1B02
4
5
12
1B01
P1B01
5
6
11
1B00
P1B00
6
7
10
1O17
OUT15
7
8
9
1O15
OUT16
8
9
10
RN5
VALUE
RN4
VALUE
36
34
40
38
P1
P1
P1
P1
P1
P1
P1
P1
33
35
37
39
1O16
25 26 27 28 29 30 31 32 33 34 35 36
U1
AD9751/53/55
48
47
46
45
44
43
42
41
40
39
38
37
13
14
15
16
17
18
19
20
22
23
24
21
12 11 10 9
8
3
2
1
7
6
5
4
P2B12
MSB
P2B13
P2B11
P2B10
P2B09
P2B08
P2B07
P2B06
P2B05
P2B04
P2B03
P2B02
P2B01
P2B00 LSB
S
P
1
2
3
JP9
AVDD PLANE
1
2
3
JP8
AVDD_PLANE
DIV0
DIV1
A B
A B
REFIO
FSADJ
C12
0.1 F
WHT
TP2
WHT
TP1
R1
1.91k
DVDD PLANE
P1B12
P1B11
P1B10
P1B09
P1B08
P1B07
P1B06
P1B05
P1B04
P1B03
P1B02
P1B01
P1B00 LSB
P1B13 MSB
CLKVDD
IA
IB
P
LPF
C11
1.0 F
R5
392
R10
OPT
4
6
1
2
3
T1
BL1
BL2
1
2
IOUT
J5
1
2
DGND: 3,4,5
J1
DVDD PLANE
CLK+
CLK
RESET
1
2
3
JP5
RESET
A
B
R4
50
WHT
TP3
OUT16
EDGE
EXT
R3
50
C10
10pF
R2
50
C9
10pF
NOTE:
SHIELD AROUND
R5 AND C11 ARE
CONNECTED TO
PLLVDD PLANE
PLLVDD PLANE
1
2
3
4
5
6
7
8
9
10
1B13
1B12
1B11
1B10
1B09
1B08
1B07
1B06
RN3
VALUE
1B05
1B04
1B03
1B02
1B01
1B00
1O16
RN6
VALUE
1
2
3
4
5
6
7
8
9
10
1O17
1O15
JP10
BLK
TP4
BLK
TP5
BLK
TP6
1
16
2B05
18
20
22
24
28
26
32
30
P2
P2
P2
P2
P2
P2
P2
P2
17
19
21
23
25
27
29
31
P2
P2
P2
P2
P2
P2
P2
P2
P2B05
1
2
15
2B04
P2B04
2
3
14
2B03
P2B03
3
4
13
2B02
P2B02
4
5
12
2B01
P2B01
5
6
11
2B00
P2B00
6
7
10
7
8
9
8
9
10
RN11
VALUE
RN10
VALUE
36
34
40
38
P2
P2
P2
P2
P2
P2
P2
P2
33
35
37
39
P2OUT15
P2OUT16
2B05
2B04
2B03
2B02
2B01
2B00
RN12
VALUE
1
2
3
4
5
6
7
8
9
10
2OUT15
2OUT16
NOTES:
1. ALL DIGITAL INPUTS FROM RN1 RN12
MUST BE OF EQUAL LENGTH.
2. ALL DECOUPLING CAPS TO BE LOCATED
AS CLOSE AS POSSIBLE TO DUT,
PREFERABLY UNDER DUT ON BOTTOM
SIGNAL LAYER.
3. CONNECT GNDS UNDER DUT USING
BOTTOM SIGNAL LAYER.
4. CREATE PLANE CAPACITOR WITH 0.007"
DIELECTRIC BETWEEN LAYERS 2 AND 3.
BLK
TP7
BLK
TP8
BLK
TP9
P
BLK
TP10
BLK
TP12
Figure 34. Evaluation Board Circuitry
REV. 0
AD9755
22
S
P
4
6
1
2
3
T2
1
2
1
2
3
JP6
CLK+
A
B
C16
0.1 F
P
R9
1k
JP4
DF
J3
CLK
P
1
2
3
JP2
A
B
SE
DF
JP1
EDGE
OUT15
CKLVDD
R7
1k
P
1
2
3
JP3
A
B
SE
DF
PGND: 3, 4, 5
P
R8
50
CLK
C13
10 F
10V
TP13
DVDD PLANE
BLK
P
DVDD
J8
L1
FBEAD
1
1
2
TP14
RED
DGND
J8
1
C14
10 F
10V
TP15
AVDD PLANE
BLK
AVDD
J10
L2
FBEAD
1
1
2
TP16
RED
AGND
J11
1
C15
10 F
10V
TP17
CLKVDD
BLK
CLKVDD
J12
L3
FBEAD
1
1
2
TP11
RED
CLKGND
J13
1
1
2
3
JP7
A
B
PLLVDD PLANE
C1
0.1 F
DVDD PLANE
PINS 5, 6
C2
1 F
C3
0.1 F
C4
1 F
PINS 21, 22
C5
0.1 F
PINS 41, 44
C6
1 F
AVDD PLANE
C7
0.1 F
PINS 45, 47
C8
1 F
CLKVDD
P
U1 BYPASS CAPS
Figure 35. Evaluation Board Clock Circuitry
Figure 36. Evaluation Board, Assembly--Top
REV. 0
AD9755
23
Figure 37. Evaluation Board, Assembly--Bottom
Figure 38. Evaluation Board, Top Layer
REV. 0
AD9755
24
Figure 39. Evaluation Board, Layer 2, Ground Plane
Figure 40. Evaluation Board, Layer 3, Power Plane
REV. 0
AD9755
25
Figure 41. Evaluation Board, Bottom Layer
REV. 0
AD9755
26
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
48-Lead LQFP Package
(ST-48)
TOP VIEW
(PINS DOWN)
1
12
13
25
24
36
37
48
0.019 (0.5)
BSC
0.276
(7.00)
BSC
SQ
0.011 (0.27)
0.006 (0.17)
0.354 (9.00) BSC SQ
0.063 (1.60)
MAX
0.030 (0.75)
0.018 (0.45)
0.008 (0.2)
0.004 (0.09)
0
MIN
COPLANARITY
0.003 (0.08)
SEATING
PLANE
0.006 (0.15)
0.002 (0.05)
7
0
0.057 (1.45)
0.053 (1.35)
C022522.51/01 (rev. 0)
PRINTED IN U.S.A.
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