AD7853_15 datasheet(16/34 Pages) AD

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REV. B

–16–

AD7853/AD7853L

AIN(+)

AIN(–)

AMODE

AD7853/AD7853L

BIPOLAR

ANALOG

INPUT RANGE

SELECTED

DOUT

COMPLEMENT

FORMAT

VIN = 0 TO VREF

TRACK AND HOLD

AMPLIFIER

VREF/2

DVDD

Figure 15.

±V

REF/2 about VREF/2 Bipolar Input Configuration

+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 =

4096

Figure 16. Unipolar Transfer Characteristic

Figure 15 shows the AD7853/AD7853L’s

±V

REF/2 bipolar ana-

log input configuration (where AIN(+) cannot go below 0 V so

for the full bipolar range then 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 = 4096 = 3.3 V/4096 =

0.8 mV. The ideal input/output transfer characteristic is shown

in Figure 17.

– 1 LSB

FS = VREFV

1LSB =

4096

OUTPUT

CODE

VREF/2

011...111

011...110

000...001

000...000

100...001

100...000

100...010

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

+ FS

111...111

(VREF/2) –1 LSB

(VREF/2) +1 LSB

Figure 17. Bipolar Transfer Characteristic

IC1

AD820

AD820-3V

0.1 F

10 F

V+

V–

10k

10nF

(NPO)

TO AIN(+) OF

AD7853/AD7853L

VIN

–VREF/2 TO +VREF/2

VREF/2

10k

+3V TO +5V

Figure 13. Analog Input Buffering

Input Ranges

The analog input range for the AD7853/AD7853L 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 14

DGND

±V

REF/2

VIN

VREF/2

Figure 15

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 AD7853/AD7853L 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 am-

plifier 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

AD7853/AD7853L

UNIPOLAR ANALOG

INPUT RANGE

SELECTED

DOUT

STRAIGHT

BINARY

FORMAT

VIN = 0 TO VREF

TRACK AND HOLD

AMPLIFIER

Figure 14. 0 to VREF Unipolar Input Configuration

Transfer Functions

For the unipolar 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 for the unipolar range with 1 LSB = FS/4096 =

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

transfer characteristic for the unipolar range is shown in

Figure 16.

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