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.

AD7888

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

Š Analog Devices, Inc., 1999

+2.7 V to +5.25 V, Micropower, 8-Channel,

125 kSPS, 12-Bit ADC in 16-Lead TSSOP

FUNCTIONAL BLOCK DIAGRAM

SAR + ADC

CONTROL LOGIC

AGND

REF IN/REF OUT

AIN8

AIN1

SCLK

DOUT

DIN

AGND

AD7888

T/H

I/P

MUX

BUF

2.5V
REF

CHARGE

REDISTRIBUTION

DAC

SPORT

COMP

FEATURES
Specified for V

of +2.7 V to +5.25 V

Flexible Power/Throughput Rate Management
Shutdown Mode: 1 A Max
Eight Single-Ended Inputs
Serial Interface: SPITM/QSPITM/MICROWIRETM/DSP

Compatible

16-Lead Narrow SOIC and TSSOP Packages

APPLICATIONS
Battery-Powered Systems (Personal Digital Assistants,

Medical Instruments, Mobile Communications)

Instrumentation and Control Systems
High Speed Modems

GENERAL DESCRIPTION

The AD7888 is a high speed, low power, 12-bit ADC that oper-
ates from a single +2.7 V to +5.25 V power supply. The AD7888
is capable of a 125 kSPS throughput rate. The input track-and-
hold acquires a signal in 500 ns and features a single-ended
sampling scheme. The AD7888 contains eight single-ended
analog inputs, AIN1 through AIN8. The analog input on each
of these channels is from 0 to V

REF

. The part is capable of con-

verting full power signals up to 2.5 MHz.

The AD7888 features an on-chip 2.5 V reference that can be
used as the reference source for the A/D converter. The REF
IN/REF OUT pin allows the user access to this reference. Alter-
natively, this pin can be overdriven to provide an external refer-
ence voltage for the AD7888. The voltage range for this external
reference is from 1.2 V to V

CMOS construction ensures low power dissipation of typically
2 mW for normal operation and 3

W in power-down mode.

The part is available in a 16-lead narrow body small outline
(SOIC) and a 16-lead thin shrink small outline (TSSOP) package.

PRODUCT HIGHLIGHTS

1. Smallest 12-bit 8-channel ADC; 16-lead TSSOP is the same

area as an 8-lead SOIC and less than half the height.

2. Lowest Power 12-bit 8-channel ADC.

3. Flexible power management options including automatic

power-down after conversion.

4. Analog input range from 0 V to V

REF

5. Versatile serial I/O port (SPI/QSPI/MICROWIRE/DSP

Compatible).

SPI and QSPI are trademarks of Motorola, Inc.
MICROWIRE is a trademark of National Semiconductor Corporation.

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AD7888SPECIFICATIONS

= +2.7 V to +5.25 V, REFIN/REFOUT = +2.5 V External/Internal Reference unless

otherwise noted; f

SCLK

= 2 MHz (V

= +2.7 V to +5.25 V); T

= T

MIN

to T

MAX

, unless otherwise noted)

Parameter

A Version

B Version

Units

Test Conditions/Comments

DYNAMIC PERFORMANCE

Signal to Noise + Distortion Ratio

2, 3

(SNR)

dB typ

= 10 kHz Sine Wave, f

SAMPLE

= 125 kSPS

Total Harmonic Distortion

(THD)

dB typ

= 10 kHz Sine Wave, f

SAMPLE

= 125 kSPS

Peak Harmonic or Spurious Noise

dB typ

= 10 kHz Sine Wave, f

SAMPLE

= 125 kSPS

Intermodulation Distortion

(IMD)

Second Order Terms

dB typ

fa = 9.983 kHz, fb = 10.05 kHz, f

SAMPLE

= 125 kSPS

Third Order Terms

dB typ

fa = 9.983 kHz, fb = 10.05 kHz, f

SAMPLE

= 125 kSPS

Channel-to-Channel Isolation

dB typ

= 25 kHz

Full Power Bandwidth

2.5

MHz typ

@ 3 dB

DC ACCURACY

Any Channel

Resolution

Bits

Integral Nonlinearity

LSB max

Differential Nonlinearity

LSB max

Guaranteed No Missed Codes to 11 Bits (A Grade)
Guaranteed No Missed Codes to 12 Bits (B Grade)

Offset Error

LSB max

= 4.75 V to 5.25 V (Typically

3 LSB)

4.5

LSB max

= 2.7 V to 3.6 V (Typically

2 LSB)

Offset Error Match

LSB typ

Gain Error

LSB max

Typically 30 LSB with Internal Reference

Gain Error Match

LSB max

ANALOG INPUT

Input Voltage Ranges

0 to V

REF

0 to V

REF

Volts

Leakage Current

A max

Input Capacitance

pF typ

When in Track

pF typ

When in Hold

REFERENCE INPUT/OUTPUT

REFIN Input Voltage Range

2.5/V

V min/max

Functional from 1.2 V

Input Impedance

typ

Very High Impedance If Internal Reference Disabled

REFOUT Output Voltage

2.45/2.55

V min/max

REFOUT Tempco

ppm/

C typ

LOGIC INPUTS

Input High Voltage, V

INH

2.4

V min

= +4.75 V to +5.25 V

2.1

V min

= +2.7 V to +3.6 V

Input Low Voltage, V

INL

0.8

V max

= +2.7 V to +5.25 V

Input Current, I

A max

Typically 10 nA, V

= 0 V or V

Input Capacitance, C

pF max

LOGIC OUTPUTS

Output High Voltage, V

SOURCE

= 200

0.5

V min

= +2.7 V to +5.25 V

Output Low Voltage, V

0.4

V max

SINK

= 200

Floating-State Leakage Current

A max

Floating-State Output Capacitance

pF max

Output Coding

Straight (Natural) Binary

CONVERSION RATE

Throughput Time

SCLK Cycles

Conversion Time + Acquisition Time. 125 kSPS with
2 MHz Clock

Track/Hold Acquisition Time

1.5

SCLK Cycles

Conversion Time

14.5

SCLK Cycles

7.25

s (2 MHz Clock)

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AD7888

Parameter

A Version

B Version

Units

Test Conditions/Comments

POWER REQUIREMENTS

+2.7/+5.25

V min/max

Normal Mode

(Static)

700

A max

Normal Mode

(Operational)

700

A typ

SAMPLE

= 125 kSPS

Using Standby Mode

450

A typ

SAMPLE

= 50 kSPS

Using Shutdown Mode

A typ

SAMPLE

= 10 kSPS

A typ

SAMPLE

= 1 kSPS

Standby Mode

200

A max

= +2.7 V to +5.25 V

Shutdown Mode

A max

= +4.75 V to +5.25 V (0.5

A typ)

A max

= +2.7 V to +3.6 V

Normal-Mode Power Dissipation

3.5

mW max

= +5 V

2.1

mW max

= +3 V

Shutdown Power Dissipation

W max

= +5 V

W max

= +3 V

Standby Power Dissipation

mW max

= +5 V

600

W max

= +3 V

NOTES

Temperature ranges as follows: A Version: 40

C to +105

C; B Version: 0

C to +105

See Terminology.

