REV. B

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.

AD7887

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, 2-Channel,

125 kSPS, 12-Bit ADC in 8-Lead SOIC

FUNCTIONAL BLOCK DIAGRAM

SAR + ADC

CONTROL LOGIC

GND

AIN0

SCLK

DOUT

DIN

AD7887

T/H

I/P

MUX

BUF

CHARGE

REDISTRIBUTION

DAC

COMP

REF

/AIN1

SOFTWARE

CONTROL

LATCH

REF

AIN1

2.5V
REF

SPORT

FEATURES
Specified for V

of +2.7 V to +5.25 V

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

Compatible

8-Lead Narrow SOIC and SOIC Packages

APPLICATIONS
Battery-Powered Systems (Personal Digital Assistants,

Medical Instruments, Mobile Communications)

Instrumentation and Control Systems
High Speed Modems

GENERAL DESCRIPTION

The AD7887 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 AD7887
is capable of 125 kSPS throughput rate. The input track-and-
hold acquires a signal in 500 ns and features a single-ended
sampling scheme. The output coding for the AD7887 is straight
binary and the part is capable of converting full power signals up to
2.5 MHz.

The AD7887 can be configured for either dual or single chan-
nel operation, via the on-chip Control Register. There is a
default single-channel mode that allows the AD7887 to be
operated as a read-only ADC. In single-channel operation,
there is one analog input (AIN0) with the V

REF

/AIN1 pin as-

suming its V

REF

function. This V

REF

pin allows the user access

to the part's internal +2.5 V reference, or the V

REF

pin can be

overdriven by an external reference to provide the reference
voltage for the part. This external reference voltage has a range
of +2.5 V to V

. The analog input range on AIN0 is 0 to +V

REF

In dual-channel operation, the V

REF

/AIN1 pin assumes its AIN1

function, providing a second analog input channel. In this case,
the reference voltage for the part is provided via the V

pin. As

a result, the input voltage range on both the AIN0 and AIN1
inputs is 0 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 an 8-lead, 0.15-inch-wide narrow body
SOIC and an 8-lead

SOIC package.

PRODUCT HIGHLIGHTS

1. Smallest 12-bit dual/single-channel ADC; 8-lead

SOIC

package.

2. Lowest power 12-bit dual/single-channel ADC.

3. Flexible power management options including automatic

power-down after conversion.

4. Read-Only ADC capability.

5. Analog input range from 0 V to V

REF

6. 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.

REV. B

AD7887SPECIFICATIONS

= +2.7 V to +5.25 V, V

REF

= +2.5 V External/Internal Reference unless otherwise

noted, f

SCLK

= 2 MHz; 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 Missing Codes to 11 Bits (A Grade)

Offset Error

LSB max

= 5 V, Dual-Channel Mode

LSB max

= 3 V, Dual-Channel Mode

LSB typ

Single-Channel Mode

Offset Error Match

0.5

LSB max

Gain Error

LSB max

Dual-Channel Mode

LSB max

Single-Channel Mode, External Reference

LSB typ

Single-Channel Mode, 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

REFERENCE INPUT/OUTPUT

REF

Input Voltage Range

2.5/V

V min/max

Functional from 1.2 V

Input Impedance

typ

Very High Impedance If Internal Reference Disabled

REF

OUT

Output Voltage

2.45/2.55

V min/max

REF

OUT

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)

REV. B

AD7887

Parameter

A Version

B Version

Units

Test Conditions/Comments

POWER REQUIREMENTS

+2.7/+5.25

V min/max

Normal Mode

(Mode 2)

Static

700

A max

Operational (f

SAMPLE

= 125 kSPS) 850

850

A typ

Internal Reference Enabled

700

A typ

Internal Reference Disabled

Using Standby Mode (Mode 4)

450

A typ

SAMPLE

= 50 kSPS

Using Shutdown Mode (Modes 1, 3)

120

A typ

SAMPLE

= 10 kSPS

A typ

SAMPLE

= 1 kSPS

Standby Mode

210

A max

= +2.7 V to +5.25 V

Shutdown Mode

A max

= +2.7 V to +3.6 V

A max

= +4.75 V to +5.25 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

1.05

mW max

= +5 V

630

W max

= +3 V

NOTES

Temperature ranges as follows: A, B Versions: 40

C to +125

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

REF

/REF

OUT

to AGND . . . . . . . . . . 0.3 V to V

+ 0.3 V

Input Current to Any Pin Except Supplies

. . . . . . .

