AD7656 datasheet(16/25 Pages) AD

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AD7658/AD7657/AD7656

Preliminary Technical Data

Rev. PrI | Page 16 of 25

ADC TRANSFER FUNCTION

The output coding of the AD7658/AD7657/AD7656 is two’s

Complement. The designed code transitions occur midway

between successive integer LSB values, i.e., 1/2 LSB, 3/2 LSBs.

The LSB size is FSR/4096 for the AD7658, FSR/16384 for the

AD7657 and FSR/65536 for the AD7656. The ideal transfer

characteristic for the AD7658/AD7657/AD7656 is shown in

Figure 4.

000...000

-FSR/2 +

1/2LSB

ANALOG INPUT

011...111

100...001

100...010

011...110

000...001

111...111

+FSR/2 -

3/2LSB

100...000

AGND - 1LSB

Figure 4. AD7658/AD7657/AD7656 Transfer Characteristic

The LSB size is dependant on the Analog Input Range selected.

See Table 7.

Reference Section

The VREF pin either provides access to the

AD7658/AD7657/AD7656’s own 2.5V reference or allows for an

external reference to be connected providing the reference

source for the AD7658/AD7657/AD7656 conversions. The

AD7658/AD7657/AD7656 can accommodate a 2.5V to 3V

external reference range. When using an external reference the

internal reference needs to be disabled. After a RESET the

AD7658/AD7657/AD7656 defaults to operating in external

Reference mode. The internal reference can be enabled in either

hardware or software mode. To enable the internal reference in

hardware mode, H/S SEL pin =0 and the REF EN/DISABLE = 1.

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

=1, a write to the control register is necessary to make DB1 of

the register = 1. The REFIN/OUT pin should be decoupled

using 10 µF and 100 nF capacitors.

The AD7656 contains three on-chip reference buffers. Each of

the three ADC pairs has an associated reference buffer. These

reference buffers require external decoupling caps on REFCAPA,

REFCAPB, and REFCAPC pins. 10 µF and 100 nF decoupling

capacitor should be placed on these REFCAP pins.

Table 7. LSB sizes for each Analog Input Range

AD7656

AD7657

AD7658

Input Range

±10V

±5V

±10V

±5V

±10V

±5V

FS Range

20V/65536

10V/65536

20V/16384

10V/16384

20V/4096

10V/4096

LSB Size

0.305 mV

0.152 mV

1.22 mV

0.61 mV

4.88 mV

2.44 mV

10 µF

100 nF

10 µF

Analog Supply

Voltage 5V

Note 1

10 µF

DVCC

100 nF

+ 10 µF

100 nF

DVCC

AVCC AGND

DGND VDRIVE DGND

Digital Supply

Voltage 3V or 5V

VDD

AGND

10 µF

100 nF

VSS

AGND

10 µF

100 nF

10 µF

100 nF

REFCAPA/B/C

AGND

2.5V

REF

REFIN/OUT

+9.5V to +16.5V

Supply

-9.5V to -16.5V

Supply

AGND

D0 to D15 PARALLEL

INTERFACE

CONVST A/B/C

BUSY

µP/µC/DSP

SER/PAR

H/S

W/B

RANGE

RESET

STDBY

Six Analog

Inputs

VDRIVE

AD7658/7/6

Figure 5. AD7658/AD7657/AD7656 Typical connection diagram.

Note 1: Decoupling shown on the

AVCC pin applies to each AVCC pin.

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