AD7858LBR datasheet(17/32 Pages) AD

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AD7858/AD7858L

REV. B

–17–

Input Range

The analog input range for the AD7858/AD7858L is 0 V to

VREF. The AIN(–) pin on the AD7858/AD7858L can be biased

up above AGND, if required. The advantage of biasing the

lower end of the analog input range away from AGND is that

the user does not need to have the analog input swing all the

way down to AGND. This has the advantage in true single-

supply applications that the input amplifier does not need to

swing all the way down to AGND. The upper end of the analog

input range is shifted up by the same amount. Care must be

taken so that the bias applied does not shift the upper end of the

analog input above the AVDD supply. In the case where the

reference is the supply, AVDD, the AIN(–) must be tied to

AGND.

AD7858/

AD7858L

AIN(+)

AIN(–)

TRACK AND HOLD

AMPLIFIER

DOUT

STRAIGHT

BINARY

FORMAT

VIN = 0 TO VREF

Figure 14. 0 to VREF Input Configuration

Transfer Function

For the AD7858/AD7858L input range the designed code tran-

sitions 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/4096 = 3.3 V/

4096 = 0.8 mV when VREF = 3.3 V. The ideal input/output

transfer characteristic is shown in Figure 15.

OUTPUT

CODE

111...111

111...110

111...101

111...100

000...011

000...010

000...001

000...000

1LSB = FS

4096

0V 1LSB

+FS –1LSB

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

Figure 15. AD7858/AD7858L Transfer Characteristic

REFERENCE SECTION

For specified performance, it is recommended that when using

an external reference this reference should be between 2.3 V

and the analog supply AVDD. The connections for the relevant

reference 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

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.

0.1 F

0.01 F

0.1 F

10 F

0.1 F

AD7858/

AD7858L

ANALOG SUPPLY

+3V TO +5V

AVDD

DVDD

CREF1

CREF2

REFIN/REFOUT

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, the 100 nF

capacitor 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.

0.1 F

0.01 F

0.1 F

10 F

0.1 F

AD7858/

AD7858L

ANALOG SUPPLY

+3V TO +5V

AVDD

DVDD

CREF1

CREF2

REFIN/REFOUT

Figure 17. Relevant Connections When Using AVDD as the

Reference

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