SNR calculation includes distortion and noise components.

Sample tested @ +25

C to ensure compliance.

All digital inputs @ GND except

CS @ V

. No load on the digital outputs. Analog inputs @ GND.

SCLK @ GND when SCLK off. All digital inputs @ GND except for

CS @ V

. No load on the digital outputs. Analog inputs @ GND.

Specifications subject to change without notice.

ABSOLUTE MAXIMUM RATINGS

= +25

C unless otherwise noted)

to AGND . . . . . . . . . . . . . . . . . . . . . . . . . 0.3 V to +7 V

Analog Input Voltage to AGND . . . . . 0.3 V to V

+ 0.3 V

Digital Input Voltage to AGND . . . . . . 0.3 V to V

+ 0.3 V

Digital Output Voltage to AGND . . . . 0.3 V to V

+ 0.3 V

REFIN/REFOUT to AGND . . . . . . . . 0.3 V to V

+ 0.3 V

Input Current to Any Pin Except Supplies

. . . . . . . .

10 mA

Operating Temperature Range

Commercial

(A Version) . . . . . . . . . . . . . . . . . . . . . . 40

C to +105

(B Version) . . . . . . . . . . . . . . . . . . . . . . . 0

C to +105

Storage Temperature Range . . . . . . . . . . . 65

C to +150

Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . +150

SOIC, TSSOP Package, Power Dissipation . . . . . . . . 450 mW

Thermal Impedance . . . . . . . . . . . . 124.9

C/W (SOIC)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150.4

C/W (TSSOP)

Thermal Impedance . . . . . . . . . . . . . 42.9

C/W (SOIC)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27.6

C/W (TSSOP)

Lead Temperature, Soldering

Vapor Phase (60 secs) . . . . . . . . . . . . . . . . . . . . . +215

Infrared (15 secs) . . . . . . . . . . . . . . . . . . . . . . . . +220

ESD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 kV

NOTES

Stresses above those listed under Absolute Maximum Ratings may cause perma-

nent damage to the device. This is a stress rating only; 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.

Transient currents of up to 100 mA will not cause SCR latch-up.

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 AD7888 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

ORDERING GUIDE

Linearity
Error

Package

Model

(LSB)

Descriptions

Options

AD7888AR

SOIC

R-16A

AD7888BR

SOIC

R-16A

AD7888ARU

TSSOP

RU-16

AD7888BRU

TSSOP

RU-16

EVAL-AD7888CB

EVAL-CONTROL BOARD

NOTES

Linearity error here refers to integral linearity error.

Contact factory for availability.

This can be used as a stand-alone evaluation board or in conjunction with the

EVAL-CONTROL BOARD for evaluation/demonstration purposes.

This board is a complete unit allowing a PC to control and communicate with all

Analog Devices evaluation boards ending in the CB designators.

REV. 0

AD7888

Limit at T

MIN

, T

MAX

(A, B Versions)

Parameter

+4.75 V to +5.25 V

+2.7 V to +3.6 V

Units

Description

SCLK

MHz max

CONVERT

14.5 t

SCLK

14.5 t

SCLK

ACQ

1.5 t

SCLK

1.5 t

SCLK

Throughput Time = t

CONVERT

+ t

ACQ

= 16 t

SCLK

ns min

CS to SCLK Setup Time

ns max

Delay from

CS until DOUT 3-State Disabled

100

ns max

Data Access Time after SCLK Falling Edge

ns min

Data Setup Time Prior to SCLK Rising Edge

ns min

Data Valid to SCLK Hold Time

0.4 t

SCLK

0.4 t

SCLK

ns min

SCLK High Pulsewidth

0.4 t

SCLK

0.4 t

SCLK

ns min

SCLK Low Pulsewidth

ns max

CS Rising Edge to DOUT High Impedance

s typ

Power-Up Time from Shutdown

NOTES

Sample tested at +25

C to ensure compliance. All input signals are specified with tr = tf = 5 ns (10% to 90% of V

) and timed from a voltage level of 1.6 V.

Mark/Space ratio for the SCLK input is 40/60 to 60/40.

Measured with the load circuit of Figure 1 and defined as the time required for the output to cross 0.8 V or 2.4 V with V

= 5 V

10% and time for an output to

cross 0.4 V or 2.0 V with V

= 3 V

10%.

is derived from 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

, quoted in the timing characteristics is the true bus relinquish

time of the part and is independent of the bus loading.

Specifications subject to change without notice.

TIMING SPECIFICATIONS

= T

MIN

to T

MAX

, unless otherwise noted)

+1.6V

200 A

OUTPUT

PIN

50pF

Figure 1. Load Circuit for Digital Output Timing Specifications

REV. 0

AD7888

PIN CONFIGURATIONS

SOIC AND TSSOP

TOP VIEW

(Not to Scale)

REF IN/REF OUT

AGND

AIN1

AIN2

AIN3

AIN4

SCLK

DOUT

DIN

AGND

AIN8

AIN7

AIN6

AIN5

AD7888

PIN FUNCTION DESCRIPTIONS

Pin

No.

Mnemonic

Function

Chip Select. Active low logic input. This input provides the dual function of initiating conversions on
the AD7888 and also frames the serial data transfer.

REF IN/REF OUT

Reference Input/Output. The on-chip reference is available on this pin for use external to the AD7888.
Alternatively, the internal reference can be disabled and an external reference applied to this input.
The voltage range for the external reference is from +1.2 V to V

Power Supply Input. The V

range for the AD7888 is from +2.7 V to +5.25 V.

