AD7658BSTZ datasheet(21/32 Pages) AD

21 / 32 page

Data Sheet

AD7656/AD7657/AD7658

Rev. D | Page 21 of 32

ADC TRANSFER FUNCTION

The output coding of the AD7656/AD7657/AD7658 is twos

complement. The designed code transitions occur midway

between successive integer LSB values, that is, 1/2 LSB, 3/2 LSB.

The LSB size is FSR/65536 for the AD7656, FSR/16384 for the

AD7657, and FSR/4096 for the AD7658. The ideal transfer

characteristic is shown in Figure 25.

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 25. AD7656/AD7657/AD7658 Transfer Characteristic

The LSB size is dependent on the analog input range selected

(see Table 9).

REFERENCE SECTION

The RFIN/REFOUT pin either allows access to the AD7656/

AD7657/AD7658’s 2.5 V reference or it allows an external

reference to be connected, providing the reference source for

each part’s conversions. The AD7656/AD7657/AD7658 can

accommodate a 2.5 V to 3 V external reference range. When

using an external reference, the internal reference needs to be

disabled. After a reset, the AD7656/AD7657/AD7658 default 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, the H/S SEL pin = 0 and the REFEN/DIS pin = 1. To

enable the internal reference in software mode, H/S SEL = 1 and

a write to the control register is necessary to make DB9 of the

pin should be decoupled using 10 µF and 100 nF capacitors.

The AD7656/AD7657/AD7658 contain three on-chip reference

buffers. Each of the three ADC pairs has an associated reference

buffer. These reference buffers require external decoupling

capacitors on REFCAPA, REFCAPB, and REFCAPC pins,

and 10 µF and 100 nF decoupling capacitors should be placed

on these REFCAP pins. The internal reference buffers can be

disabled in software mode by writing to Bit DB8 in the internal

control register. If operating the devices in serial mode,

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, an external

buffered reference should be applied to the REFCAP pins.

TYPICAL CONNECTION DIAGRAM

Figure 26 shows the typical connection diagram for the

AD7656/AD7657/AD7658. There are eight AVCC supply pins

on the parts. The AVCC supply is the supply that is used for the

AD7656/AD7657/AD7658 conversion process; therefore, it

should be well decoupled. Each AVCC supply pin should be

individually decoupled with a 10 µF tantalum capacitor and a

100 nF ceramic capacitor. The AD7656/AD7657/AD7658 can

operate with the internal reference or an externally applied

reference. In this configuration, the parts are 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 REFCAP

pins are decoupled with the 10 µF and 100 nF capacitor pair.

Six of the AVCC supply pins are used as the supply to the six

ADC cores on the AD7656/AD7657/AD7658 and, as a result,

are used for the conversion process. Each analog input pin is

surrounded by an AVCC supply pin and an AGND pin. These

AVCC and AGND pins are the supply and ground for the indi-

vidual ADC cores. For example, Pin 33 is V1, Pin 34 is the AVCC

supply for ADC Core 1, and Pin 32 is the AGND for ADC Core

1. An alternative reduced decoupling solution is to group these

six AVCC supply pins into three pairs, Pin 34 and Pin 35, Pin 40

and Pin 41, and Pin 46 and Pin 47.

For the AD7656, a 100 µF decoupling capacitor can be placed

on each of the pin pairs. All of the other supply and reference

pins should be decoupled with a 10 µF decoupling capacitor.

When the AD7657 is configured in this reduced decoupling

configuration, each of the three AVCC pin pairs should be

decoupled with a 33 µF capacitor. When the AD7658 is con-

figured in this same configuration, each of the three AVCC

pin pairs should be decoupled with a 22 µF capacitor.

If the same supply is being used for the AVCC supply and DVCC

supply, a ferrite or small RC filter should be placed between the

supply pins.

The AGND pins are connected to the AGND plane of the system.

The DGND pins are connected to the digital ground plane in

the system. The AGND and DGND planes should be connected

together at one place in the system. This connection should be

made as close as possible to the AD7656/AD7657/AD7658 in

the system.

Table 9. LSB Size for Each Analog Input Range

Range

AD7656

AD7657

AD7658

Input Range

±10 V

±5 V

±10 V

±5 V

±10 V

±5 V

LSB Size

0.305 mV

0.152 mV

1.22 mV

0.610 mV

4.88 mV

2.44mV

FS Range

20 V/65536

10 V/65536

20 V/16384

10 V/16384

20 V/4096

10 V/4096

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