AD7864ASZ-2 datasheet(21/29 Pages) AD

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AD7864

Rev. D | Page 20 of 28

OFFSET AND FULL-SCALE ADJUSTMENT

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. Invariably, some applications require that the

input signal spans the full analog input dynamic range. In such

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

Figure 15 shows a circuit that can be used to adjust the offset

and full-scale errors on the AD7864 (VINxA on the AD7864-1

version is shown for example purposes only). 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 AD7864 while the input voltage is 1/2

LSB below analog ground. The trim procedure is as follows:

apply a voltage of −2.44 mV (−1/2 LSB) at V1 in Figure 15 and

adjust the op amp offset voltage until the ADC output code

flickers between 1111 1111 1111 and 0000 0000 0000.

Adjust gain error 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.

POSITIVE FULL-SCALE ADJUST

Apply a voltage of 9.9927 V (FS − 3/2 LSB) at V1 and 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 + 1/2 LSB) at V1 and adjust

R2 until the ADC output code flickers between 1000 0000 0000

and 1000 0000 0001.

An alternative scheme for adjusting full-scale error in systems

that use an external reference is to adjust the voltage at the VREF

pin until the full-scale error for any of the channels is adjusted

out. Good full-scale matching of the channels ensures small

full-scale errors on the other channels.

10kΩ

500Ω

10kΩ

AGND

AD7864-1*

*ADDITIONAL PINS OMITTED FOR CLARITY.

INPUT

RANGE = ±10V

10kΩ

VINxA

Figure 15. Full-Scale Adjust Circuit

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