Datasheet

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 (V
INxA
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 V
1
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 V
1
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 V
1
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 V
REF
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.
V
1
R1
10kΩ
R2
500Ω
R3
10kΩ
AGND
AD7864-1*
*ADDITIONAL PINS OMITTED FOR CLARITY.
INPUT
RANGE = ±10V
10kΩ
R5
10kΩ
R4
V
INxA
01341-015
Figure 15. Full-Scale Adjust Circuit