AD7851ARS datasheet(17/36 Pages) AD

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REV. B

AD7851

Transfer Functions

For the unipolar range, the designed code transitions occur mid-

way 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 for the unipolar range with 1 LSB = FS/16384 =

4.096 V/16384 = 0.25 mV when VREF = 4.096 V. The ideal

input/output transfer characteristic for the unipolar range is

shown in Figure 16.

+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 16. AD7851 Unipolar Transfer Characteristic

Figure 15 shows the AD7851’s

±V

REF/2 bipolar analog input con-

figuration (where AIN(+) cannot go below 0 V, so for the full bipo-

lar range the AIN(–) pin should be biased to +VREF/2). Once again

the designed code transitions occur midway between successive

integer LSB values. The output coding is twos complement with

1 LSB = 16384 = 4.096 V/16384 = 0.25 mV. The ideal input/

output transfer characteristic is shown in Figure 17.

FS = VREFV

1LSB =

16384

OUTPUT

CODE

VREF/2

011...111

011...110

000...001

000...000

100...001

100...000

100...010

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

+ FS – 1 LSB

111...111

(VREF/2) – 1 LSB

(VREF/2) + 1 LSB

Figure 17. AD7851 Bipolar Transfer Characteristic

Input Ranges

The analog input range for the AD7851 is 0 V to VREF in both

the unipolar and bipolar ranges.

The only difference between the unipolar range and the bipolar

range is that in the bipolar range the AIN(–) has to be biased up

to +VREF/2 and the output coding is twos complement (see

Table V and Figures 14 and 15). The unipolar or bipolar mode

is selected by the AMODE pin (0 for the unipolar range and 1

for the bipolar range).

Table V. Analog Input Connections

Analog Input

Input Connections Connection

Range

AIN(+)

AIN(–)

Diagram

AMODE

0 V to VREF

VIN

AGND

Figure 8

DGND

±V

REF/2

VIN

VREF/2

Figure 9

DVDD

NOTES

1Output code format is straight binary.

2Range is

± V

REF/2 biased about VREF/2. Output code format is twos complement.

Note that the AIN(–) pin on the AD7851 can be biased up above

AGND in the unipolar mode also, if required. The advantage of

biasing the lower end of the analog input range away from

AGND is that the user does not have 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

have 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 in unipolar mode.

AIN(+)

AIN(–)

AMODE

AD7851

UNIPOLAR

ANALOG

INPUT RANGE

SELECTED

DOUT

STRAIGHT

BINARY

FORMAT

VIN = 0 TO VREF

TRACK AND HOLD

AMPLIFIER

Figure 14. 0 V to VREF Unipolar Input Configuration

TWOS

COMPLEMENT

FORMAT

VREF/2

DVDD

AIN(+)

AIN(–)

AMODE

AD7851

UNIPOLAR

ANALOG

INPUT RANGE

SELECTED

DOUT

VIN = 0 TO VREF

TRACK AND HOLD

AMPLIFIER

Figure 15.

±VREF/2 about VREF/2 Bipolar Input Configuration

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