10 mA

Operating Temperature Range

Commercial

A, B Versions . . . . . . . . . . . . . . . . . . . . 40

C to +125

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

C to +150

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

SOIC,

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

Thermal Impedance . . . . . . . . . . . . . . 157

C/W (SOIC)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.9

C/W (

SOIC)

Thermal Impedance . . . . . . . . . . . . . . . 56

C/W (SOIC)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.74

C/W (

SOIC)

Lead Temperature, Soldering

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

Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . +220

ESD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 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.

ORDERING GUIDE

Model

Linearity Error (LSB)

Package Options

Branding

AD7887AR

SO-8

AD7887AR

AD7887ARM

RM-8

C5A

AD7887BR

SO-8

AD7887BR

EVAL-AD7887CB

Evaluation Board

EVAL-CONTROL BOARD

Controller Board

NOTES

Linearity error here refers to integral linearity error.

SO = SOIC; RM =

ľSOIC.

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. B

AD7887

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 Three-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 volts.

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 2.0 V.

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

+1.6V

200 A

OUTPUT

PIN

50pF

Figure 1. Load Circuit for Digital Output Timing Specifications

REV. B

AD7887

PIN FUNCTION DESCRIPTIONS

Pin

No.

Mnemonic

Function

Chip Select. Active low logic input. This input provides the dual function of initiating conversions on the
AD7887 and also frames the serial data transfer. When the AD7887 operates in its default mode, the CS pin
also acts as the shutdown pin such that with the CS pin high, the AD7887 is in its power-down mode.

Power Supply Input. The V

range for the AD7887 is from +2.7 V to +5.25 V. When the AD7887 is con-

figured for two-channel operation, this pin also provides the reference source for the part.

GND

Ground Pin. This pin is the ground reference point for all circuitry on the AD7887. In systems with separate
AGND and DGND planes, these planes should be tied together as close as possible to this GND pin. Where
this is not possible, this GND pin should connect to the AGND plane.

AIN1/V

REF

Analog Input 1/Voltage Reference Input. In single-channel mode, this pin becomes the reference input/
output. In this case, the user can either access the internal +2.5 V reference or overdrive the internal refer-
ence with the voltage applied to this pin. The reference voltage range for an externally-applied reference is
+1.2 V to V

. In two-channel mode, this pin provides the second analog input channel AIN1. The input

voltage range on AIN1 is 0 to V

AIN0

Analog Input 0. In single-channel mode, this is the analog input and the input voltage range is 0 to V

REF

. In

dual-channel mode, it has an analog input range of 0 to V

DIN

Data In. Logic Input. Data to be written to the AD7887's Control Register is provided on this input and is
clocked into the register on the rising edge of SCLK (see Control Register section). The AD7887 can be
operated as a single-channel read-only ADC by tying the DIN line permanently to GND.

DOUT

Data Out. Logic Output. The conversion result from the AD7887 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 AD7887's conversion
process.

PIN CONFIGURATION

TOP VIEW

(Not to Scale)

SCLK

AD7887

GND

AIN1/V

REF

DOUT

DIN

AIN0

REV. B

AD7887

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 AD7887, 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 are 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 AD7887 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 the nonselected input channel
and determining how much that signal is attenuated in the se-
lected channel. The figure given is the worst case across both
channels for the AD7887.

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. B

AD7887

CONTROL REGISTER

The Control Register on the AD7887 is an 8-bit, write-only register. Data is loaded from the DIN pin of the AD7887 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 eight 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 contents
of the Control Register on power up is all zeros.

