a
REV. PrF (
6/04)
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4
3MSPS,12-/10-/8-Bit
ADCs in 6-Lead TSOT
Preliminary Technical Data
AD7276/AD7277/AD7278
PRELIMINARY TECHNICAL DATA
FUNCTIONAL BLOCK DIAGRAM
FEATURES
Fast Throughput Rate: 3MSPS
Specified for V
DD
of 2.35 V to 3.6V
Low Power:
13.5 mW max at 3MSPS with 3V Supplies
TBD mW typ at 1.5MSPS with 3V Supplies
Wide Input Bandwidth:
70dB SNR at 1MHz Input Frequency
Flexible Power/Serial Clock Speed Management
No Pipeline Delays
High Speed Serial Interface
SPI
TM
/QSPI
TM
/MICROWIRE
TM
/DSP Compatible
Power Down Mode: 1µA max
6-Lead TSOT Package
8-lead MSOP Package
AD7476 and AD7476A pin compatible
APPLICATIONS
Battery-Powered Systems
Personal Digital Assistants
Medical Instruments
Mobile Communications
Instrumentation and Control Systems
Data Acquisition Systems
High-Speed Modems
Optical Sensors
PRODUCT HIGHLIGHTS
1. 3MSPS ADCs in a 6-lead TSOT package.
2. AD7476/77/78 and AD7476A/77A/78A pin compatible.
3. High Throughput with Low Power Consumption.
4. Flexible Power/Serial Clock Speed Management.
The conversion rate is determined by the serial clock
allowing the conversion time to be reduced through the
serial clock speed increase. This allows the average
power consumption to be reduced when a power-down
mode is used while not converting. The part also
features a power-down mode
to maximize power effi-
ciency at lower throughput rates. Current consumption is
1
µ
A max when in Power-Down mode.
5. Reference derived from the power supply.
6. No Pipeline Delay.
The parts feature a standard successive-approximation
ADC with accurate control of the sampling instant via a
CS input and once-off conversion control.
GENERAL DESCRIPTION
The AD7276/AD7277/AD7278 are 12-bit, 10-bit and 8-
bit, high speed, low power, successive-approximation
ADCs respectively. The parts operate from a single 2.35V
to 3.6 V power supply and feature throughput rates up to 3
MSPS. The parts contain a low-noise, wide bandwidth
track/hold amplifier which can handle input frequencies in
excess of TBD MHz.
The conversion process and data acquisition are controlled
using
CS and the serial clock, allowing the devices to
interface with microprocessors or DSPs. The input signal
is sampled on the falling edge of
CS and the conversion is
also initiated at this point. There are no pipeline delays
associated with the part.
The AD7276/AD7277/AD7278 use advanced design
techniques to achieve very low power dissipation at high
throughput rates.
The reference for the part is taken internally from V
DD.
This allows the widest dynamic input range to the ADC.
Thus the analog input range for the part is 0 to V
DD
. The
conversion rate is determined by the SCLK.
T/H
8-/10-/12-BIT
SUCCESSIVE
APPROXIMATION
ADC
CONTROL
LOGIC
V
DD
V
IN
GND
SCLK
SDATA
&6
AD7276/AD7277/AD7278
2
REV. PrF
PRELIMINARY TECHNICAL DATA
Parameter
B Grade
1
Units
Test Conditions/Comments
DYNAMIC PERFORMANCE
f
IN
= 1MHz Sine Wave
Signal-to-Noise + Distortion (SINAD)
2
49
dB min
Total Harmonic Distortion (THD)
2
-65
dB max
Peak Harmonic or Spurious Noise (SFDR)
2
-65
dB max
Intermodulation Distortion (IMD)
2
Second Order Terms -76
dB typ
fa= TBD kHz, fb= TBD kHz
Third Order Terms -76
dB typ
fa= TBD kHz, fb= TBD kHz
Aperture Delay
T B D
ns typ
Aperture Jitter
T B D
ps typ
Full Power Bandwidth
T B D
MHz typ
@ 3 dB
Full Power Bandwidth
T B D
MHz typ
@ 0.1dB
DC ACCURACY
Resolution
8
Bits
Integral Nonlinearity
2
±0.3
LSB max
Differential Nonlinearity
2
±0.3
LSB max
Guaranteed No Missed Codes to 8 Bits
Offset Error
2
±0.5
LSB max
± T B D
LSB typ
Gain Error
2
±0.5
LSB max
± T B D
LSB typ
Total Unadjusted Error (TUE)
2
± T B D
LSB max
ANALOG INPUT
Input Voltage Ranges 0 to V
DD
Volts
DC Leakage Current
±0.5
µA max
Input Capacitance
T B D
pF typ
LOGIC INPUTS
Input High Voltage, V
INH
0.7(V
D D
)
V min
2.35V Vdd 2.7V
2
V min
2.7V < Vdd 3.6V
Input Low Voltage, V
INL
0.2(V
D D
)
V max
2.35V Vdd< 2.7V
0.8
V max
2.7V Vdd 3.6V
Input Current, I
IN
, SCLK Pin
±0.5
µA max
Typically TBD nA, V
IN
= 0 V or V
DD
Input Current, I
IN
,
CS Pin
± T B D
µA max
Input Capacitance, C
IN
3
10
pF max
LOGIC OUTPUTS
Output High Voltage, V
OH
V
DD
- 0.2
V min
I
SOURCE
= 200 µA,V
DD
= 2.35 V to 3.6V
Output Low Voltage, V
OL
0.2
V max
I
SINK
= 200µA
Floating-State Leakage Current
± 1
µA max
Floating-State Output Capacitance
3
10
pF max
Output Coding Straight (Natural) Binary
CONVERSION RATE
Conversion Time
192
ns max
10 SCLK Cycles with SCLK at 52 MHz
Track/Hold Acquisition Time
2
50
ns max
Throughput Rate
3
MSPS max
POWER REQUIREMENTS
V
DD
2.35/3.6
Vmin/max
I
D D
Digital I/Ps= 0V or V
DD
Normal Mode(Static)
2.5
mA typ
V
DD
= 2.35V to 3.6V, SCLK On or Off
Normal Mode (Operational)
4.5
mA max
V
DD
= 2.35V to 3.6V, f
SAMPLE
= 3MSPS
Full Power-Down Mode (Static)
1
µA max
SCLK On or Off, typically TBD nA
Full Power-Down Mode (Dynamic)
T B D
mA typ
V
DD
= 3V, f
SAMPLE
= 1MSPS
Power Dissipation
4
Normal Mode (Operational)
13.5
mW max
V
DD
= 3V, f
SAMPLE
= 3MSPS
Full Power-Down
3
µW max
V
DD
= 3V
AD7278-SPECIFICATIONS
(V
DD
=+2.35 V to +3.6 V, f
SCLK
=52 MHz, f
SAMPLE
=3 MSPS unless otherwise noted;
T
A
=T
MIN
to T
MAX
, unless otherwise noted.)
N O T E S
1
Temperature range from 40°C to +85°C.
2
See Terminology.
3
Guaranteed by characterization.
4
See Power Versus Throughput Rate section.
Specifications subject to change without notice.