4, 13

AGND

Analog Ground. Ground reference point for all circuitry on the AD7888. All analog input signals and
any external reference signals should be referred to this AGND voltage. Both of these pins should
connect to the AGND plane of a system.

512

AIN1AIN8

Analog Input 1 through Analog Input 8. Eight single-ended analog input channels that are multiplexed
into the on-chip track/hold. The analog input channel to be converted is selected by using the ADD0
through ADD2 bits of the Control Register. The input range for all input channels is 0 to V

REF

. Any

unused input channels should be connected to AGND to avoid noise pickup.

DIN

Data In. Logic Input. Data to be written to the AD7888's Control Register is provided on this input
and is clocked into the register on the rising edge of SCLK (see Control Register section).

DOUT

Data Out. Logic Output. The conversion result from the AD7888 is provided on this output as a serial
data stream. The bits are clocked out on the falling edge of the SCLK input. The data stream consists
of four leading zeros followed by the 12 bits of conversion data, which is provided MSB first.

SCLK

Serial Clock. Logic Input. SCLK provides the serial clock for accessing data from the part and writing
serial data to the Control Register. This clock input is also used as the clock source for the AD7888's
conversion process.

REV. 0

AD7888

TERMINOLOGY
Integral Nonlinearity

This is the maximum deviation from a straight line passing
through the endpoints of the ADC transfer function. The end-
points 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.

Offset Error Match

This is the difference in offset error between any two channels.

Gain Error

This is the deviation of the last code transition (111 . . . 110) to
(111 . . . 111) from the ideal (i.e., V

REF

1.5 LSB) after the

offset error has been adjusted out.

Gain Error Match
This is the difference in gain error between any two channels.

Track/Hold Acquisition Time

The track/hold amplifier returns into track mode at the end of
conversion. Track/Hold acquisition time is the time required for
the output of the track/hold amplifier to reach its final value,
within

1/2 LSB, after the end of conversion.

Signal to (Noise + Distortion) Ratio

This is the measured ratio of signal to (noise + distortion) at the
output of the A/D converter. The signal is the rms amplitude of
the fundamental. Noise is the sum of all nonfundamental sig-
nals up to half the sampling frequency (f

/2), excluding dc. The

ratio is dependent on the number of quantization levels in the
digitization process; the more levels, the smaller the quantiza-
tion noise. The theoretical signal to (noise + distortion) ratio for
an ideal N-bit converter with a sine wave input is given by:

Signal to (Noise + Distortion) = (6.02 N + 1.76) dB

Thus for a 12-bit converter, this is 74 dB.

Total Harmonic Distortion

Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7888, it is defined as:

THD dB

(

) log

where V

is the rms amplitude of the fundamental and V

, V

and V

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

/2 and excluding dc) to the rms value of the

fundamental. Normally, the value of this specification is deter-
mined 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 is equal to zero. For example,
the second order terms include (fa + fb) and (fa fb), while the
third order terms include (2fa + fb), (2fa fb), (fa + 2fb) and
(fa 2fb).

The AD7888 is tested using the CCIF standard where two
input frequencies near the top end of the input bandwidth are
used. 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 sepa-
rately. 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.

Channel-to-Channel Isolation

Channel-to-channel isolation is a measure of the level of crosstalk
between channels. It is measured by applying a full-scale 25 kHz
sine wave signal to all nonselected input channels and determin-
ing how much that signal is attenuated in the selected channel.
The figure given is the worst case across all four or eight chan-
nels for the AD7888.

PSR (Power Supply Rejection)

Variations in power supply will affect the full-scale transition,
but not the converter's linearity. Power supply rejection is the
maximum change in the full-scale transition point due to a
change in power-supply voltage from the nominal value.

REV. 0

AD7888

CONTROL REGISTER

The Control Register on the AD7888 is an 8-bit, write-only register. Data is loaded from the DIN pin of the AD7888 on the rising
edge of SCLK. The data is transferred on the DIN line at the same time as the conversion result is read from the part. This requires
16 serial clocks for every data transfer. Only the information provided on the first 8 rising clock edges (after

CS falling edge) is loaded

to the Control Register. MSB denotes the first bit in the data stream. The bit functions are outlined in Table I. The default contents
of the Control Register on power-up is all zeros.

Table I. Control Register Bit Function Description

MSB

Bit

Mnemonic

Comment

DONTC

Don't Care. The value written to this bit of the Control Register is a don't care, i.e., it doesn't matter if the bit is
0 or 1.

ZERO

A zero must be written to this bit to ensure correct operation of the AD7888.

ADD2

These three address bits are loaded at the end of the present conversion sequence and select which analog input

ADD1

channel is converted for the next conversion. The selected input channel is decoded as shown in Table II.

ADD0

REF

Reference Bit. With a 0 in this bit, the on-chip reference is enabled. With a 1 in this bit, the on-chip reference
is disabled. To obtain best performance from the AD7888, the internal reference should be disabled when
using an externally applied reference source. (See On-Chip Reference section.)

1, 0

PM1, PM0

Power Management Bits. These two bits decode the mode of operation of the AD7888 as shown in Table III.

PERFORMANCE CURVES

Figure 2 shows a typical FFT plot for the AD7888 at 100 kHz
sample rate and 10 kHz input frequency.

FREQUENCY kHz

10

48.83

12.21

24.41

36.62

30

50

70

90

110

4096 POINT FFT
SAMPLING
100kSPS
f

= 10kHz

SNR = 70dB

Figure 2. Dynamic Performance

Figure 3 shows a typical plot for the SNR vs. frequency for a
5 V supply and with a 5 V external reference.

INPUT FREQUENCY kHz

71.0

42.14

SNR dB

73.0

72.5

72.0

71.5

10.89

31.59

= +5V

5V EXT REFERENCE

21.14

Figure 3. SNR vs. Input Frequency

REV. 0

AD7888

Figure 4 shows the typical power supply rejection ratio vs.
frequency for the part. The power supply rejection ratio is de-
fined as the ratio of the power in the ADC output at frequency f
to the power of a full-scale sine wave applied to the ADC of
frequency f

PSRR (dB) = 10 log (Pf/Pfs)

Pf = Power at frequency f in ADC output, Pfs = power at fre-
quency fs in ADC full scale input. Here a 100 mV peak-to-peak
sine wave is coupled onto the V

supply. Both the +2.7 V and

+5.5 V supply performances are shown.

