AD7886

–7–

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:

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

AD7886*

SUM

+

–

AD586

+V

IN

GND

+V

C1

10

µF

C2

0.1

µF

AD707

VIN1

VIN2**

AGND

V

SS

V

DD

AIN

R1

82k

R2

56k

R3

5k

0 TO 5V

OR

0 TO 10V

–

+

AD845

A1

5V

+

5REF

+

5V

+

3.5V

–

V

REF

5V

–

*ADDITIONAL PINS OMITTED FOR CLARITY

**0 TO 5V RANGE: CONNECT VIN2 TO VIN1

0 TO 10V RANGE: CONNECT VIN2 TO AGND

Figure 6. Unipolar Operation with Gain Error Adjust

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.

AD7886*

SUM

+

–

AD586

V

OUT

+V

IN

GND

+V

C1

10

µF

C2

0.1

µF

AD707

VIN1

VIN2

AGND

V

SS

V

DD

AIN

*ADDITIONAL PINS OMITTED FOR CLARITY

5V

+

5REF

+

V

REF

5V

±

5V

+

5V

–

3.5V

–

Figure 7. Bipolar Operation

00...000

00...001

01...101

01...110

11...111

11...110

11...101

10...000

10...001

10...010

01...111

FS = 10V

1LSB =

FS

4096

VIN, INPUT VOLTAGE – LSBs

OUTPUT

CODE

1LSB

–

+1LSB

– 1LSB

2

FS

+

2

FS

–

+1LSB

Figure 8. Ideal Input/Output Characteristics for

Bipolar Operation