ad7886kp Analog Devices, Inc., ad7886kp Datasheet - Page 7

• Manufacturer Part Number: ad7886kp
• Description: Lc2mos 12-bit, 750 Khz/1 Mhz, Sampling Adc
• Manufacturer: Analog Devices, Inc.

REV. B
OFFSET AND GAIN ADJUSTMENT
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
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:
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:
0 V to 10 V Range: FS–1 LSB = 9.99756 V
Figure 6. Unipolar Operation with Gain Error Adjust
AD586
0 TO 10V
+V
GND
0 TO 5V
+V
AIN
IN
OR
V
OUT
**0 TO 5V RANGE: CONNECT VIN2 TO VIN1
*ADDITIONAL PINS OMITTED FOR CLARITY
56k
0 TO 10V RANGE: CONNECT VIN2 TO AGND
R3
R2
5k
+
5V
1 LSB = 1.22 mV
FS–1 LSB = 4.99878 V
R1
82k
AD845
+
–
10 F
A1
C1
AD707
–
+
0.1 F
C2
–
3.5V
VIN1
VIN2**
AGND
+
V
SUM
5REF
REF
AD7886*
–
V
5V
SS
+
V
5V
DD
–7–
BIPOLAR OPERATION
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.
OUTPUT
11...110
10...010
10...000
01...110
00...000
11...111
11...101
10...001
01...111
01...101
00...001
CODE
Figure 8. Ideal Input/Output Characteristics for
Bipolar Operation
AD586
*ADDITIONAL PINS OMITTED FOR CLARITY
+V
GND
+V
–
IN
FS
2
V
Figure 7. Bipolar Operation
OUT
+1LSB
10 F
C1
+
VIN, INPUT VOLTAGE – LSBs
5V
AD707
AIN
+
–
5V
–
0.1 F
1LSB
C2
–
3.5V
+1LSB
VIN1
VIN2
V
+
SUM
AGND
REF
5REF
1LSB =
AD7886*
FS = 10V
+
–
V
V
5V
DD
5V
SS
+
4096
FS
AD7886
2
FS
– 1LSB