REV. C

AD624

–12–

225.3

124

4445.7

80.2

50

50

20k

10k

10k

10k

AD624

G = 100

G = 200

G = 500

–INPUT

(+INPUT)

20k

10k

+INPUT

(–INPUT)

AD7528

1/2

AD712

256:1

DATA

INPUTS

CS

WR

DAC A/DAC B

DB0

DB7

DAC A

DAC B

1/2

AD712

Figure 40. Programmable Output Gain Using a DAC

AUTOZERO CIRCUITS

In many applications it is necessary to provide very accurate

data in high gain configurations. At room temperature the offset

effects can be nulled by the use of offset trimpots. Over the

operating temperature range, however, offset nulling becomes a

problem. The circuit of Figure 41 shows a CMOS DAC operat-

ing in the bipolar mode and connected to the reference terminal

to provide software controllable offset adjustments.

AD624

G = 100

G = 200

G = 500

+INPUT

–INPUT

DATA

INPUTS

CS

WR

MSB

LSB

AD7524

C1

OUT1

OUT2

1/2

AD712

R3

20k

R4

10k

R5

20k

R6

5k

GND

AD589

39k

1/2

AD712

Figure 41. Software Controllable Offset

In many applications complex software algorithms for autozero

applications are not available. For these applications Figure 42

provides a hardware solution.

AD624

1k

12 11

910

0.1 F LOW

LEAKAGE

CH

15

16

14

13

GND

A1

A2

A3

A4

AD7510DIKD

200 s

ZERO PULSE

AD542

Figure 42. Autozero Circuit

The microprocessor controlled data acquisition system shown in

Figure 43 includes includes both autozero and autogain capabil-

ity. By dedicating two of the differential inputs, one to ground

and one to the A/D reference, the proper program calibration

cycles can eliminate both initial accuracy errors and accuracy

errors over temperature. The autozero cycle, in this application,

converts a number that appears to be ground and then writes

that same number (8 bit) to the AD624 which eliminates the

zero error since its output has an inverted scale. The autogain

cycle converts the A/D reference and compares it with full scale.

A multiplicative correction factor is then computed and applied

to subsequent readings.

AD624

1/2

AD712

AD583

AGND

AD574A

AD7507

EN A1

A2

A0

ADDRESS BUS

5k

10k

20k

LATCH

20k

1/2

AD712

CONTROL

DECODE

AD7524

MICRO-

PROCESSOR

Figure 43. Microprocessor Controlled Data Acquisition

System