3
REV. PrF
PRELIMINARY TECHNICAL DATA
Parameter
B Grade
1
Units
Test Conditions/Comments
DYNAMIC PERFORMANCE
f
IN
= 1 MHz Sine Wave
Signal-to-Noise + Distortion (SINAD)
2
61
dB min
Total Harmonic Distortion (THD)
2
-73
dB max
Peak Harmonic or Spurious Noise (SFDR)
2
-74
dB max
Intermodulation Distortion (IMD)
2
Second Order Terms -82
dB typ
fa= TBD kHz, fb= TBD kHz
Third Order Terms -82
dB typ
fa= TBD kHz, fb= TBD kHz
Aperture Delay
T B D
ns typ
Aperture Jitter
T B D
ps typ
Full Power Bandwidth
T B D
MHz typ
@ 3 dB
Full Power Bandwidth
T B D
MHz typ
@ 0.1dB
DC ACCURACY
Resolution
10
Bits
Integral Nonlinearity
2
±0.5
LSB max
Differential Nonlinearity
2
±0.5
LSB max
Guaranteed No Missed Codes to 10 Bits
Offset Error
2
± 1
LSB max
± T B D
LSB typ
Gain Error
2
± 1
LSB max
± T B D
LSB typ
Total Unadjusted Error (TUE)
2
± T B D
LSB max
ANALOG INPUT
Input Voltage Ranges 0 to V
DD
Volts
DC Leakage Current
±0.5
µA max
Input Capacitance
T B D
pF typ
LOGIC INPUTS
Input High Voltage, V
INH
0.7(V
D D
) V min
2.35V Vdd 2.7V
2
V min
2.7V <Vdd 3.6V
Input Low Voltage, V
INL
0.2(V
D D
) V max
2.35V Vdd< 2.7V
0.8
V max
2.7V Vdd 3.6V
Input Current, I
IN
, SCLK Pin
±0.5
µA max
Typically TBD nA, V
IN
= 0 V or V
DD
Input Current, I
IN
,
CS Pin
± T B D
µA max
Input Capacitance, C
IN
3
10
pF max
LOGIC OUTPUTS
Output High Voltage, V
OH
V
DD
- 0.2
V min
I
SOURCE
= 200 µA,V
DD
= 2.35 V to 3.6 V
Output Low Voltage, V
OL
0.2
V max
I
SINK
= 200µA
Floating-State Leakage Current ±1
µA max
Floating-State Output Capacitance
3
10
pF max
Output Coding Straight (Natural) Binary
CONVERSION RATE
Conversion Time
230
ns max
12 SCLK cycles with SCLK at 52 MHz
Track/Hold Acquisition Time
2
50
ns max
Throughput Rate
3
MSPS max
POWER REQUIREMENTS
V
DD
2.35/3.6
V min/max
I
D D
Digital I/Ps 0V or V
DD
Normal Mode(Static)
2.5
mA typ
V
DD
= 2.35V to 3.6V, SCLK On or Off
Normal Mode (Operational)
4.5
mA max
V
DD
= 2.35V to 3.6V, f
SAMPLE
= 3MSPS
Full Power-Down Mode(Static)
1
µA max
SCLK On or Off, typically TBD nA
Full Power-Down Mode(Dynamic)
T B D
mA typ
V
DD
= 3V, f
SAMPLE
= 1MSPS
Power Dissipation
4
Normal Mode (Operational)
13.5
mW max
V
DD
= 3V, f
SAMPLE
= 3MSPS
Full Power-Down
3
µW max
V
D D
= 3 V
AD7277-SPECIFICATIONS
N O T E S
1
Temperature range from 40°C to +85°C.
2
See Terminology.
3
Guaranteed by Characterization.
4
See Power Versus Throughput Rate section.
Specifications subject to change without notice.
(V
DD
=+2.35 V to +3.6 V, f
SCLK
=52 MHz, f
SAMPLE
=3MSPS unless otherwise noted;
T
A
=T
MIN
to T
MAX
, unless otherwise noted.)
4
REV. PrF
PRELIMINARY TECHNICAL DATA
AD7276-SPECIFICATIONS
Parameter B Grade
1
Units
Test Conditions/Comments
DYNAMIC PERFORMANCE
f
IN
= 1 MHz Sine Wave
Signal-to-Noise + Distortion (SINAD)
2
70
dB min
Signal-to-Noise Ratio (SNR)
71
dB min
Total Harmonic Distortion (THD)
2
-80
dB typ
Peak Harmonic or Spurious Noise (SFDR)
2
-82
dB typ
Intermodulation Distortion (IMD)
2
Second Order Terms -84
dB typ
fa= TBD kHz, fb= TBD kHz
Third Order Term -84
dB typ
fa= TBD kHz, fb= TBD kHz
Aperture Delay
T B D
ns typ
Aperture Jitter
T B D
ps typ
Full Power Bandwidth
T B D
MHz typ
@ 3 dB
Full Power Bandwidth
T B D
MHz typ
@ 0.1dB
DC ACCURACY
Resolution
12
Bits
Integral Nonlinearity
2
± 1
LSB max
Differential Nonlinearity
2
± 1
LSB max Guaranteed No Missed Codes to 12 Bits
Offset Error
2
± T B D
LSB max
Gain Error
2
± T B D
LSB max
Total Unadjusted Error (TUE)
2
± T B D
LSB max
ANALOG INPUT
Input Voltage Ranges
0 to V
DD
Volts
DC Leakage Current
±0.5
µA max
Input Capacitance
TBD
pF typ
LOGIC INPUTS
Input High Voltage, V
INH
0.7(V
DD
)
V min
2.35V Vdd 2.7V
2
V min
2.7V < Vdd 3.6V
Input Low Voltage, V
INL
0.2(V
DD
)
V max
2.35V Vdd< 2.7V
0.8
V max
2.7V Vdd 3.6V
Input Current, I
IN
,SCLK Pin ±0.5
µA max
Typically TBDnA, V
IN
= 0 V or V
DD
Input Current, I
IN
,
CS Pin
± T B D
µA max
Input Capacitance, C
IN
3
10
pF max
LOGIC OUTPUTS
Output High Voltage, V
OH
V
DD
- 0.2
V min
I
SOURCE
= 200 µA;V
DD
= 2.35 V to 3.6V
Output Low Voltage, V
OL
0.2
V max
I
SINK
=200 µA
Floating-State Leakage Current
±1
µA max
Floating-State Output Capacitance
3
10
pF max
Output Coding Straight (Natural) Binary
CONVERSION RATE
Conversion Time
270
ns max
14 SCLK Cycles with SCLK at 52 MHz
Track/Hold Acquisition Time
2
50
ns max
Throughput Rate
3
MSPS max See Serial Interface Section
POWER REQUIREMENTS
V
DD
2.35/3.6
V min/max
I
D D
Digital I/Ps 0V or V
DD
Normal Mode(Static)
2.5
mA typ V
DD
= 2.35V to 3.6V, SCLK On or Off
Normal Mode (Operational)
4.5
mA max V
DD
= 2.35V to 3.6V, f
SAMPLE
= 3MSPS
Full Power-Down Mode(Static)
1
µA max SCLK On or Off, typically TBD nA
Full Power-Down Mode(Dynamic)
T B D
mA typ V
DD
= 3V, f
SAMPLE
= 1MSPS
Power Dissipation
4
Normal Mode (Operational)
13.5 mW max V
DD
= 3V, f
SAMPLE
= 3MSPS
Full Power-Down
3 µW max V
DD
=3V
(V
DD
=+2.35 V to +3.6 V, f
SCLK
=52 MHz, f
SAMPLE
=3MSPS unless otherwise noted;
T
A
=T
MIN
to T
MAX
, unless otherwise noted.)
