AD7859AS datasheet(17/29 Pages) AD

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AD7859/AD7859L

REV. A

–16–

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/4096 =

3.3 V/4096 = 0.8 mV when VREF = 3.3 V. Figure 12 shows the

unipolar analog input configuration. The ideal input/output

transfer characteristic for the unipolar range is shown in

Figure 14.

1LSB =

4096

OUTPUT

CODE

111...111

111...110

111...101

111...100

000...011

000...010

000...001

000...000

0V 1LSB

+FS –1LSB

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

Figure 14. AD7859/AD7859L Unipolar Transfer

Characteristic

Figure 13 shows the AD7859/AD7859L’s

±V

REF/2 bipolar ana-

log input configuration. AIN(+) cannot go below 0 ,V so for

the full bipolar range, AIN(–) should be biased to at least

+VREF/2. Once again the designed code transitions occur mid-

way between successive integer LSB values. The output coding

is 2s complement with 1 LSB = 4096 = 3.3 V/4096 = 0.8 mV.

The ideal input/output transfer characteristic is shown in Fig-

ure 15.

1LSB =

4096

FS = VREFV

OUTPUT

CODE

011...111

011...110

000...001

111...111

000...010

000...001

000...000

+FS –1LSB

VIN = (AIN(+) –AIN(–)), INPUT VOLTAGE

000...000

VREF/2

(VREF/2) +1LSB

(VREF/2) –1LSB

Figure 15. AD7859/AD7859L Bipolar Transfer Characteristic

IC1

+3V TO +5V

10k

Ω

10k

Ω

10k

Ω

V+

V–

10k

Ω

AD820

AD820-3V

VIN

(–VREF/2 TO +VREF/2)

VREF/2

10µF

0.1µF

10nF

(NPO)

TO AIN(+) OF

AD7854/AD7854L

Figure 11. Analog Input Buffering

Input Ranges

The analog input range for the AD7859/AD7859L is 0 V to

VREF in both the unipolar and bipolar ranges.

The difference between the unipolar range and the bipolar range

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

least +VREF/2 and the output coding is 2s complement (See

Table VI and Figures 14 and 15).

Table VI. Analog Input Connections

Analog Input

Input Connections

Connection

Range

AIN(+)

AIN(–)

Diagram

0 V to VREF

VIN

AGND

Figure 12

±V

REF/2

VIN

VREF/2

Figure 13

NOTES

1Output code format is straight binary.

2Range is

±V

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

Note that the AIN(–) channel on the AD7859/AD7859L can be

biased up above AGND in the unipolar mode, or above VREF/2

in bipolar mode if required. The advantage of biasing the lower

end of the analog input range away from AGND is that the ana-

log input does not have to swing all the way down to AGND.

Thus, in single supply applications 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(–) should be tied to

AGND in unipolar mode or to AVDD/2 in bipolar mode.

TRACK AND HOLD

AMPLIFIER

AIN(+)

AIN(–)

DB0

DB15

VIN = 0 TO VREF

AD7859/AD7859L

STRAIGHT

BINARY

FORMAT

Figure 12. 0 to VREF Unipolar Input Configuration

TRACK AND HOLD

AMPLIFIER

AIN(+)

AIN(–)

DB0

DB15

VIN = 0 TO VREF

AD7859/AD7859L

2'S

COMPLEMENT

FORMAT

VREF/2

Figure 13.

±V

REF/2 about VREF/2 Bipolar Input Configuration

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