AD7870LP datasheet(8/20 Pages) AD

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REV. B

AD7870/AD7875/AD7876

–8–

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

The ideal input/output transfer function is shown in Figure 7.

Figure 5c. AD7875 Analog Input

Figure 6. AD7870/AD7876 Transfer Function

Figure 7. 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 applica-

tions, offset and full-scale error will have to be adjusted to zero.

Where adjustment is required, offset error must be adjusted be-

fore 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

8 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 posi-

tive full scale). The trim procedures for both cases are as follows

(see Figure 8).

Figure 8. Offset and Full-Scale Adjust Circuit

Positive Full-Scale Adjust

Apply a voltage of 2.9978 V (FS/2 – 3/2 LSBs) at V1. Adjust R2

until the ADC output code flickers between 0111 1111 1110

and 0111 1111 1111.

Negative Full-Scale Adjust

Apply a voltage of –2.9993 V (–FS/2 + 1/2 LSB) at V1 and ad-

just R2 until the ADC output code flickers between 1000 0000

0000 and 1000 0000 0001.

OFFSET AND FULL-SCALE ADJUSTMENT—AD7876

The offset and full-scale adjustment for the AD7876 is similar

to that just outlined for the AD7870. The trim procedure, for

those applications that do require adjustment, is as follows:

apply a voltage of –2.44 mV (–1/2 LSB) at V1 and adjust the op

amp offset voltage until the ADC output code flickers between

1111 1111 1111 and 0000 0000 0000. Full-scale 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 procedure for both case is as follows (see Figure 8):

Positive Full-Scale Adjust

Apply a voltage of 9.9927 V (FS/2 –3/2 LSBs) at V1. Adjust R2

until the ADC output code flickers between 0111 1111 1110

and 0111 1111 1111.

Negative Full-Scale Adjust

Apply a voltage of –9.9976 V (FS/2 + 1/2 LSB) at V1 and adjust

R2 until the ADC output code flickers between 1000 0000 0000

and 1000 0000 0001.

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