N O T E S
1
Temperature range from 40°C to +85°C.
2
See Terminology.
3
Guranteed by Characterization.
4
See Power Versus Throughput Rate section.
Specifications subject to change without notice.
5
REV. PrF
PRELIMINARY TECHNICAL DATA
Preliminary Technical Data
AD7276/AD7277/AD7278
Limit at T
MIN
, T
MAX
Parameter AD7276/AD7277/AD7278
Units
Description
f
SCLK
2
20
KHz min
3
52
MHz max
t
CONVERT
14 x t
SCLK
AD7276
12 x t
SCLK
AD7277
10 x t
SCLK
AD7278
t
QUIET
T B D
ns min
Minimum Quiet Time required between Bus Relinquish
and start of Next Conversion
t
1
10
ns min
Minimum
CS Pulse Width
t
2
T B D
ns min
CS to SCLK Setup Time
t
3
4
T B D
ns max
Delay from
CS Until SDATA Three-State Disabled
t
4
4
T B D
ns max
Data Access Time After SCLK Falling Edge
t
5
0.4t
SCLK
ns min
SCLK Low Pulse Width
t
6
0.4t
SCLK
ns min
SCLK High Pulse Width
t
7
4
T B D
ns min
SCLK to Data Valid Hold Time
t
8
5
T B D
ns max
SCLK Falling Edge to SDATA Three-State
T B D
ns min
SCLK Falling Edge to SDATA Three-State
t
power-up
6
T B D
µ
s max
Power Up Time from Full Power-down
N O T E S
1
Guaranteed by Characterization. All input signals are specified with tr=tf=5ns (10% to 90% of V
DD
) and timed from a voltage level of 1.6 Volts.
2
Mark/Space ratio for the SCLK input is 40/60 to 60/40.
3
Minimum
f
sclk
at which specifications are guaranteed.
4
Measured with the load circuit of Figure 1 and defined as the time required for the output to cross the Vih or Vil voltage.
5
t
8
is derived form the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of Figure 1. The measured number
is then extrapolated back to remove the effects of charging or discharging the 50 pF capacitor. This means that the time, t
8
, quoted in the
timing characteristics is the true bus relinquish time of the part and is independent of the bus loading.
6
See Power-up Time section.
Specifications subject to change without notice.
TIMING SPECIFICATIONS
1
(V
DD
= +2.35 V to +3.6 V; T
A
= T
MIN
to T
MAX
, unless otherwise noted.)
Figure 1. Load Circuit for Digital Output
Timing Specifications
+1.6V
IOL
200µA
200µA
IOH
TO
OUTPUT
PIN
CL
25pF
V
IH
V
IL
t7
SCLK
SDATA
V
I H
V
IL
t4
SCLK
SDATA
1.4 V
t
8
SCLK
SDATA
Figure 2. Access time after SCLK falling edge
Figure 3. Hold time after SCLK falling edge
Figure 4. SCLK falling edge to SDATA Three-State
6
REV. PrF
PRELIMINARY TECHNICAL DATA
AD7276/AD7277/AD7278
Preliminary Technical Data
Figure 5. AD7276 Serial Interface Timing Diagram
Timing Example 1
From Figure 6, having f
SCLK
= 52 MHz and a throughput of 3MSPS, gives a cycle time of t
2
+ 12.5(1/f
SCLK
) + t
ACQ
=
333 ns. With t
2
= TBD ns min, this leaves t
ACQ
to be TBD ns. This TBD ns satisfies the requirement of TBD ns for
t
ACQ
. Figure 6 shows that, t
ACQ
comprises of 2.5(1/f
SCLK
) + t
8
+ t
QUIET
, where t
8
= TBD ns max. This allows a value of
TBD ns for t
QUIET
satisfying the minimum requirement of TBD ns.
Timing Example 2
Having f
SCLK
= 20 MHz and a throughput of 1.5 MSPS, gives a cycle time of t
2
+ 12.5(1/f
SCLK
) + t
ACQ
= 666 ns.
With t
2
= TBD ns min, this leaves t
ACQ
to be TBD ns. This TBD ns satisfies the requirement of TBD ns for t
ACQ
. From
Figure 6, t
ACQ
comprises of 2.5(1/f
SCLK
) + t
8
+ t
QUIET
, where t
8
= TBD ns max. This allows a values of TBD ns for
t
QUIET
satisfying the minimum requirement of TBD ns.
Figure 6. Serial Interface Timing Example
Figures 5 and 6 show some of the timing parameters from the Timing Specifications table.
&6
SCLK
1
5
13
15
SDATA
2 LEADING
ZERO'S
THREE-
STATE
t
4
2
3
4
16
t
5
t
3
t
quiet
t
convert
t
2
THREE-STATE
DB10
t
6
t
7
t
8
14
ZERO
Z
B
t
1
1/ THROUGHPUT
DB11
DB9
ZERO
ZERO
DB0
DB1
2 TRAILING
ZERO'S
SCLK
1
5
2
3
4
&6
13
t
qu iet
t
2
t
8
14
B
t
acquisition
12.5(1/fSCLK)
1/THROUGHPUT
t
convert
t
1
15
16
12
7
REV. PrF
PRELIMINARY TECHNICAL DATA
Preliminary Technical Data
AD7276/AD7277/AD7278
ABSOLUTE MAXIMUM RATINGS
1
(T
A
= +25°C unless otherwise noted)
V
DD
to GND......................................-0.3 V to TBD V
Analog Input Voltage to GND......0.3 V to V
DD
+ 0.3 V
Digital Input Voltage to GND..............0.3 V to TBD V
Digital Output Voltage to GND....0.3 V to V
DD
+ 0.3 V
Input Current to Any Pin Except Supplies
2
..........±10 mA
Operating Temperature Range
Commercial (B Grade)......................40°C to +85°C
Storage Temperature Range..............65°C to +150°C
Junction Temperature..........................................150°C
6-lead TSOT Package
JA
Thermal Impedance..................................TBD°C/W
JC
Thermal Impedance................................TBD°C/W
N O T E S
1
Stresses above those listed under "Absolute Maximum Ratings" may
cause permanent damage to the device. This is a stress rating only and
functional operation of the device at these or any other conditions above
those listed in the operational sections of this specification is not implied.
Exposure to absolute maximum rating conditions for extended periods
may affect device reliability.
2
Transient currents of up to 100 mA will not cause SCR latch up.
