AD7656ABSTZ datasheet(16/28 Pages) ETC2

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AD7656A

Data Sheet

ADC TRANSFER FUNCTION

The output coding of the AD7656A is twos complement. The

designed code transitions occur midway between successive

integer LSB values, that is, 1/2 LSB and 3/2 LSB. The LSB size is

FSR/65,536 for the AD7656A. The ideal transfer characteristic

is shown in Figure 21.

011...111

011...110

000...001

000...000

111...111

–FSR/2 + 1/2LSB

+FSR/2 – 3/2LSB

AGND – 1LSB

ANALOG INPUT

100...010

100...001

100...000

Figure 21. Transfer Characteristic

The LSB size is dependent on the analog input range selected

(see Table 7).

Table 7. LSB Size for Each Analog Input Range

Input Range (V)

LSB Size (mV)

Full Scale Range

±10

0.305

20 V/65,536

±5

0.152

10 V/65,536

REFERENCE SECTION

Either the REFIN/REFOUT pin allows access to the 2.5 V reference

of the AD7656A, or it allows an external reference to be connected,

providing the reference source for the conversions of the device.

The AD7656A can accommodate a 2.5 V to 3 V external reference

range. When using an external reference, the internal reference

must be disabled. After a reset, the AD7656A defaults to

operating in external reference mode with the internal reference

buffers enabled. The internal reference can be enabled in either

hardware or software mode. To enable the internal reference in

hardware mode, set the H/S SEL pin to 0 and the REFEN/DIS pin to 1.

To enable the internal reference in software mode, set the H/S SEL

pin to 1 and write to the control register to set DB9 of the

control register to 1. For the internal reference mode, decouple

the REFIN/REFOUT pin using 10 µF and 100 nF capacitors.

The AD7656A contains three on-chip reference buffers. Each of

the three ADC pairs has an associated reference buffer. These

reference buffers require external decoupling capacitors on the

REFCAPA, REFCAPB, and REFCAPC pins.Place 10 µF and

100 nF decoupling capacitors on these REFCAPx pins. The

internal reference buffers can be disabled in software mode by

writing to Bit DB8 in the internal control register. If the serial

interface is selected, the internal reference buffers can be disabled

in hardware mode by setting the DB14/REFBUFEN/DIS pin high. If

the internal reference and its buffers are disabled, apply an

external buffered reference to the REFCAPx pins.

TYPICAL CONNECTION DIAGRAM

Figure 22 shows the typical connection diagram for the AD7656A.

There are eight AVCC supply pins on the device. The AVCC supply

is the supply that is used for the AD7656A conversion process;

therefore, it must be well decoupled. Individually decouple each

AVCC supply pin with a 10 µF tantalum capacitor and a 100 nF

ceramic capacitor. The AD7656A can operate with the internal

reference or an externally applied reference. In this configuration,

the device is configured to operate with the external reference.

The REFIN/REFOUT pin is decoupled with a 10 µF and 100 nF

capacitor pair. The three internal reference buffers are enabled.

Each of the REFCAPx pins is decoupled with the 10 µF and

100 nF capacitor pair.

100nF

DVCC

100nF

DVCC

AVCC

AGND

DGND

VDRIVE DGND

VDD

AGND

100nF

VSS

AGND

100nF

REFCAPA, REFCAPB, REFCAPC

AGND

REFIN/REFOUT

AGND

DB0 TO DB15

CONVST A, CONVST B, CONVST C

BUSY

SER/PAR/SEL

H/S SEL

W/B

RANGE

RESET

STBY

VDRIVE

AD7656A

10µF

DIGITAL SUPPLY

VOLTAGE +3V OR +5V

ANA L OG SUPPLY

VOLTAGE 5V1

1DECOUPLING SHOWN ON THE AVCC PIN APPLIES TO EACH AVCC PIN.

2SEE THE POWER SUPPLY SEQUENCING SECTION.

+11.0V TO +16.5V2

SUPPLY

2.5V

REF

SIX ANALOG

INPUTS

–11.0V TO –16.5V2

SUPPLY

PARALLEL

MICROPROCESSOR/

MICROCONTROLLER/

DSP

INTERFACE

Figure 22. Typical Connection Diagram

Rev. 0 | Page 16 of 28

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