Table I. Control Register

MSB

DONTC

ZERO

REF

SIN/DUAL

ZERO

PM1

PM0

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 AD7887.

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.

SIN/DUAL

Single/Dual Bit. This bit determines whether the AD7887 operates in single-channel or dual-channel
mode. A 0 in this bit selects single-channel operation and the AIN1/V

REF

pin assumes its V

REF

function.

A 1 in this bit selects dual-channel mode and the reference voltage for the ADC is internally connected
to V

and the AIN1/V

REF

pin assumes its AIN1 function as the second analog input channel. To

obtain best performance from the AD7887, the internal reference should be disabled when operating in
the dual channel mode, i.e., REF = 1.

Channel Bit. When the part is selected for dual-channel mode, this bit determines which channel will be
converted for the next conversion. A 0 in this bit selects the AIN0 input while a 1 in this bit selects the
AIN1 input. In single-channel mode, this bit should always be 0.

ZERO

A zero must be written to this bit to ensure correct operation of the AD7887.

1, 0

PM1, PM0

Power Management Bits. These two bits decode the mode of operation of the AD7887 as described
below.

Table II. Power Management Options

PM1

PM0

Mode

Mode 1

. In this mode, the AD7887 enters shutdown if the CS input is 1 and is in full power mode when

CS is 0. Thus the part comes out of shutdown on the falling edge of CS and enters shutdown on the
rising edge of CS.

Mode 2

. In this mode, the AD7887 is always fully powered up, regardless of the status of any of the logic

inputs.

Mode 3

. In this mode, the AD7887 automatically enters shutdown mode at the end of each conversion,

regardless of the state of CS.

Mode 4

. In this standby mode, portions of the AD7887 are powered down but the on-chip reference

voltage remains powered up. This mode is similar to Mode 3, but allows the part to power up much
faster. The REF bit should be 0 to ensure the on-chip reference is enabled.

REV. B

AD7887

PERFORMANCE CURVES

Figure 2 shows a typical FFT plot for the AD7887 at 125 kHz
sample rate and 10 kHz input frequency.

6.103516

12.20703

18.31055

24.41406

30.51758

36.62109

42.72461

48.82813

54.93164

61.03516

110

10

30

50

70

90

4096 POINT FFT
SAMPLING
125kSPS
f

= 10kHz

SNR = 71dB

Figure 2. Dynamic Performance

Figure 3 shows the SNR vs. Frequency for a 5 V supply with a
5 V external reference.

INPUT FREQUENCY kHz

71.0

0.15

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

Figure 4 shows the power supply rejection ratio versus fre-
quency for the part. The power supply rejection ratio is defined
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 fre-
quency f

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

Pf = Power at frequency f in ADC output, Pfs = power at fre-
quency f

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 AD7887 is a fast, low power, 12-bit, single supply, single-
channel/dual-channel A/D converter. The part can be operated
from a +3 V (+2.7 V to +3.6 V) supply or from a +5 V (+4.75 V to
+5.25 V) supply. When operated from either a +5 V or +3 V
supply, the AD7887 is capable of throughput rates of 125 kSPS
when provided with a 2 MHz clock.

The AD7887 provides the user with an on-chip track/hold, A/D
converter, reference and serial interface housed in an 8-lead
package. The serial clock input accesses data from the part and
also provides the clock source for the successive approximation
A/D converter. The part can be configured for single-channel or
dual-channel operation. When configured as a single-channel
part, the analog input range is 0 to V

REF

(where the externally-

applied V

REF

can be between +1.2 V and V

). When the

AD7887 is configured for two input channels, the input range is
determined by internal connections to be 0 to V

If single-channel operation is required, the AD7887 can be
operated in a read-only mode by tying the DIN line permanently to
GND. For applications where the user wants to change the
mode of operation or wants to operate the AD7887 as a dual-
channel A/D converter, the DIN line can be used to clock data
into the part's control register.

CONVERTER OPERATION

The AD7887 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.

