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AD7856

–17–

REV. A

AIN(+)

AIN(–)

AD7856

DOUT

TRACK AND HOLD

AMPLIFIER

STRAIGHT

BINARY

FORMAT

VIN = 0 TO VREF

Figure 14. 0 to VREF Input Configuration

Transfer Function

For the AD7856 input range the designed code transitions occur

midway between successive integer LSB values (i.e., 1/2 LSB,

3/2 LSBs, 5/2 LSBs . . . FS – 3/2 LSBs). The output coding

is straight binary, with 1 LSB = FS/16384 = 4.096 V/16384 =

0.25 mV when VREF = 4.096 V. The ideal input/output transfer

characteristic is shown in Figure 15.

+FS –1LSB

OUTPUT

CODE

111...111

111...110

111...101

111...100

000...011

000...001

000...000

000...010

VIN = (AIN(+) – AIN(–)), INPUT VOLTAGE

1LSB

1LSB =

16384

Figure 15. Transfer Characteristic

REFERENCE SECTION

For specified performance, it is recommended that when using

an external reference this reference should be between 4 V and

the analog supply AVDD. The connections for the relevant refer-

ence pins are shown in the typical connection diagrams. If the

internal reference is being used, the REFIN/REFOUT pin should

have a 100 nF capacitor connected to AGND very close to the

REFIN/REFOUT pin. These connections are shown in Figure 16.

If the internal reference is required for use external to the ADC,

it should be buffered at the REFIN/REFOUT pin and a 100 nF

capacitor connected from this pin to AGND. The typical noise

performance for the internal reference, with 5 V supplies is

150 nV/

√Hz @ 1 kHz and dc noise is 100 µV p-p.

REFIN/REFOUT

AD7856

ANALOG

SUPPLY

+5V

AVDD

DVDD

0.01 F

0.1 F

10 F

CREF1

CREF2

0.01 F

0.1 F

470nF

Figure 16. Relevant Connections When Using Internal

Reference

The other option is that the REFIN/REFOUT pin be overdriven

by connecting it to an external reference. This is possible due to

the series resistance from the REFIN/REFOUT pin to the internal

reference. This external reference can have a range that includes

AVDD. When using AVDD as the reference source or when an

externally applied reference approaches AVDD, the 100 nF ca-

pacitor from the REFIN/REFOUT pin to AGND should be as

close as possible to the REFIN/REFOUT pin, and also the CREF1

pin should be connected to AVDD to keep this pin at the same

level as the reference. The connections for this arrangement are

shown in Figure 17. When using AVDD it may be necessary to

add a resistor in series with the AVDD supply. This will have the

effect of filtering the noise associated with the AVDD supply.

REFIN/REFOUT

AD7856

ANALOG

SUPPLY

+5V

AVDD

DVDD

0.01 F

0.1 F

10 F

CREF1

CREF2

0.01 F

0.1 F

470nF

Figure 17. Relevant Connections When Using AVDD as the

Reference

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