AD7886
OFFSET AND GAIN ADJUSTMENT
BIPOLAR OPERATION
In most digital signal processing (DSP) applications, offset and
full-scale errors have little or no effect on system performance.
Offset error can usually be eliminated in the analog domain by
ac coupling. Full-scale errors do not cause problems as long as
the input signal is within the full dynamic range of the ADC.
For applications requiring that the input signal range match the
full analog input dynamic range of the ADC, offset and full-
scale errors must be adjusted to zero.
UNIPOLAR OFFSET AND GAIN ERROR ADJUSTMENT
Bipolar operation is achieved by providing a +10 V span on
the VIN1 input while offsetting the VIN2 input by +5 V. A
typical circuit is shown in Figure 7. The output code is off-
set binary. The ideal input/output transfer characteristic is
shown in Figure 8. The LSB size is (10/4096) V = 2.44 mV.
+ 5V
V
DD
If absolute accuracy is an application requirement, offset and
gain can be adjusted to zero. Offset error must be adjusted be-
fore gain error. Zero offset is achieved by adjusting the offset of
the op amp driving the analog input (i.e., A1 in Figure 6). For
zero offset error, apply a voltage of 1 LSB to AIN and adjust
the op amp offset until the ADC output code flickers between
0000 0000 0000 and 0000 0000 0001.
0 V to 5 V Range: 1 LSB = 1.22 mV
0 V to 10 V Range: 1 LSB = 2.44 mV
For zero gain, error apply an analog input voltage equal to
FS–1 LSB (last code transition) at AIN and adjust R3 until the
ADC output code flickers between 1111 1111 1110 and 1111
1111 1111.
0 V to 5 V Range:
FS–1 LSB = 4.99878 V
0 V to 10 V Range: FS–1 LSB = 9.99756 V
+ 5V
AIN
0 TO 5V
OR
0 TO 10V
AIN
±
5V
+V
+V
IN
V
OUT
VIN1
VIN2
AGND
+ 5V
+ 5REF
AD586
GND
SUM
AD707
–
+
–
3.5V
AD7886*
V
REF
V
SS
C1
10µF
C2
0.1µF
– 5V
AD845
+
A1
VIN1
V
DD
–
VIN2**
OUTPUT
CODE
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 7. Bipolar Operation
+V
AGND
+V
IN
V
OUT
+ 5V
+ 5REF
R1
82k
SUM
11...111
11...110
11...101
10...010
10...001
– FS
+1LSB
2
AD586
GND
AD707
R3
5k
–
+
– 3.5V
–1LSB
+1LSB
+ FS
– 1LSB
2
FS = 10V
1LSB =
FS
4096
V
REF
10...000
01...111
01...110
AD7886*
R2
56k
C1
10µF
C2
0.1µF
V
SS
01...101
00...001
00...000
– 5V
*ADDITIONAL PINS OMITTED FOR CLARITY
**0 TO 5V RANGE: CONNECT VIN2 TO VIN1
0 TO 10V RANGE: CONNECT VIN2 TO AGND
VIN, INPUT VOLTAGE – LSBs
Figure 6. Unipolar Operation with Gain Error Adjust
Figure 8. Ideal Input/Output Characteristics for
Bipolar Operation
REV. B
–7–
AD [ ANALOG DEVICES ]
