AD7870LP datasheet(12/28 Pages) AD

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AD7870/AD7875/AD7876

Rev. C | Page 12 of 2

AD7876

TRACK-AND-HOLD

AMPLIFIER

TO INTERNAL

REFERENCE

2.1R

VIN

REF OUT

AGND

TO INTERNAL

COMPARATOR

TO INTERNAL AGND

Figure 9. AD7876 Analog Input

Figure 10 shows the analog input for the AD7875. The input

range is 0 V to +5 V into an input resistance of typically 25 kΩ.

Once again, the designed code transitions occur midway

between successive integer LSB values. The output code is

straight binary with 1 LSB = FS/4096 = 5 V/4096 = 1.22 mV.

The ideal input/output transfer function is shown in Figure 12.

AD7875

TRACK-AND-HOLD

AMPLIFIER

TO INTERNAL AGND

VIN

AGND

TO INTERNAL

COMPARATOR

Figure 10. AD7875 Analog Input

011…111

OUTPUT

CODE

AD7870 (AD7876)

011…110

100…001

100…000

VIN – INPUT VOLTAGE

000…010

000…001

000…000

111…111

111…110

–FS

+FS – 1LSB

1LSB =

FS = 6V (20V)

4096

Figure 11. AD7870/AD7876 Transfer Function

OUTPUT

CODE

000…011

000…010

000…001

000…000

VIN – INPUT VOLTAGE

111…101

111…100

111…111

111…110

1LSB =

FS = 5V

+FS – 1LSB

4096

Figure 12. AD7875 Transfer Function

OFFSET AND FULL-SCALE ADJUSTMENT—

AD7870

In most digital signal processing (DSP) applications, offset and

full-scale errors have little or no effect on system performance.

Offset error can always be eliminated in the analog domain

by ac coupling. Full-scale error effect is linear and does not

cause problems as long as the input signal is within the full

dynamic range of the ADC. Some applications will require

that the input signal span the full analog input dynamic range.

In such applications, offset and full-scale error have to be

adjusted to zero.

Where adjustment is required, offset error must be adjusted

before full-scale error. This is achieved by trimming the offset

of the op amp driving the analog input of the AD7870 while the

input voltage is 1/2 LSB below ground. The trim procedure is as

follows: apply a voltage of −0.73 mV(−1/2 LSB) at V1 in Figure 13

and adjust the op amp offset voltage until the ADC output code

flickers between 1111 1111 1111 and 0000 0000 0000. Gain

error can be adjusted at either the first code transition (ADC

negative full-scale) or the last code transition (ADC positive full

scale). The trim procedures for both cases are as follows (see

Figure 13).

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