INPUT FREQUENCY kHz

93

2.65

64.15

PSRR dB

12.85

33.65

= +5.5V/+2.7V

100mV p-p SINE WAVE ON V

REF

= 2.488V EXT REFERENCE

23.15

91

89

87

85

83

81

79

77

75

43.85

54.35

Figure 4. PSRR vs. Frequency

CIRCUIT INFORMATION

The AD7888 is a fast, low power, 12-bit, single supply, 8-
channel A/D converter. The part can be operated from +3 V
(+2.7 V to +3.6 V) supply or from +5 V (+4.75 V to +5.25 V)
supply. When operated from either a +5 V supply or a +3 V
supply, the AD7888 is capable of throughput rates of 125 kSPS
when provided with a 2 MHz clock.

The AD7888 provides the user with an 8-channel multiplexer,
on-chip track/hold, A/D converter, reference and serial interface
housed in a tiny 16-lead TSSOP package, which offers the user
considerable space saving advantages over alternative solutions.
The serial clock input accesses data from the part and also
provides the clock source for the successive-approximation
A/D converter. The analog input range is 0 to V

REF

(where

the externally-applied V

REF

can be between +1.2 V and V

The 8-channel multiplexer is controlled by the part's Control
Register. This Control Register also allows the user to power-off
the internal reference and to determine the Modes of Operation.

CONVERTER OPERATION

The AD7888 is a successive-approximation analog-to-digital
converter based around a charge redistribution DAC. Figures 5
and 6 show simplified schematics of the ADC. Figure 5 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 AIN.

CHARGE

REDISTRIBUTION

DAC

(REF IN/REF OUT)/2

SAMPLING

CAPACITOR

COMPARATOR

CONTROL

LOGIC

ACQUISITION

PHASE

SW1

SW2

AGND

AIN

Figure 5. ADC Acquisition Phase

When the ADC starts a conversion, (see Figure 6), SW2 will
open and SW1 will move to Position B causing the comparator
to become unbalanced. The control logic and the charge redis-
tribution DAC are used to add and subtract fixed amounts of
charge from the sampling capacitor to bring the comparator
back into a balanced condition. When the comparator is rebal-
anced, the conversion is complete. The control logic generates
the ADC output code. Figure 7 shows the ADC transfer
function.

CONVERSION

PHASE

CHARGE

REDISTRIBUTION

DAC

(REF IN/REF OUT)/2

SAMPLING

CAPACITOR

COMPARATOR

CONTROL

LOGIC

SW1

SW2

AGND

VIN

Figure 6. ADC Conversion Phase

ADC TRANSFER FUNCTION

The output coding of the AD7888 is straight binary. The de-
signed code transitions occur at successive integer LSB values
(i.e., 1 LSB, 2 LSBs, etc.). The LSB size is = V

REF

/4096. The

ideal transfer characteristic for the AD7888 is shown in Figure 7
below.

ADC CODE

ANALOG INPUT

111...000

011...111

0.5LSB

REF

 1.5LSB

1LSB = V

REF

/4096

111...111

111...110

000...010

000...001

000...000

Figure 7. Transfer Characteristic

REV. 0

AD7888

TYPICAL CONNECTION DIAGRAM

Figure 8 shows a typical connection diagram for the AD7888.
Both AGND pins are connected to the analog ground plane of
the system. V

REF

is connected to a well decoupled V

pin to

provide an analog input range of 0 V to V

. The conversion

result is output in a 16-bit word with four leading zeroes fol-
lowed by the MSB of the 12-bit result. For applications where
power consumption is of concern, the automatic power down at
the end of conversion should be used to improve power perfor-
mance. See Modes of Operation section of the data sheet.

DOUT

DIN

SCLK

C/ P

AIN1

AIN2

AIN8

AGND

0.1 F

10 F

SUPPLY +2.7V

TO +5.25V

0V TO

REF IN/

REF OUT

INPUT

SERIAL
INTERFACE

REF IN/

REF OUT

AD7888

Figure 8. Typical Connection Diagram

Analog Input

Figure 9 shows an equivalent circuit of the analog input structure
of the AD7888. The two diodes D1 and D2 provide ESD pro-
tection for the analog inputs. Care must be taken to ensure that
the analog input signal never exceeds the supply rails by more
than 200 mV. This will cause these diodes to become forward-
biased and start conducting current into the substrate. 20 mA is
the maximum current these diodes can conduct without causing
irreversible damage to the part. However, it is worth noting that
a small amount of current (1 mA) being conducted into the
substrate due to an overvoltage on an unselected channel, can
cause inaccurate conversions on a selected channel. The capaci-
tor C1 in Figure 9 is typically about 4 pF and can primarily be
attributed to pin capacitance. The resistor R1 is a lumped com-
ponent made up of the on resistance of a multiplexer and a switch.
This resistor is typically about 100

. The capacitor C2 is the

ADC sampling capacitor and has a capacitance of 20 pF typically.

Note: The analog input capacitance seen when the track and
hold is in track mode is typically 38 pF, while in hold mode it is
typically 4 pF.

C1
4pF

CONVERSION PHASE SWITCH OPEN

TRACK PHASE SWITCH CLOSED

20pF

Figure 9. Equivalent Analog Input Circuit

For ac applications, removing high frequency components from
the analog input signal is recommended by use of an RC low-
pass filter on the relevant analog input pin. In applications
where harmonic distortion 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.

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.
Figure 10 shows a graph of the total harmonic distortion versus
analog input signal frequency for different source impedances.

INPUT FREQUENCY kHz

90

0.15

42.14

THD dB

10.89

31.59

21.14

85

80

75

70

65

49.86

THD vs. FREQUENCY FOR DIFFERENT
SOURCE IMPEDANCES

= +5V

5V EXT REFERENCE

= 1k , C

= 100pF

= 50 , C

= 2.2nF

= 10 , C

= 10nF

Figure 10. THD vs. Analog Input Frequency

Analog Input Selection

On power-up, the default AIN selection is AIN1. When return-
ing to normal operation from power-down, the AIN selected
will be the same one that was selected prior to power-down
being initiated. Table II below shows the multiplexer address
corresponding to each analog input from AIN1 to AIN8 for the
AD7888.