(REF IN/REF OUT)/2

SAMPLING

CAPACITOR

COMPARATOR

ACQUISITION

PHASE

SW1

SW2

AGND

AIN

CHARGE

REDISTRIBUTION

DAC

CONTROL

LOGIC

Figure 5. ADC Acquisition Phase

REV. B

AD7887

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

REF IN/REF OUT/2

SAMPLING

CAPACITOR

COMPARATOR

CONTROL

LOGIC

SW1

SW2

AGND

CHARGE

REDISTRIBUTION

DAC

Figure 6. ADC Conversion Phase

ADC TRANSFER FUNCTION

The output coding of the AD7887 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 AD7887 is shown in Figure 7.

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

TYPICAL CONNECTION DIAGRAM

Figure 8 shows a typical connection diagram for the AD7887.
The GND pin is connected to the analog ground plane of the
system. The part is in dual-channel mode so V

REF

is internally

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 zeros followed 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 performance. See Modes of
Operation section of the data sheet.

DOUT

DIN

SCLK

AIN1

AIN2

GND

0.1 F

10 F

SUPPLY +2.7V

TO +5.25V

SERIAL
INTERFACE

AD7887

0V TO V

INPUT

C/ P

Figure 8. Typical Connection Diagram

Analog Input

Figure 9 shows an equivalent circuit of the analog input structure
of the AD7887. 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
component 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 typically has a capacitance
of 20 pF.

Note: The analog input capacitance seen when 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

REV. B

AD7887

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

On-Chip Reference

The AD7887 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 then it must be buffered before it is
applied elsewhere. If an external reference is applied to the
device, then the internal reference is automatically overdriven.
However, it is advised to disable the internal reference by setting
the REF bit in the control register when an external reference is
applied in order to obtain optimum performance from the de-
vice. When the internal reference is disabled, SW1 in Figure 11
will open and the input impedance seen at the AIN1/V

REF

pin is

the input impedance of the reference buffer, which is in the
region of gigaohms. When the internal reference is enabled the
input impedance seen at the pin is typically 10 k

. When the

AD7887 is operated in two-channel mode, the reference is taken
from V

internally and not from the on-chip 2.5 V reference.

2.5V

10k

SW1

AIN1/V

REF

Figure 11. On-Chip Reference Circuitry

POWER-DOWN OPTIONS

The AD7887 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 II summarizes the available options. When the
power management bits are programmed for either of the auto
power-down modes, the part will enter 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 AD7887 is in Mode 1, i.e.,

PM1 = PM0 = 0, the part will enter shutdown on the rising
edge of CS and power up from shutdown on the falling edge of
CS. If CS is brought high during the conversion in this mode,
the part will immediately enter shutdown.

Power-Up Times

The AD7887 has an approximate 1

ľs power-up time when

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

is first connected the AD7887 will power up in

Mode 1, i.e., PM1 = PM0 = 0. The part is put into shutdown
on the rising edge of CS in this mode. A subsequent power-up
from shutdown will take approximately 5

ľs. The AD7887

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

POWER VS. THROUGHPUT RATE

By operating the AD7887 in autoshutdown, autostandby mode
or Mode 1, the average power consumption of the AD7887
decreases at lower throughput rates. Figure 12 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 AD7887 is operated in a continuous sam-
pling 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

ľs, and the remaining conversion plus

acquisition time is 15.5 t

SCLK

, i.e., approximately 7.75

ľs, (see

Figure 15a), the AD7887 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, then the power
dissipation during normal operation is 2.1 mW. The AD7887
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 aver-
age power dissipated during each cycle is (12.75/100)

× (2.1 mW)

= 267.75

ľW. 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. B

AD7887

MODES OF OPERATION

The AD7887 has a number of different modes of operation.
These are designed to provide flexible power management op-
tions. These options can be chosen to optimize the power dissi-
pation/throughput rate ratio for differing application requirements.
The modes of operation are controlled by the PM1 and PM0 bits
of the Control Register as previously outlined. For read-only
operation of the AD7887, the default mode of all 0s in the Con-
trol Register can be set up by tying the DIN line permanently
low.