8-lead MSOP Package
JA
Thermal Impedance.................................205.9°C/W
JC
Thermal Impedance...............................43.74°C/W
Lead Temperature Soldering
Reflow (10- 30 sec) ...................................+TBD°C
ESD................................................................. TBD KV
6-Lead TSOT
PIN CONFIGURATION
AD7276/AD7277/AD7278
ORDERING GUIDE
Temperature
Linearity
Package
Package
Branding
Model
Range
Error (LSB)
1
Option
Description Information
AD7276BUJ-REEL 40°C to +85°C
±1 max
UJ-6
T S O T
T B D
AD7276BRM
40°C to +85°C
±1 max
RM-8
M S O P
T B D
AD7277BUJ-REEL 40°C to +85°C
±0.5 max
UJ-6
T S O T
T B D
AD7277BRM
40°C to +85°C
±0.5 max
RM-8
M S O P
T B D
AD7278BUJ-REEL 40°C to +85°C
±0.3 max
UJ-6
T S O T
T B D
AD7278BRMJ
40°C to +85°C
±0.3 max
RM-8
M S O P
T B D
N O T E S
1
Linearity error here refers to integral nonlinearity.
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 AD7276/AD7277/AD7278 feature proprietary ESD protection circuitry, perma-
nent damage may occur on devices subjected to high energy electrostatic discharges. There-
fore, proper ESD precautions are recommended to avoid performance degradation or loss of
functionality.
&6
SDATA
SCLK
VIN
VDD
GND
TOP VIEW
1
2
3
4
5
6
(Not to Scale)
AD7276/
AD7277/
AD7278
TOP VIEW
1
2
3
4
5
6
&6
SDATA
SCLK
VIN
(Not to Scale)
VDD
GND
AD7276/
AD7277/
AD7278
7
8
NC
NC
8-Lead MSOP
8
REV. PrF
PRELIMINARY TECHNICAL DATA
AD7276/AD7277/AD7278
Preliminary Technical Data
PIN FUNCTION DESCRIPTION
Pin
Mnemonic
Function
C S
Chip Select. Active low logic input. This input provides the dual function of initiating
conversion on the AD7276/AD7277/AD7278 and also frames the serial data transfer.
V
D D
Power Supply Input. The V
DD
range for the AD7276/AD7277/AD7278 is from +2.35V to
+3.6V.
G N D
Analog Ground. Ground reference point for all circuitry on the AD7276/AD7277/AD7278.
All analog input signals should be referred to this GND voltage.
V
I N
Analog Input. Single-ended analog input channel. The input range is 0 to V
DD
.
SDATA
Data Out. Logic Output. The conversion result from the AD7276/AD7277/AD7278 is pro-
vided on this output as a serial data stream. The bits are clocked out on the falling edge of
the SCLK input. The data stream from the AD7276 consists of two leading zeros followed
by the 12 bits of conversion data followed by two trailing zeros, which is provided MSB
first. The data stream from the AD7277 consists of two leading zeros followed by the 10 bits
of conversion data followed by four trailing zeros, which is provided MSB first. The data
stream from the AD7278 consists of two leading zeros followed by the 8 bits of conversion
data followed by six trailing zeros, which is provided MSB first.
SCLK
Serial Clock. Logic input. SCLK provides the serial clock for accessing data from the part.
This clock input is also used as the clock source for the AD7276/AD7277/AD7278's conver-
sion process.
9
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PRELIMINARY TECHNICAL DATA
Preliminary Technical Data
AD7276/AD7277/AD7278
THD (dB )
=
20 log
V
2
2
+
V
3
2
+
V
4
2
+
V
5
2
+
V
6
2
V
1
TERMINOLOGY
Integral Nonlinearity
This is the maximum deviation from a straight line pass-
ing through the endpoints of the ADC transfer function.
For the AD7276/AD7277/AD7278, the endpoints of the
transfer function are zero scale, a point 1/2 LSB below
the first code transition, and full scale, a point 1/2 LSB
above the last code transition.
Differential Nonlinearity
This is the difference between the measured and the
ideal 1 LSB change between any two adjacent codes in
the ADC.
Offset Error
This is the deviation of the first code transition (00 . . .
000) to (00 . . . 001) from the ideal, i.e, AGND + 0.5 LSB.
Gain Error
This is the deviation of the last code transition (111 . . .
110) to (111 . . . 111) from the ideal, i.e, V
REF
1.5LSB after the offset error has been adjusted out.
Total Unadjusted Error
This is a comprehensive specification which includes gain,
linearity and offset errors.
Track/Hold Acquisition Time
The Track/Hold acquisition time is the time required
for the output of the track/hold amplifier to reach its
final value, within ±0.5 LSB, after the end of
conversion. See Serial Interface section for more details.
Signal to Noise Ratio (SNR)
This is the measured ratio of signal to noise at the
output to the A/D converter. The signal is the rms value
of the sine wave input. Noise is the rms quantization
error within the Nyquist bandwitdh (fs/2). The rms
value of a sine wave is one half its peak to peak value
divided by
2 and the rms value for the quantization
noise is q/
12. The ratio is dependant on the number of
quantization levels in the digitization process; the more
levels, the smaller the quantization noise. For an ideal
N-bit converter, the SNR is defined as:
SNR = 6.02 N + 1.76 dB
Thus for a 12-bit converter this is 74 dB, for a 10-bit
converter it is 62dB and for an 8-bit converter it is
50dB.
Practically, though, various error sources in the ADC
cause the measured SNR to be less than the theoretical
value. These errors occur due to integral and differential
nonlinearities, internal AC noise sources, etc.
Signal-to- (Noise + Distortion) Ratio (SINAD)
This is the measured ratio of signal to (noise +
distortion) at the output of the A/D converter. The
signal is the rms value of the sine wave and noise is the
rms sum of all nonfundamentals signals up to half the
sampling frequency (fs/2), including harmonics but
excluding dc.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms
sum of harmonics to the fundamental. It is defined as:
where V
1
is the rms amplitude of the fundamental and V
2
,
V
3
, V
4
, V
5
and V
6
are the rms amplitudes of the second
through the sixth harmonics.
Peak Harmonic or Spurious Noise
Peak harmonic or spurious noise is defined as the ratio of
the rms value of the next largest component in the ADC
output spectrum (up to f
S
/2 and excluding dc) to the rms
value of the fundamental. Normally, the value of this
specification is determined by the largest harmonic in the
spectrum, but for ADCs where the harmonics are buried
in the noise floor, it will be a noise peak.
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa
and fb, any active device with nonlinearities will create
distortion products at sum and difference frequencies of
mfa ± nfb where m, n = 0, 1, 2, 3, etc. Intermodulation
distortion terms are those for which neither m nor n are
equal to zero. For example, the second order terms in-
clude (fa + fb) and (fa fb), while the third order terms
include (2fa + fb), (2fa fb), (fa + 2fb) and (fa 2fb).
The AD7276/AD7277/AD7278 are tested using the CCIF
standard where two input frequencies are used (see fa and
fb in the specification page). In this case, the second order
terms are usually distanced in frequency from the original
sine waves while the third order terms are usually at a
frequency close to the input frequencies. As a result, the
second and third order terms are specified separately. The
calculation of the intermodulation distortion is as per the
THD specification where it is the ratio of the rms sum of
the individual distortion products to the rms amplitude of
the sum of the fundamentals expressed in dBs.
Aperture Delay
This is the measured interval between the leading edge of the
sampling clock and the point at which the ADC actually takes
the sample.
Aperture Jitter
This is the sample-to-sample variation in the effective point
in time at which the sample is taken.