Table II. Channel Configurations

ADD2

ADD1

ADD0

Analog Input Channel

AIN1

AIN2

AIN3

AIN4

AIN5

AIN6

AIN7

AIN8

On-Chip Reference

The AD7888 has an on-chip 2.5 V reference. This reference can
be enabled or disabled by clearing or setting the REF bit in the
Control Register, respectively. If the on-chip reference is to be
used externally in a system, it must be buffered before it is ap-
plied elsewhere. If an external reference is applied to the device,
the internal reference is automatically overdriven. However, in

REV. 0

AD7888

order to obtain optimum performance from the device it is
advised to disable the internal reference by setting the REF bit
in the Control Register when an external reference is applied.
When the internal reference is disabled, SW1 in Figure 11 will
open and the input impedance seen at the REF IN/REF OUT
pin is the input impedance of the reference buffer, which is in
the region of giga

. When the reference is enabled, the input

impedance seen at the pin is typically 5 k

SW1

2.5V

REF IN/REF OUT

Figure 11. On-Chip Reference Circuitry

Table III. Power Management Options

PM1

PM0

Mode

Normal Operation. In this mode, the AD7888
remains in full power mode regardless of the
status of any of the logic inputs. This mode
allows the fastest possible throughput rate from
the AD7888.

Full Shutdown. In this mode, the AD7888 is
in full shutdown mode with all circuitry on the
AD7888, including the on-chip reference, en-
tering its power-down mode. The AD7888
retains the information in the control Register
bits while in full shutdown. The part remains in
full shutdown until these bits are changed.

Autoshutdown. In this mode, the AD7888
automatically enters full shutdown mode at the
end of each conversion. Wake-up time from full
shutdown is 5

s and the user should ensure

that 5

s have elapsed before attempting to

perform a valid conversion on the part in this
mode.

Autostandby. In this standby mode, portions
of the AD7888 are powered down but the on-
chip reference voltage remains powered up. The
REF bit should be 0 to ensure the on-chip refer-
ence is enabled. This mode is similar to auto-
shutdown but allows the part to power-up much
faster.

POWER-DOWN OPTIONS

The AD7888 provides flexible power management to allow the
user to achieve the best power performance for a given through-
put rate.

The power management options are selected by programming
the power management bits (i.e., PM1 and PM0) in the control
register. Table III summarizes the options available. When the
power management bits are programmed for either of the auto
power-down modes, the part will enter the power-down mode
on the 16th rising SCLK edge after the falling edge of

CS. The

first falling SCLK edge after the

CS falling edge will cause the

part to power up again. When the AD7888 is in full shutdown,

the only way to fully power it up again is to reprogram the
power management bits to PM1 = PM0 = 0, i.e., normal
mode. In this case the device will power up on the 16th SCLK
rising edge after the

CS falling edge as this is when the power

management bits become effective.

Power-Up Times

The AD7888 has an approximate 1

s power-up time when

powering up from standby or when using an external reference.
When V

is first connected, the AD7888 will fully power up,

i.e., it powers up in normal mode. If the part is put into shut-
down, a subsequent power-up will take approximately 5

s. The

AD7888 wake-up time is very short in the autostandby mode so
it is possible to wake up the part and carry out a valid conver-
sion in the same read/write operation.

POWER VS. THROUGHPUT RATE

By operating the AD7888 in autoshutdown or autostandby
mode the average power consumption of the AD7888 decreases
at lower throughput rates. Figure 12 shows how as the through-
put 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 AD7888 were operated in a continuous
sampling mode, with a throughput rate of 10 kSPS and a SCLK
of 2 MHz (V

= 5 V), and if PM1 = 1 and PM0 = 0, i.e., the

device is in autoshutdown mode and the on-chip reference is
used, the power consumption is calculated as follows. The
power dissipation during normal operation is 3.5 mW (V

5 V). If the power-up time is 5

s and the remaining conversion-

plus-acquisition time is 15.5 t

SCLK

, i.e., approximately 7.75

(see Figure 14a), the AD7888 can be said to dissipate 3.5 mW
for 12.75

s during each conversion cycle. If the throughput rate

is 10 kSPS, the cycle time is 100

s and the average power dissi-

pated during each cycle is (12.75/100)

(3.5 mW) = 446.25

If V

= 3 V SCLK = 2 MHz, and the device is again in auto-

shutdown mode using the on-chip reference, the power dissipa-
tion during normal operation is 2.1 mW. The AD7888 can now
be said to dissipate 2.1 mW for 12.75

s during each conversion

cycle. With a throughput rate of 10 kSPS, the average power
dissipated during each cycle is (12.75/100)

(2.1 mW) =

267.75

W. Figure 12 shows the power vs. throughput rate for

automatic shutdown with both 5 V and 3 V supplies.

THROUGHPUT kSPS

POWER mW

0.1

0.01

= +5V

SCLK = 2MHz

= +3V

SCLK = 2MHz

Figure 12. Power vs. Throughput

REV. 0

AD7888

MODES OF OPERATION

The AD7888 has a number of different modes of operation.
These are designed to provide flexible power management
options. These options can be chosen to optimize the power
dissipation/throughput rate ratio for differing application re-
quirements. The modes of operation are controlled by the PM1
and PM0 bits of the Control Register as outlined previously.

Normal Mode (PM1 = 0, PM0 = 0)

This mode is intended for fastest throughput rate performance
as the user does not have to worry about any power-up times
with the AD7888 remaining fully powered all the time. Figure
13 shows the general diagram of the operation of the AD7888 in
this mode.

The data presented to the AD7888 on the DIN line during the
first eight clock cycles of the data transfer are loaded to the
Control Register. The part will remain powered up at the end of
the conversion as long as PM1 and PM0 were set to zero in the
write during that conversion. To continue to operate in this
mode, the user must ensure that PM1 and PM0 are both loaded
with 0 on every data transfer.

The falling edge of

CS initiates the sequence and the input

signal is sampled on the second rising edge of the SCLK input.
Sixteen serial clock cycles are required to complete the conver-
sion and access the conversion result. Once a data transfer is
complete (

CS has returned high), another conversion can be

initiated immediately by bringing

CS low again.