Mode 1 (PM1 = 0, PM0 = 0)

This mode allows the user to control the powering down of the
part via the CS pin. Whenever CS is low, the AD7887 is fully
powered up; whenever CS is high, the AD7887 is in full shut-
down. When CS goes from high to low, all on-chip circuitry
starts to power up. It takes approximately, 5

ľs for the AD7887

internal circuitry to be fully powered up. As a result, a conver-
sion (or sample-and-hold acquisition) should not be initiated
during this 5

ľs.

Figure 13 shows a general diagram of the operation of the AD7887
in this mode. The input signal is sampled on the second rising
edge of SCLK following the CS falling edge. The user should
ensure that 5

ľs elapses between the falling edge of CS and the

second rising edge of SCLK. 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 micro-
controllers serial port) is not initiated for 5

ľs. In DSP applica-

tions, where the CS is generally derived from the serial frame
synchronization line, it is usually not possible to separate the CS

falling edge and second SCLK rising edge by up to 5

s without

affecting the speed of the rest of the serial clock. Therefore, the
user will need to write to the Control Register to exit this mode
and (by writing PM1 = 0 and PM0 = 1) put the part into Mode
2, i.e., normal mode. A second conversion will then need to be
initiated when the part is powered-up to get a conversion result.
The write operation which takes place in conjunction with this
second conversion can put the part back into Mode 1 and the
part will go into power-down mode when CS returns high.

Mode 2 (PM1 = 0, PM0 = 1)

In this mode of operation, the AD7887 remains fully powered
up regardless of the status of the CS line. It is intended for
fastest throughput rate performance as the user does not have to
worry about the 5

ľs power-up time previously mentioned.

Figure 14 shows the general diagram of the operation of the
AD7887 in this mode.

The data presented to the AD7887 on the DIN line during the
first eight clock cycles of the data transfer are loaded to the
Control Register. To continue to operate in this mode, the user
must ensure that PM1 is loaded with 0 and PM0 is loaded with
1 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.

SCLK

DOUT

DIN

CONTROL REGISTER DATA IS LOADED ON THE FIRST 8 CLOCKS.

PM1 AND PM0 = 0 TO KEEP THE PART IN THIS MODE

4 LEADING ZEROS + CONVERSION RESULT

DATA IN

THE PART POWERS UP ON

FALLING EDGE AS PM1 AND PM0 = 0

THE PART POWERS DOWN ON

RISING EDGE AS PM1 AND PM0 = 0

Figure 13. Mode 1 Operation

SCLK

DOUT

DIN

THE PART REMAINS POWERED UP

AT ALL TIMES AS

PM1 = 0 AND PM0 = 1

CONTROL REGISTER DATA IS LOADED ON THE FIRST 8 CLOCKS.

PM1 = 0 AND PM0 = 1 TO KEEP THE PART IN THIS MODE

4 LEADING ZEROS + CONVERSION RESULT

DATA IN

Figure 14. Mode 2 Operation

REV. B

AD7887

Mode 3 (PM1 = 1, PM0 = 0)

In this mode, the AD7887 automatically enters its full shutdown
mode at the end of every conversion. It is similar to Mode 1
except that the status of CS does not have any effect on the
power-down status of the AD7887.

Figure 15a shows the general diagram of the operation of the
AD7887 in this mode. On the first falling SCLK edge after CS
goes low, all on-chip circuitry starts to power up. It takes ap-
proximately, 5

s for the AD7887 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

ľs elapses

between the first falling edge of SCLK and the second rising
edge of SCLK after the CS falling edge as shown in Figure 15a.
In microcontroller applications (or with a slow serial clock) this
is readily achievable by driving the CS input from one of the
port lines and ensuring that the serial data read (from the micro-
controller's serial port) is not initiated for 5

s. However, for

higher speed serial clocks it will not be possible to have a 5

delay between powering up and the first rising edge of the SCLK.
Therefore, the user will need to write to the Control Register to
exit this mode and (by writing PM1 = 0 and PM0 = 1) put the

part into Mode 2. A second conversion will then need to be
initiated when the part is powered up to get a conversion result, as
shown in Figure 15b. The write operation that takes place in
conjunction with this second conversion can put the part back
into Mode 3 and the part will go into power-down mode when
the conversion sequence ends.