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PRELIMINARY TECHNICAL DATA
AD7276/AD7277/AD7278
Preliminary Technical Data
PERFORMANCE CURVES
Dynamic Performance curves
TPC 1, TPC 2 and TPC 3 show typical FFT plots for the
AD7276, AD7277 and AD7278 respectively, at 3 MSPS
sample rate and TBD KHz input tone.
TPC 4 shows the Signal-to-(Noise+Distortion) Ratio
performance versus Input frequency for various supply
voltages while sampling at 3 MSPS with a SCLK frequency
of 52 MHz for the AD7276.
TPC 5 shows the Signal to Noise Ratio (SNR) performance
versus Input frequency for various supply voltages while
sampling at 3 MSPS with a SCLK frequency of 52 MHz for
the AD7276.
TPC 6 shows a graph of the Total Harmonic Distortion
versus Analog input signal frequency for various supply
voltages while sampling at 3 MSPS with a SCLK frequency
of 52 MHz for the AD7276.
TPC 7 shows a graph of the Total Harmonic Distortion
versus Analog input frequency for different source impedances
when using a supply voltage of TBD V,
SCLK frequency of
52 MHz
and sampling at a rate of 3 MSPS for the AD7276.
See Analog Input section.
DC Accuracy curves
TPC 8 and TPC 9 show typical INL and DNL performance
for the AD7276.
Power Requirements curves
TPC10 shows Maximum current versus Supply voltage for
the AD7276 with different SCLK frequencies.
See also Power versus Throughput Rate section.
Typical Performance Characteristics
TPC 1. AD7276 Dynamic performance at 3 MSPS
TPC 2. AD7277 Dynamic performance at 3 MSPS
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PRELIMINARY TECHNICAL DATA
Preliminary Technical Data
AD7276/AD7277/AD7278
TPC 8. AD7276 INL performance
TPC 5. AD7276 SNR vs Analog Input Frequency
at 3 MSPS for various Supply Voltages
TPC 3. AD7278 Dynamic performance at 3 MSPS
TPC 6. THD vs. Analog Input Frequency at 3 MSPS
for various Supply Voltages
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for various Source Impedance
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TPC 4. AD7276 SINAD vs Analog Input Frequency
at 3 MSPS for various Supply Voltages
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PRELIMINARY TECHNICAL DATA
AD7276/AD7277/AD7278
Preliminary Technical Data
TPC 10. Maximum current vs Supply voltage for
different SCLK frequencies.
TPC 9. AD7276 DNL performance
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PRELIMINARY TECHNICAL DATA
Preliminary Technical Data
AD7276/AD7277/AD7278
CIRCUIT INFORMATION
The AD7276/AD7277/AD7278 are fast, micropower, 12-/
10-/8-Bit, single supply, A/D converters respectively. The
parts can be operated from a +2.35V to +3.6V supply.
When operated from any supply voltage within this range,
the AD7276/AD7277/AD7278 are capable of throughput
rates of 3 MSPS when provided with a 52 MHz clock.
The AD7276/AD7277/AD7278 provide the user with an
on-chip track/hold, A/D converter, and a serial interface
housed in a tiny 6-lead TSOT or 8-lead MSOP package,
which offers the user considerable space saving advantages
over alternative solutions. The serial clock input accesses
data from the part but also provides the clock source for
the successive-approximation A/D converter. The analog
input range is 0 to V
DD
. An external reference is not
required for the ADC and neither is there a reference on-
chip. The reference for the AD7276/AD7277/AD7278 is
derived from the power supply and thus gives the widest
dynamic input range.
The AD7276/AD7277/AD7278 also feature a power down
option to allow power saving between conversions. The
Power-Down feature is implemented across the standard
serial interface as described in the Modes of Operation
section.
CONVERTER OPERATION
The AD7276/AD7277/AD7278 is a successive-
approximation analog-to-digital converter based around a
charge redistribution DAC. Figures 7 and 8 show
simplified schematics of the ADC. Figure 7 shows the
ADC during its acquisition phase. SW2 is closed and SW1
is in position A, the comparator is held in a balanced
condition and the sampling capacitor acquires the signal on
V
IN
.
When the ADC starts a conversion, see Figure 8, SW2
will open and SW1 will move to position B causing the
comparator to become unbalanced. The Control Logic
and the Charge Redistribution DAC are used to add and
subtract fixed amounts of charge from the sampling ca-
pacitor to bring the comparator back into a balanced con-
dition. When the comparator is rebalanced the conversion
is complete. The Control Logic generates the ADC out-
put code. Figure 9 shows the ADC transfer function.
Figure 7. ADC Acquisition Phase
CHA R GE
RE DI ST R I B UT I ON
D A C
VI N
V DD / 2
SA MP LI NG
CAP AC I TO R
CO M PA R AT O R
CO N T RO L
LO GI C
ACQ UI SI TI ON
PH AS E
SW1
A
B
SW2
AG N D
ADC TRANSFER FUNCTION
The output coding of the AD7276/AD7277/AD7278 is
straight binary. The designed code transitions occur
midway between succesive integer LSB values, i.e,
0.5LSB, 1.5LSBs, etc. The LSB size is V
DD
/4096 for the
AD7276, V
DD
/1024 for the AD7277 and V
DD
/256 for the
AD7278. The ideal transfer characteristic for the AD7276/
AD7277/AD7278 is shown in Figure 9.
Figure 8. ADC Conversion Phase
CHARGE
REDISTRIBUTION
DAC
VIN
VDD / 2
SAMP LING
CAPACIT OR
COMPARATOR
CONTROL
LOGIC
CONVERSION
PHASE
S W1
A
B
SW2
AGND
Figure 9. AD7276/AD7277/AD7278 Transfer Characteristic
000...000
0V
A
D
C
C
O
D
E
ANALOG INPUT
111...111
000...001
000...010
111...110
111...000
011...111
0.5LSB
+VDD-1.5LSB
1LSB = VDD/1024 (AD7277)
1LSB = VDD/256 (AD7278)
1LSB = VDD/4096 (AD7276)
14
REV. PrF
PRELIMINARY TECHNICAL DATA
AD7276/AD7277/AD7278
Preliminary Technical Data
TYPICAL CONNECTION DIAGRAM
Figure 10 shows a typical connection diagram for the
AD7276/AD7277/AD7278. V
REF
is taken internally from
V
DD
and as such V
DD
should be well decoupled. This
provides an analog input range of 0V to V
DD
. The
conversion result is output in a 16-bit word with two
leading zeros followed by the 12-bit, 10-bit or 8-bit result.
The 12-bit result from the AD7276 will be followed by
two trailing zeros and the 10-bit and 8-bit result from the
AD7277 and AD7278 will be followed by four and six
trailing zeros respectively.
Alternatively, because the supply current required by the
AD7276/AD7277/AD7278 is so low, a presision reference
can be used as the supply source to the AD7276/AD7277/
AD7278. A REF19x voltage reference (REF193 for 3V)
can be used to supply the required voltage to the ADC
-see Figure 10. This configuration is especially useful if
the power supply is quite noisy or if the system supply
voltages are at some value other than 3V (e.g. 5V or 15V).
