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AD7871 Datasheet PDF : 24 Pages
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AD7871/AD7872
BIPOLAR OFFSET SCALE ADJUSTMENT
When the offset and full-scale errors of the AD7871/AD7872
need to be adjusted, offset error must be adjusted first. This is
achieved by trimming the offset of the op amp driving the
analog input of the AD7871/AD7872 while the input voltage is
½ LSB below AGND. The trim procedure is as follows: apply a
voltage of −0.183 mV (−½ LSB) at V1 in Figure 11 and adjust
the op amp offset voltage until the ADC output code flickers
between 11 1111 1111 1111 and 00 0000 0000 0000.
R1
10kΩ
V1
INPUT RANGE = ±3V
R2
500Ω
AD711
VIN
R3
10kΩ
R4
10kΩ
R5
10kΩ
AD7871/
AD7872*
AGND
*ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 11. Bipolar Adjust Circuit
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 in the
following sections (see Figure 11).
Positive Full-Scale Adjust
Apply a voltage of 2.9995 V (FS/2 −3/2 LSBs) at V1 and adjust
R2 until the ADC output code flickers between 01 1111 1111
1110 and 01 1111 1111 1111.
Negative Full-Scale Adjust
Apply a voltage of −2.9998 V (−FS/2 + 1/2 LSB) at V1 and adjust
R2 until the ADC output code flickers between 10 0000 0000
0000 and 10 0000 0000 0001.
UNIPOLAR OPERATION
A typical unipolar circuit is shown in Figure 12. The
AD7871/AD7872 REF OUT is used to offset the analog input
by 3 V. The analog input range is determined by the ratio of R3
to R4. The minimum range with which the circuit will work is
0 V to 3 V. The resistor values are given in Figure 12 for input
ranges of 0 to 5 V and 0 to 10 V. R5 and R6 are included for
offset and full scale adjust only and should be omitted if
adjustment is not required.
Data Sheet
R3
10kΩ (9.1kΩ)
V1
INPUT RANGE = 0V TO 5V
(0V TO 10V
R5
200Ω
R4
15kΩ (3.9kΩ)
R6
100Ω
AD711
R2
10kΩ
R1
10kΩ
VIN
AD7871/
AD7872*
REF OUT
AGND
*ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 12. Unipolar Circuit
The ideal input/output transfer function is shown in Figure 13.
The output can be converted to straight binary by inverting the
MSB.
OUTPUT
CODE
011…111
011…110
011…101
011…100
100…011
100…010
FS = 5V (10V)
1LSB
=
FS
10384
100…001
100…000
0V
V1, INPUT VOLTAGE
+FS – 1LSB
Figure 13. Unipolar Transfer Function
UNIPOLAR OFFSET AND FULL-SCALE
ADJUSTMENT
When absolute accuracy is required, offset and full-scale error
can be adjusted to zero. Offset must be adjusted before
full scale. This is achieved by applying an input voltage of ½
LSB to V1 and adjusting R6 until the ADC output code flickers
between 10 0000 0000 0000 and 10 0000 0000 0001. For full-
scale adjustment, apply an input voltage of (FS −3/2 LSBs) to V1
and adjust R5 until the output code flickers between 01 1111
1111 1110 and 01 1111 1111 1111.
Rev. E | Page 10 of 24
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