Full Shutdown (PM1 = 0, PM0 = 1)

In this mode, all internal circuitry on the AD7888, including the
on-chip reference, is powered-down. The part retains the infor-
mation in the Control Register during full shutdown. The part
remains in full shutdown until the power management bits are
changed. If the power management bits are changed to PM1 = 1
and PM0 = 0, i.e., the autoshutdown mode, the part will remain
in shutdown (now in autoshutdown) but will power up once a
conversion is initiated after that (see Power-Up Times section).
The part changes mode as soon as the control register has been
updated, so if the part is in full shutdown mode and the power
management bits are changed to PM1 = PM0 = 0, i.e., normal
mode, then the part will power up on the 16th SCLK rising edge.

Autoshutdown (PM1 = 1, PM0 = 0)

In this mode, the AD7888 automatically enters its power-down
mode at the end of every conversion. Figure 14a shows the
general diagram of the operation of the AD7888 in this mode.

When

CS goes from high to low, all on-chip circuitry will start

to power up on the next falling edge of SCLK. On the sixteenth
SCLK rising edge the part will power down again. It takes ap-
proximately 5

s for the AD7888 internal circuitry to be fully

powered up. As a result, a conversion (or sample-and-hold
acquisition) should not be initiated during this 5

s. The input

signal is sampled on the second rising edge of SCLK following
the

CS falling edge. The user should ensure that 5

s elapse

between the first falling edge of SCLK after the falling edge of

SCLK

4 LEADING ZEROES + CONVERSION RESULT

DOUT

DATA IN

DIN

THE PART REMAINS POWERED UP

AT ALL TIMES AS PM1 AND PM0 = 0

CONTROL REGISTER DATA IS LOADED ON THE

FIRST 8 CLOCKS. PM1 AND PM0 = 0 TO KEEP

THE PART IN THIS MODE

Figure 13. Normal-Mode Operation

SCLK

4 LEADING ZEROES + CONVERSION RESULT

DOUT

DATA IN

DIN

THE PART POWERS UP FROM

SHUTDOWN ON SCLK FALLING EDGE AS

PM1 = 1 AND PM0 = 0

4 LEADING ZEROES + CONVERSION RESULT

DATA IN

= 5 s

THE PART ENTERS

SHUTDOWN AT THE END OF

CONVERSION AS PM1 = 1 AND PM0 = 0

CONTROL REGISTER DATA IS LOADED ON THE

FIRST 8 CLOCKS. PM1 = 1 AND PM0 = 0

PM1 = 1 AND PM0 = 0 TO KEEP THE

PART IN THIS MODE

Figure 14a. Autoshutdown Operation

REV. 0

AD7888

CS and the second rising edge of SCLK as shown in Figure 14a.
In microcontroller applications, this is readily achievable by
driving the

CS input from one of the port lines and ensuring

that the serial data read (from the microcontrollers serial port) is
not initiated for 5

s. In DSP applications, where the

CS is

generally derived from the serial frame synchronization line, it is
not possible to separate the first falling edge and second rising
edge of SCLK after the

CS falling edge by up to 5

s. There-

fore, the user will need to write to the Control Register to exit
this mode and (by writing PM1 = 0 and PM0 = 0) put the part
into normal mode. A second conversion will then need to be
initiated when the part is powered up to obtain a conversion
result as shown in Figure 14b.

SCLK

4 LEADING ZEROES

+ CONVERSION RESULT

DOUT

DATA IN

DIN

4 LEADING ZEROES

+ CONVERSION RESULT

DATA IN

4 LEADING ZEROES

+ CONVERSION RESULT

DATA IN

PM1 AND PM0 = 0 TO PLACE
THE PART IN NORMAL MODE

PM1 = 1 AND PM0 = 0 TO

PLACE THE PART BACK IN

AUTOSHUTDOWN MODE

CONTROL REGISTER DATA IS LOADED ON

THE FIRST 8 CLOCKS. PM1 = 1 AND PM0 = 0

THE PART ENTERS

SHUTDOWN AT THE END

OF CONVERSION AS

PM1 = 1 AND PM0 = 0

THE PART REMAINS POWERED

UP AS PM1 AND PM0 = 0

THE PART BEGINS TO POWER-

UP FROM SHUTDOWN

THE PART ENTERS

SHUTDOWN AT THE END OF

CONVERSION AS PM1 = 1

AND PM0 = 0

Figure 14b. Autoshutdown Operation

SCLK

4 LEADING ZEROES + CONVERSION RESULT

DOUT

DATA IN

DIN

CONTROL REGISTER DATA IS LOADED ON

THE FIRST 8 CLOCKS. PM1 = 1 AND PM0 = 1

4 LEADING ZEROES + CONVERSION RESULT

DATA IN

PM1 = 1 AND PM0 = 1 TO KEEP

THE PART IN THIS MODE

THE PART POWERS UP

FROM STANDBY ON SCLK

FALLING EDGE AS PM1 = 1

AND PM0 = 1

THE PART ENTERS

STANDBY AT THE END OF

CONVERSION AS

PM1 = 1 AND PM0 = 1

Figure 15. Autostandby Operation

Autostandby (PM1 = 1, PM0 = 1)

In this mode, the AD7888 automatically enters a standby (or
sleep) mode at the end of every conversion. In this standby
mode, all on-chip circuitry, apart from the on-chip reference, is
powered down. This mode is similar to the autoshutdown but
in this case, the power-up time is much shorter as the on-chip
reference remains powered up at all times.

Figure 15 shows the general diagram of the operation of the
AD7888 in this mode. On the first falling SCLK edge after

goes low, the AD7888 comes out of standby. The AD7888
wake-up time is very short in this mode so it is possible to wake
up the part and carry out a valid conversion in the same read/
write operation. The input signal is sampled on the second
rising edge of SCLK following the

CS falling edge. At the end

of conversion (last rising edge of SCLK) the part automatically
enters its standby mode.

REV. 0

AD7888

SERIAL INTERFACE

Figure 16 shows the detailed timing diagram for serial interfac-
ing to the AD7888. The serial clock provides the conversion
clock and also controls the transfer of information to and from
the AD7888 during conversion.