The REF19x will output a steady voltage to the AD7276/
7277/7278. If the low dropout REF193 is used, the
current it needs to supply to the AD7276/AD7277/
AD7278 is typically TBD mA. When the ADC is
converting at a rate of 3 MSPS the REF193 will need to
supply a maximum of TBD mA to the AD7276/AD7277/
AD7278. The load regulation of the REF193 is typically
10 ppm/mA (REF193, V
S
= 5V), which results in an error
of TBD ppm (TBD
µ
V) for the TBD mA drawn from it.
This corresponds to a TBD LSB error for the AD7276
with V
DD
= 3V from the REF193, a TBD LSB error for
the AD7277, and a TBD LSB error for the AD7278. For
applications where power consumption is of concern, the
Power-Down mode of the ADC and the sleep mode of the
REF19x reference should be used to improve power per-
formance. See Modes of Operation section.
Figure 10. REF193 as Power Supply to AD7276/
AD7277/AD7278
V
DD
VIN
GND
+5V
SUPPLY
0.1µF
10µF
REF193
0.1µF
1µF
TANT
+3V
0V toVDD
INPUT
SDATA
DSP/
µC/µP
SCLK
SERIAL
INTERFACE
&6
TBD mA
680nF
AD7276/
AD7277/
AD7278
Table I provides some typical performance data with
various references used as a V
DD
source under the same
set-up conditions.
Reference Tied AD7276 SNR Performance
To V
DD
TBD kHz Input
AD780@3V
TBD dB
ADR423
TBD dB
AD780@2.5V TBD dB
REF192
TBD dB
ADR421
TBD dB
ADR291
TBD dB
Table I. AD7276 performance for various Voltage
References IC
Analog Input
Figure 11 shows an equivalent circuit of the analog input
structure of the AD7276/AD7277/AD7278. The two
diodes D1 and D2 provide ESD protection for the analog
inputs. Care must be taken to ensure that the analog input
signal never exceeds the supply rails by more than 300mV.
This will cause these diodes to become forward biased and
start conducting current into the substrate. 10mA is the
maximum current these diodes can conduct without
causing irreversable damage to the part. The capacitor C1
in Figure 11 is typically about 4pF and can primarily be
attributed to pin capacitance. The resistor R1 is a lumped
component made up of the on resistance of a switch. This
resistor is typically about TBD
.
The capacitor C2 is the
ADC sampling capacitor and has a capacitance of TBD
pF typically. For ac applications, removing high
frequency components from the analog input signal is
recommended by use of a bandpass filter on the relevant
analog input pin. In applications where harmonic distor-
tion and signal to noise ratio are critical, the analog input
should be driven from a low impedance source. Large
source impedances will significantly affect the ac perfor-
mance of the ADC. This may necessitate the use of an
input buffer amplifier. The choice of the op-amp will be a
function of the particular application.
Figure 11. Equivalent Analog Input Circuit
VIN
D1
VDD
D2
R1
C2
TBD PF
C1
4pF
CONVERSION PHASE - SWITCH OPEN
TRACK PHASE - SWITCH CLOSED
15
REV. PrF
PRELIMINARY TECHNICAL DATA
Preliminary Technical Data
AD7276/AD7277/AD7278
Table II provides some typical performance data with
various op-amps used as the input buffer under the same
set-up conditions.
Op-amp in the AD7276 SNR Performance
input buffer
TBD kHz Input
AD8510
TBD dB
AD8610
TBD dB
AD8038 TBD dB
AD8519
TBD dB
When no amplifier is used to drive the analog input, the
source impedance should be limited to low values. The
maximum source impedance will depend on the amount
of total harmonic distortion (THD) that can be
tolerated. The THD will increase as the source
impedance increases and performance will degrade. TPC
7 shows a graph of the Total Harmonic Distortion
versus Analog input frequency for different source
impedances when using a supply voltage of TBD V and
sampling at a rate of 3 MSPS.
Table II. AD7276 performance for various Input Buffers
Digital Inputs
The digital inputs applied to the AD7276/AD7277/
AD7278 are not limited by the maximum ratings which
limit the analog inputs. Instead, the digitals inputs applied
can go to TBDV and are not restricted by the V
DD
+ 0.3V
limit as on the analog inputs. For example, if the
AD7276/AD7277/AD7278 were operated with a V
DD
of
3V then 5V logic levels could be used on the digital
inputs. However, it is important to note that the data
output on SDATA will still have 3V logic levels when
V
DD
= 3V. Another advantage of SCLK and
CS not being
restricted by the V
DD
+ 0.3V limit is the fact that power
supply sequencing issues are avoided. If
CS or SCLK are
applied before V
DD
then there is no risk of latch-up as
there would be on the analog inputs if a signal greater
than 0.3V was applied prior to V
DD
.
MODES OF OPERATION
The mode of operation of the AD7276/AD7277/AD7278
is selected by controlling the logic state of the
CS signal
during a conversion. There are two possible modes of
operation, Normal Mode and Power-Down Mode. The
point at which
CS is pulled high after the conversion has
been initiated will determine whether the AD7276/
AD7277/AD7278 will enter Power-Down Mode or not.
Similarly, if already in Power-Down then
CS can control
whether the device will return to Normal operation or
remain in Power-Down. These modes of operation are
designed to provide flexible power management options.
These options can be chosen to optimize the power dissi-
pation/throughput rate ratio for different application
requirements.
Normal Mode
This mode is intended for fastest throughput rate perfor-
mance as the user does not have to worry about any
power-up times with the AD7276/AD7277/AD7278
remaining fully powered all the time. Figure 12 shows the
general diagram of the operation of the AD7276/AD7277/
AD7278 in this mode.
The conversion is iniated on the falling edge of
CS as
described in the Serial Interface section. To ensure the
part remains fully powered up at all times
CS must remain
low until at least 10 SCLK falling edges have elapsed after
the falling edge of
CS. If CS is brought high any time
after the 10th SCLK falling, the part will remain powered
up but the conversion will be terminated and SDATA will
go back into three-state.
For the AD7276 a minimum of 14 serial clock cycles are
required to complete the conversion and access the
complete conversion result. For the AD7277 and AD7278
a minimum of 12 and 10 serial clock cycles are required
to complete the conversion and access the complete con-
version result, respectively.
CS may idle high until the next conversion or may idle
low until
CS returns high sometime prior to the next
conversion (effectively idling
CS low).
Once a data transfer is complete (SDATA has returned to
three-state), another conversion can be initiated after the
quiet time, t
QUIET
, has elapsed by bringing
CS low again.
Figure 12. Normal Mode Operation
12
14
16
AD7276/77/78
VALID DATA
SDATA
SCLK
&6
1
10
16
REV. PrF
PRELIMINARY TECHNICAL DATA
AD7276/AD7277/AD7278
Preliminary Technical Data
Figure 13. Entering Power Down Mode
Figure 14. Exiting Power Down Mode
THREE-STATE
1
16
10
2
SDATA
SCLK
&6
INVALID DATA
SCLK
&6
INVALID DATA
VALID DATA
1
10
16
16
1
THE PART BEGINS
TO POWER UP
THE PART IS FULLY
POWERED UP WITH VIN
FULLY ACQUIRED
A
SDATA
Power-Down Mode
This mode is intended for use in applications where
slower throughput rates are required; either the ADC is
powered down between each conversion, or a series of
conversions may be performed at a high throughput rate
and then the ADC is powered down for a relatively long
duration between these bursts of several conversions.