Mode 4 (PM1 = 1, PM0 = 1)

In this mode, the AD7887 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 Mode 3 but in this case,
the power-up time is much shorter as the on-chip reference
remains powered up at all times.

Figure 16 shows the general diagram of the operation of the
AD7887 in this mode. On the first falling SCLK edge after CS
goes low, the AD7887 comes out of standby. The AD7887
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.

SCLK

DOUT

DIN

THE PART POWERS UP FROM

SHUTDOWN ON SCLK FALLING EDGE AS

PM1 = 1 AND PM0 = 0

= 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

DATA IN

4 LEADING ZEROS + CONVERSION RESULT

Figure 15a. Mode 3 Operation

SCLK

DOUT

DIN

PM1 = 0 AND PM0 = 1 TO PLACE

THE PART IN NORMAL MODE

PM1 = 1 AND PM0 = 0 TO PLACE

THE PART BACK IN MODE 3

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 = 0 AND PM0 = 1

THE PART BEGINS TO POWER

UP FROM SHUTDOWN

THE PART ENTERS

SHUTDOWN AT THE END OF

CONVERSION AS PM1 = 1

AND PM0 = 0

4 LEADING ZEROS

+ CONVERSION RESULT

DATA IN

4 LEADING ZEROS

+ CONVERSION RESULT

DATA IN

4 LEADING ZEROS

+ CONVERSION RESULT

DATA IN

Figure 15b. Mode 3 Operation

REV. B

AD7887

SCLK

DOUT

DIN

CONTROL REGISTER DATA IS LOADED ON

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

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

4 LEADING ZEROS + CONVERSION RESULT

DATA IN

4 LEADING ZEROS + CONVERSION RESULT

DATA IN

Figure 16. Mode 4 Operation

SERIAL INTERFACE

Figure 17 shows the detailed timing diagrams for serial interfac-
ing to the AD7887. The serial clock provides the conversion
clock and also controls the transfer of information to and from
the AD7887 during conversion.

CS initiates the data transfer and conversion process. For some
modes, the falling edge of CS wakes up the part. In all cases, it
gates the serial clock to the AD7887 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 are when the acquisition of the input signal takes place. This
time is denoted as the acquisition time (t

ACQ

). In modes where

the falling edge of CS wakes up the part, 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 con-
version 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 conversion will be initiated.

In dual-channel operation, the input channel that is sampled is
the one that was selected in the previous write to the Control
Register. Thus, in dual-channel operation the user must write
ahead 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 Control
Register is always written to when a data transfer takes place.
However, the AD7887 can be operated in a read-only mode by
tying DIN low, thereby loading all 0s to the Control Register
every time. When operating the AD7887 in write/read mode, 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 conversion
process and to access data from the AD7887. In applications
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 leading
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 correctly.
However, subsequent bits are clocked out on the falling edge of
SCLK so that they are provided to the processor on the follow-
ing 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 sixteenth rising edge, having
being clocked out on the previous falling edge.

DONTC

ZERO

REF

SIN/DUAL

PM1

PM0

SCLK

DOUT

DIN

ACQ

CONVERT

DB11

DB0

DB10

DB9

THREE-

STATE

4 LEADING ZEROS

THREE-

STATE

Figure 17. Serial Interface Timing Diagram

REV. B

AD7887

MICROPROCESSOR INTERFACING

The serial interface on the AD7887 allows the part to be directly
connected to a range of many different microprocessors. This
section explains how to interface the AD7887 with some of the
more common microcontroller and DSP serial interface protocols.

AD7887 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 AD7887.
The CS input allows easy interfacing with an inverter between
the serial clock of the TMS320C5x and the AD7887 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 regis-
ter (SPC) must have the following setup: FO = 0, FSM = 1,
MCM = 1 and TXM = 1. The connection diagram is shown in
Figure 18.