CS initiates the data transfer and conversion process. For the
autoshutdown mode, the first falling edge of SCLK after the
falling edge of

CS wakes up the part. In all cases, it gates the

serial clock to the AD7888 and puts the on-chip track/hold into
track mode. The input signal is sampled on the second rising
edge of the SCLK input after the falling edge of

CS. Thus, the

first one and one-half clock cycles after the falling edge of

CS is

when the acquisition of the input signal takes place. This time is
denoted as the acquisition time (t

ACQ

). In autoshutdown mode,

the acquisition time must allow for the wake-up time of 5

s. The

on-chip track/hold goes from track mode to hold mode on the
second rising edge of SCLK and a conversion is also initiated on
this edge. The conversion process takes a further fourteen and
one-half SCLK cycles to complete. The rising edge of

CS will

put the bus back into three-state. If

CS is left low a new conver-

sion will be initiated.

The input channel that is sampled is the one selected in the
previous write to the Control Register. Thus, the user must

DONTC

REF

ZERO

ADD2

ADD1

ADD0

PM1

PM0

SCLK

DOUT

DIN

ACQ

CONVERT

DB11

DB0

DB10

DB9

4 LEADING ZEROS

THREE-

STATE

THREE-

STATE

Figure 16. Serial Interface Timing Diagram

write ahead of the channel for conversion. In other words, the
user must write the channel address for the next conversion
while the present conversion is in progress.

Writing of information to the Control Register takes place on
the first eight rising edges of SCLK in a data transfer. The Con-
trol Register is always written to when a data transfer takes
place. The user must be careful to always set up the correct
information on the DIN line when reading data from the part.

Sixteen serial clock cycles are required to perform the conver-
sion process and to access data from the AD7888. In applica-
tions where the first serial clock edge, following

CS going low, is

a falling edge, this edge clocks out the first leading zero. Thus,
the first rising clock edge on the SCLK clock has the first lead-
ing zero provided. In applications where the first serial clock
edge, following

CS going low, is a rising edge, the first leading

zero may not be set up in time for the processor to read it cor-
rectly. However, subsequent bits are clocked out on the falling
edge of SCLK so they are provided to the processor on the
following rising edge. Thus, the second leading zero is clocked
out on the falling edge subsequent to the first rising edge. The
final bit in the data transfer is valid on the 16th rising edge,
having being clocked out on the previous falling edge.

REV. 0

AD7888

MICROPROCESSOR INTERFACING

The serial interface on the AD7888 allows the part to be directly
connected to a range of many different microprocessors. This
section explains how to interface the AD7888 with some of the
more common microcontroller and DSP serial interface protocols.

AD7888 to TMS320C5x

The serial interface on the TMS320C5x uses a continuous serial
clock and frame synchronization signals to synchronize the data
transfer operations with peripheral devices like the AD7888.
The

CS input allows easy interfacing with an inverter between

the serial clock of the TMS320C5x and the AD7888 being the
only glue logic required. The serial port of the TMS320C5x is
set up to operate in burst mode with internal CLKX (TX serial
clock) and FSX (TX frame sync). The serial port control register
(SPC) must have the following setup: FO = 0, FSM = 1, MCM
= 1 and TXM = 1. The connection diagram is shown in Figure 17.

AD7888*

DOUT

DIN

SCLK

TMS320C5x*

*ADDITIONAL PINS OMITTED FOR CLARITY

CLKX

CLKR

FSX

FSR

Figure 17. Interfacing to the TMS320C5x

AD7888 to ADSP-21xx

The ADSP-21xx family of DSPs are interfaced to the AD7888
with an inverter between the serial clock of the ADSP-21xx
and the AD7888. This is the only glue logic required. The SPORT
control register should be set up as follows:

TFSW = RFSW = 1, Alternate Framing
INVRFS = INVTFS = 1, Active Low Frame Signal
DTYPE = 00, Right Justify Data
SLEN = 1111, 16-Bit Data Words
ISCLK = 1, Internal Serial Clock
TFSR = RFSR = 1, Frame Every Word
IRFS = 0
ITFS = 1

The connection diagram is shown in Figure 18. The ADSP-
21xx has the TFS and RFS of the SPORT tied together with
TFS set as an output and RFS set as in input. The DSP oper-
ated in Alternate Framing Mode and the SPORT Control Reg-
ister is set up as described. The frame synchronization signal
generated on the TFS is tied to

CS and, as with all signal pro-

cessing applications, equidistant sampling is necessary. How-
ever, in this example the timer interrupt is used to control the
sampling rate of the ADC and, under certain conditions, equi-
distant sampling may not be achieved.

The Timer Registers, etc., are loaded with a value that will
provide an interrupt at the required sample interval. When an
interrupt is received, a value is transmitted with TFS/DT (ADC
control word). The TFS is used to control the RFS and hence
the reading of data. The frequency of the serial clock is set in
the SCLKDIV Register. When the instruction to transmit with
TFS is given, (i.e., AX0 = TX0), the state of the SCLK is checked.
The DSP will wait until the SCLK has gone high, low and high
before transmission will start. If the timer and SCLK values are
chosen such that the instruction to transmit occurs on or near
the rising edge of SCLK, the data may be transmitted or it may
wait until the next clock edge.

For example, the ADSP-2111 has a master clock frequency of
16 MHz. If the SCLKDIV Register is loaded with the value 3, a
SCLK of 2 MHz is obtained, and eight master clock periods will
elapse for every one SCLK period. If the timer registers are
loaded with the value 803, then 100.5 SCLKs will occur between
interrupts and subsequently between transmit instructions. The
situation will result in nonequidistant sampling as the transmit
instruction is occurring on a SCLK edge. If the number of SCLKs
between interrupts is not a figure of N.5, equidistant sampling
will be implemented by the DSP.

AD7888*

DOUT

DIN

SCLK

*ADDITIONAL PINS OMITTED FOR CLARITY

SCLK

RFS

TFS

ADSP-21xx*

Figure 18. Interfacing to the ADSP-21xx

AD7888 to DSP56xxx

The connection diagram in Figure 19 shows how the AD7888
can be connected to the SSI (Synchronous Serial Interface) of
the DSP56xxx family of DSPs from Motorola. The SSI is oper-
ated in synchronous mode (SYN bit in CRB = 1) with inter-
nally generated 1-bit clock period frame sync for both TX and
RX (bits FSL1 = 1 and FSL0 = 0 in CRB). Set the word length
to 16 by setting bits WL1 = 1 and WL0 = 0 in CRA. An in-
verter is also necessary between the SCLK from the DSP56xxx
and the SCLK pin of the AD7888 as shown in Figure 19.