When the AD7276/AD7277/AD7278 is in Power-Down,
all analog circuitry is powered down.
To enter Power-Down, the conversion process must be
interrupted by bringing
CS high anywhere after the second
falling edge of SCLK and before the 10th falling edge of
SCLK as shown in Figure 13. Once
CS has been brought
high in this window of SCLKs, then the part will enter
Power-Down and the conversion that was intiated by the
falling edge of
CS will be terminated and SDATA will go
back into three-state. If
CS is brought high before the
second SCLK falling edge, then the part will remain in
Normal Mode and will not power-down. This will avoid
accidental power-down due to glitches on the
CS line.
In order to exit this mode of operation and power the
AD7276/AD7277/AD7278 up again, a dummy conversion
is performed. On the falling edge of
CS the device will
begin to power up, and will continue to power up as long
as
CS is held low until after the falling edge of the 10th
SCLK. The device will be fully powered up once 16
SCLKs have elapsed and valid data will result from the
next conversion as shown in Figure 14. If
CS is brought
high before the 10th falling edge of SCLK, then the
AD7276/AD7277/AD7278 will go back into Power-
Down again. This avoids accidental power up due to
glitches on the
CS line or an inadvertent burst of 8 SCLK
cycles while
CS is low. So, although the device may begin
to power up on the falling edge of
CS, it will power down
again on the rising edge of
CS as long as it occurs before
the 10th SCLK falling edge.
17
REV. PrF
PRELIMINARY TECHNICAL DATA
Preliminary Technical Data
AD7276/AD7277/AD7278
Power-up Time
The power-up time of the AD7276/AD7277/AD7278 is
TBD ns, which means that with any frequency of SCLK
up to 52 MHz, one dummy cycle will always be sufficient
to allow the device to power up. Once the dummy cycle is
complete, the ADC will be fully powered up and the input
signal will be acquired properly. The quite time t
QUIET
must still be allowed from the point where the bus goes
back into three-state after the dummy conversion, to the
next falling edge of
CS. When running at 3 MSPS
throughput rate, the AD7276/AD7277/AD7278 will power
up and acquire a signal within ±0.5LSB in one dummy
cycle, i.e. TBD ns.
When powering up from the Power-Down mode with a
dummy cycle, as in Figure 14, the track and hold which
was in hold mode while the part was powered down,
returns to track mode after the first SCLK edge the part
receives after the falling edge of
CS. This is shown as
point A in Figure 14. Although at any SCLK frequency
one dummy cycle is sufficient to power the device up and
acquire V
IN
, it does not necessarily mean that a full
dummy cycle of 16 SCLKs must always elapse to power
up the device and acquire V
IN
fully; TBD ns will be suffi-
cient to power the device up and acquire the input signal.
If, for example, a 25 MHz SCLK frequency was applied
to the ADC, the cycle time would be 640 ns. In one
dummy cycle, 640 ns, the part would be powered up and
V
IN
acquired fully. However after TBD ns with a 25 MHz
SCLK only TBD SCLK cycles would have elapsed. At
this stage, the ADC would be fully powered up and the
signal acquired. So, in this case the
CS can be brought
high after the 10th SCLK falling edge and brought low
again after a time t
QUIET
to initiate the conversion.
When power supplies are first applied to the AD7276/
AD7277/AD7278, the ADC may either power up in the
Power-Down mode or in Normal mode. Because of this,
it is best to allow a dummy cycle to elapse to ensure the
part is fully powered up before attempting a valid
conversion. Likewise, if it is intended to keep the part in
the Power-Down mode while not in use and the user
wishes the part to power up in Power-Down mode, then
the dummy cycle may be used to ensure the device is in
Power-Down by executing a cycle such as that shown in
Figure 13. Once supplies are applied to the AD7276/
AD7277/AD7278, the power up time is the same as that
when powering up from the Power-Down mode. It takes
approximately TBD ns to power up fully if the part
powers up in Normal mode. It is not necessary to wait
TBD
ns before executing a dummy cycle to ensure the
desired mode of operation. Instead, the dummy cycle can
occur directly after power is supplied to the ADC. If the
first valid conversion is then performed directly after the
dummy conversion, care must be taken to ensure that
adequate acquisition time has been allowed. As mentioned
earlier, when powering up from the Power-Down mode,
the part will return to track upon the first SCLK edge
applied after the falling edge of
CS. However, when the
ADC powers up initially after supplies are applied, the
track and hold will already be in track. This means,
assuming one has the facility to monitor the ADC supply
current, if the ADC powers up in the desired mode of
operation and thus a dummy cycle is not required to
change mode, then neither is a dummy cycle required to
place the track and hold into track.
POWER VERSUS THROUGHPUT RATE
By using the Power-Down mode on the AD7276/AD7277/
AD7278 when not converting, the average power con-
sumption of the ADC decreases at lower throughput rates.
Figure 15 shows how as the throughput rate is reduced,
the device remains in its Power-Down state longer and the
average power consumption over time drops accordingly.
For example, if the AD7276/AD7277/AD7278 is operated
in a continuous sampling mode with a throughput rate of
500KSPS and a SCLK of 52MHz (V
DD
= 3V), and the
device is placed in the Power-Down mode between
conversions, then the power consumption is calculated as
follows. The power dissipation during normal operation is
13.5 mW (V
DD
= 3V). If the power up time is one dummy
cycle, i.e. 333ns, and the remaining conversion time is
another cycle, i.e. 333ns, then the AD7276/AD7277/
AD7278 can be said to dissipate 13.5mW for 666ns during
each conversion cycle.If the throughput rate is 500KSPS,
the cycle time is 2
µ
s and the average power dissipated
during each cycle is (666/2000) x (13.5 mW)= 4.5mW.
Figure 15 shows the Power vs. Throughput Rate when
using the Power-Down mode between conversions at 3V.
The Power-Down mode is intended for use with
throughput rates of approximately TBD MSPS and under
as at higher sampling rates there is no power saving made
by using the Power-Down mode.
TITLE
0
0
T
I
T
L
E
TBD
Figure 15. Power vs Throughput
18
REV. PrF
PRELIMINARY TECHNICAL DATA
AD7276/AD7277/AD7278
Preliminary Technical Data
SERIAL INTERFACE
Figures 16, 17 and 18 show the detailed timing diagram
for serial interfacing to the AD7276, AD7277 and
AD7278 respectively. The serial clock provides the
conversion clock and also controls the transfer of
information from the AD7276/AD7277/AD7278 during
conversion.
The
CS signal initiates the data transfer and conversion
process. The falling edge of
CS puts the track and hold
into hold mode, takes the bus out of three-state and the
analog input is sampled at this point. The conversion is
also initiated at this point.
For the AD7276 the conversion will require 14 SCLK
cycles to complete. Once 13 SCLK falling edges have
elapsed the track and hold will go back into track on the
next SCLK rising edge as shown in Figure 16 at point B.