AD7887*

DOUT

DIN

SCLK

TMS320C5x*

*ADDITIONAL PINS OMITTED FOR CLARITY

CLKX

CLKR

FSX

FSR

Figure 18. Interfacing to the TMS320C5x

AD7887 to ADSP-21xx

The ADSP-21xx family of DSPs are easily interfaced to the
AD7887 with an inverter between the serial clock of the ADSP-
21xx and the AD7887. 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 19. The ADSP-
21xx has the TFS and RFS of the SPORT tied together, with
TFS set as an output and RFS set as an input. The DSP oper-
ates in Alternate Framing Mode and the SPORT control regis-
ter 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. In this
example however, 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 pro-
vide an interrupt at the required sample interval. When an inter-
rupt 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
then a SCLK of 2 MHz is obtained, and 8 master clock periods
will elapse for every 1 SCLK period. If the timer registers are
loaded with the value 803, then 100.5 SCLKs will occur be-
tween interrupts and subsequently between transmit instruc-
tions. This situation will result in nonequidistant sampling as
the transmit instruction is occurring on an SCLK edge. If the
number of SCLKs between interrupts is a whole integer number
of N, equidistant sampling will be implemented by the DSP.

AD7887*

DOUT

DIN

SCLK

*ADDITIONAL PINS OMITTED FOR CLARITY

SCLK

RFS

TFS

ADSP-21xx*

Figure 19. Interfacing to the ADSP-21xx

AD7887 to DSP56xxx

The connection diagram in Figure 20 shows how the AD7887
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 inverter
is also necessary between the SCLK from the DSP56xxx and
the SCLK pin of the AD7887 as shown in Figure 20.

DOUT

DIN

SCLK

*ADDITIONAL PINS OMITTED FOR CLARITY

DSP56xxx*

AD7887*

SCK

SRD

STD

SC2

Figure 20. Interfacing to the DSP56xxx

REV. B

AD7887

AD7887 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 21.

DOUT

DIN

SCLK

*ADDITIONAL PINS OMITTED FOR CLARITY

MC68HC11*

AD7887*

SCLK/PD4

MISO/PD2

MOSI/PD3

PA0

Figure 21. Interfacing to the MC68HC11

AD7887 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 22.

DOUT

DIN

SCLK

*ADDITIONAL PINS OMITTED FOR CLARITY

AD7887*

8051*

P1.3

P1.0

P1.1

P1.2

Figure 22. Interfacing to the 8051 Using I/O Ports

AD7887 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 23 shows the hardware connections needed to interface
to the PIC16C6x/7x. 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 consecu-
tive read/write operations are needed.

DOUT

DIN

SCLK

*ADDITIONAL PINS OMITTED FOR CLARITY

AD7887*

PIC16C6x / 7x*

SCK/RC3

SDO/RC5

RA1

SDI/RC4

Figure 23. Interfacing to the PIC16C6x/7x

APPLICATION HINTS
Grounding and Layout

The AD7887 has very good immunity to noise on the power
supplies as can be seen in Figure 4. However, care should still
be taken with regard to grounding and layout.

The printed circuit board that houses the AD7887 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, as close as possible to the GND pin of the
AD7887. If the AD7887 is in a system where multiple devices
require AGND-to-DGND connections, the connection should
still be made at one point only, a star ground point, which should
be established as close as possible to the AD7887.

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 AD7887 to avoid noise coupling. The power
supply lines to the AD7887 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

capacitors 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 AD7887 Performance

The recommended layout for the AD7887 is outlined in the
evaluation board for the AD7887. 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-CON-
TROL BOARD can be used in conjunction with the AD7887
Evaluation board, as well as many other Analog Devices evalua-
tion boards ending in the CB designator, to demonstrate/
evaluate the ac and dc performance of the AD7887.

The software allows the user to perform ac (fast Fourier trans-
form) and dc (histogram of codes) tests on the AD7887.

Part Number AD7887