DOUT

DIN

SCLK

*ADDITIONAL PINS OMITTED FOR CLARITY

DSP56xxx*

AD7888*

SCK

SRD

STD

SC2

Figure 19. Interfacing to the DSP56xxx

REV. 0

AD7888

AD7888 to MC68HC11

The Serial Peripheral Interface (SPI) on the MC68HC11 is
configured for Master Mode (MSTR = 1), Clock Polarity Bit
(CPOL) = 1 and the Clock Phase Bit (CPHA) = 1. The SPI is
configured by writing to the SPI Control Register (SPCR)--see
68HC11 User Manual. The serial transfer will take place as two
8-bit operations. A connection diagram is shown in Figure 20.

DOUT

DIN

SCLK

*ADDITIONAL PINS OMITTED FOR CLARITY

MC68HC11*

AD7888*

SCLK/PD4

MISO/PD2

MOSI/PD3

PA0

Figure 20. Interfacing to the MC68HC11

AD7888 to 8051

It is possible to implement a serial interface using the data
ports on the 8051. This allows a full duplex serial transfer to be
implemented. The technique involves "bit-banging" an I/O port
(e.g., P1.0) to generate a serial clock and using two other I/O
ports (e.g., P1.1 and P1.2) to shift data in and out--see
Figure 21.

DOUT

DIN

SCLK

*ADDITIONAL PINS OMITTED FOR CLARITY

AD7888*

8051*

P1.3

P1.0

P1.1

P1.2

Figure 21. Interfacing to the 8051 Using I/O Ports

AD7888 to PIC16C6x/7x

The PIC16C6x Synchronous Serial Port (SSP) is configured
as an SPI Master with the Clock Polarity Bit = 1. This is done
by writing to the Synchronous Serial Port Control Register
(SSPCON). See user PIC16/17 Microcontroller User Manual.
Figure 22 shows the hardware connections needed to interface
to the PIC16/17. In this example I/O port RA1 is being used to
pulse

CS. This microcontroller only transfers eight bits of data

during each serial transfer operation. Therefore, two consecutive
read/write operations are needed.

DOUT

DIN

SCLK

*ADDITIONAL PINS OMITTED FOR CLARITY

AD7888*

PIC16C6x / 7x*

SCK/RC3

SDO/RC5

SDI/RC4

RA1

Figure 22. Interfacing to the PIC16C6x/17x

APPLICATION HINTS
Grounding and Layout

The AD7888 has very good immunity to noise on the power
supplies as can be seen by the PSRR vs. Frequency graph.
However, care should still be taken with regard to grounding
and layout.

The printed circuit board that houses the AD7888 should be
designed so the analog and digital sections are separated and
confined to certain areas of the board. This facilitates the use of
ground planes that can be easily separated. A minimum etch
technique is generally best for ground planes as it gives the best
shielding. Digital and analog ground planes should be joined in
only one place. Both AGND pins of the AD7888 should be
sunk in the AGND plane. The AGND plane and DGND plane
connection should be made at one point only, a star ground
point that should be established as close as possible to an AGND
pin of the AD7888.

Avoid running digital lines under the device as these will couple
noise onto the die. The analog ground plane should be allowed
to run under the AD7888 to avoid noise coupling. The power
supply lines to the AD7888 should use as large a trace as pos-
sible to provide low impedance paths and reduce the effects of
glitches on the power supply line. Fast switching signals like
clocks should be shielded with digital ground to avoid radiating
noise to other sections of the board, and clock signals should
never be run near the analog inputs. Avoid crossover of digital
and analog signals. Traces on opposite sides of the board should
run at right angles to each other. This will reduce the effects of
feedthrough through the board. A microstrip technique is by far
the best but is not always possible with a double-sided board. In
this technique, the component side of the board is dedicated to
ground planes while signals are placed on the solder side.

Good decoupling is also important. All analog supplies should
be decoupled with 10

F tantalum in parallel with 0.1

F ca-

pacitors to AGND. To achieve the best from these decoupling
components, they must be placed as close as possible to the
device, ideally right up against the device.

Evaluating the AD7888 Performance

The recommended layout for the AD7888 is outlined in the
evaluation board for the AD7888. The evaluation board pack-
age includes a fully assembled and tested evaluation board,
documentation, and software for controlling the board from the
PC via the EVAL-CONTROL BOARD. The EVAL-CONTROL
BOARD can be used in conjunction with the AD7888 evalua-
tion board, as well as many other Analog Devices evaluation
boards ending in the CB designator, to demonstrate/evaluate
the ac and dc performance of the AD7888.

The software allows the user to perform ac (fast Fourier trans-
form) and dc (histogram of codes) tests on the AD7888.

REV. 0

AD7888

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

C345082/99

PRINTED IN U.S.A.

16-Lead Small Outline (SOIC)

(R-16A)

0.3937 (10.00)

0.3859 (9.80)

0.2440 (6.20)

0.2284 (5.80)

0.1574 (4.00)

0.1497 (3.80)

PIN 1

SEATING

PLANE

0.0098 (0.25)

0.0040 (0.10)

0.0192 (0.49)

0.0138 (0.35)

0.0688 (1.75)

0.0532 (1.35)

0.0500

(1.27)

BSC

0.0099 (0.25)

0.0075 (0.19)

0.0500 (1.27)

0.0160 (0.41)

8°
0°

0.0196 (0.50)

0.0099 (0.25)

x 45°

16-Lead Thin Shrink Small Outline (TSSOP)

(RU-16)

0.201 (5.10)

0.193 (4.90)

0.256 (6.50)

0.246 (6.25)

0.177 (4.50)

0.169 (4.30)

PIN 1

SEATING

PLANE

0.006 (0.15)

0.002 (0.05)

0.0118 (0.30)

0.0075 (0.19)

0.0256

(0.65)

BSC

0.0433
(1.10)
MAX

0.0079 (0.20)

0.0035 (0.090)

0.028 (0.70)

0.020 (0.50)

8°
0°

Part Number AD7888