If the rising edge of
CS occurs before 14 SCLKs have
elapsed then the conversion will be terminated and the
SDATA line will go back into three-state. If 16 SCLKs
are considered in the cycle, the last two bits will be zeros
and SDATA will return to three-state on the 16th SCLK
falling edge as shown in Figure 16.
For the AD7277 the conversion will require 12 SCLK
cycles to complete. Once 11 SCLK falling edges have
elapsed, the track and hold will go back into track on the
next SCLK rising edge, as shown in Figure 17 at point B.
If the rising edge of
CS occurs before 12 SCLKs have
elapsed then the conversion will be terminated and the
SDATA line will go back into three-state. If 16 SCLKs
are considered in the cycle, the AD7277 will clock out
four trailing zeros for the last four bits and SDATA will
return to three-state on the 16th SCLK falling edge, as
shown in Figure 17.
For the AD7278 the conversion will require 10 SCLK
cycles to complete. Once 9 SCLK falling edges have
Figure 16. AD7276 Serial Interface Timing Diagram
elapsed, the track and hold will go back into track on the
next rising edge. If the rising edge of
CS occurs before 10
SCLKs have elapsed then the part will enter Power-Down
mode. If 16 SCLKs are considered in the cycle, the
AD7278 will clock out six trailing zeros for the last six
bits and SDATA will return to three-state on the 16th
SCLK falling edge, as shown in Figure 18.
If the user considers a 14 SCLKs cycle serial interface for
the AD7276/AD7277/AD7278,
CS needs to be brought
high after the 14th SCLK falling edge, the last two
trailing zeros will be ignored and SDATA will go back
into three-state. In this case, the 3MSPS throughput could
be achieved using a 45MHz clock frequency.
CS going low clocks out the first leading zero to be read
in by the microcontroller or DSP. The remaining data is
then clocked out by subsequent SCLK falling edges
beginning with the 2nd leading zero. Thus the first falling
clock edge on the serial clock has the first leading zero
provided and also clocks out the second leading zero. The
final bit in the data transfer is valid on the 16th falling
edge, having being clocked out on the previous (15th)
falling edge.
In applications with a slower SCLK, it is possible to read
in data on each SCLK rising edge. In that case, the first
falling edge of SCLK will clock out the second leading
zero and it could be read in the first rising edge. However,
the first leading zero that was clocked out when
CS went
low will be missed unless it was not read in the first falling
edge. The 15th falling edge of SCLK will clock out the
last bit and it could be read in the 15th rising SCLK edge.
If
CS goes low just after one the SCLK falling edge has
elapsed,
CS will clock out the first leading zero as before
and it may be read in the SCLK rising edge. The next
SCLK falling edge will clock out the second leading zero
and it could be read in the following rising edge.
&6
SCLK
1
5
13
15
S DATA
2 LEADING
ZERO'S
THREE-
STATE
t
4
2
3
4
16
t
5
t
3
t
quiet
t
convert
t
2
THREE-STATE
DB10
t
6
t
7
t
8
14
ZERO
Z
B
t
1
1/ THROUGHPUT
DB11
DB9
ZERO
ZERO
DB0
DB1
2 TRAILING
ZE RO 'S
19
REV. PrF
PRELIMINARY TECHNICAL DATA
Preliminary Technical Data
AD7276/AD7277/AD7278
Figure 18. AD7278 Serial Interface Timing Diagram
&6
11
15
t
5
16
t
q uiet
THREE-STATE
t
8
14
ZERO
ZERO
6TRAILING ZERO'S
ZERO
B
SCLK
1
SDATA
2 LEADI NG
ZERO'S
THREE-
STATE
t
4
2
3
4
t
3
t
convert
t
2
DB6
t
6
t
7
ZERO
Z
t
1
1/ THROUGHPUT
DB7
DB0
DB1
8
9
10
Figure 17. AD7277 Serial Interface Timing Diagram
&6
13
15
t
5
16
t
quiet
THREE-STATE
t
8
14
ZERO
ZERO
4TRAILING ZE RO'S
ZERO
ZERO
B
SCLK
1
SDATA
2 LEADI NG
ZERO'S
THREE-
STATE
t
4
2
3
4
t
3
t
convert
t
2
DB8
t
6
t
7
ZERO
Z
t
1
1/ THROUGHPUT
DB9
DB0
DB1
10
11
12
AD7278 in a 10 SCLK's cycle Serial Interface
For the AD7278, if
CS is brought high in the 10th rising
edge after the 2 leading zeros and the 8 bits of the
conversion have been provided, the part can achieve a
4.2MSPS throughput rate. For the AD7278, the track and
hold goes back into track in the 9th rising edge. In that
case, a f
SCLK
= 52 MHz and a throughput of 4.2MSPS,
gives a cycle time of t
2
+ 8.5(1/f
SCLK
) + t
ACQ
= 238ns.
With t
2
= TBDns min, this leaves t
ACQ
to be TBDns.
This TBDns satisfies the requirement of 50 ns for t
ACQ
.
From Figure 19, t
ACQ
comprises of 0.5(1/f
SCLK
) + t
8
+
t
QUIET
, where t
8
= TBDns max. This allows a value of
TBDns for t
QUIET
satisfying the minimum requirement of
TBDns.
Figure 19. AD7278 in a 10 SCLK Cycle Serial Interface
&6
SCLK
t
1
1
5
9
SDATA
2 LEADING ZERO'S
3-STATE
3-STATE
DB7
DB6
DB0
ZERO
Z
DB1
2
3
4
t
QUIET
t
convert
t
2
t
6
t
8
10
B
1/ THROUGHPUT
t
ACQ
DB5
8.5 (1/ fSCLK)
20
REV. PrA
PRELIMINARY TECHNICAL DATA
Preliminary Technical Data
OUTLINE DIMENSIONS
Dimensions shown in millimeters
8-Lead Mini Small Outline Package [MSOP]
(RM-8)
Dimensions shown in millimeters
&
#
"
0.122 (3.10)
0.114 (2.90)
0.199 (5.05)
0.187 (4.75)
PIN 1
0.0256 (0.65) BSC
0.122 (3.10)
0.114 (2.90)
SEATING
PLANE
0.006 (0.15)
0.002 (0.05)
0.018 (0.46)
0.008 (0.20)
0.043 (1.09)
0.037 (0.94)
0.120 (3.05)
0.112 (2.84)
0.011 (0.28)
0.003 (0.08)
0.028 (0.71)
0.016 (0.41)
33°
27°
0.120 (3.05)
0.112 (2.84)
COMPLIANT TO JEDEC STANDARDS MO-187AA
AD7276/AD7277/AD7278
1
3
4
5
2
6
PIN 1
1 .6 0 BSC
2 .8 0 BSC
1 .9 0
BSC
0 .9 5 BSC
0.2 2
0.0 8
1 0 °
4 °
0 °
0 .5 0
0 .3 0
0 .1 0 MAX
0 .9 0
0 .8 7
0 .8 4
SEATING
PLANE
1 .0 0 MAX
0 .6 0
0 .4 5
0 .3 0
2 .9 0 BSC
6-Lead Thin Small Outline Transistor Package [TSOT]
(UJ-6)
Dimensions shown in millimeters
COMPLIANT TO JEDEC STANDARDS MO-193AA
PR04903-0-6/04(PrF)
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