AD625

REV. D

–9–

Any resistors in series with the inputs of the AD625 will degrade

the noise performance. For this reason the circuit in Figure 26b

should be used if the gains are all greater than 5. For gains less

than 5, either the circuit in Figure 26a or in Figure 26c can be

used. The two 1.4 k

Ω resistors in Figure 26a will degrade the

noise performance to:

RESISTOR PROGRAMMABLE GAIN AMPLIFIER

In the resistor-programmed mode (Figure 27), only three exter-

nal resistors are needed to select any gain from 1 to 10,000.

Depending on the application, discrete components or a

pretrimmed network can be used. The gain accuracy and gain

TC are primarily determined by the external resistors since the

AD625C contributes less than 0.02% to gain error and under

5 ppm/

°C gain TC. The gain sense current is insensitive to

common-mode voltage, making the CMRR of the resistor pro-

grammed AD625 independent of the match of the two feedback

Selecting Resistor Values

stage and sets the unity gain transconductance. These feedback

resistors are provided by the user. The AD625 is tested and

specified with a value of 20 k

RTO errors increases with increasing feedback resistance, values

much above 20 k

Ω are not recommended (values below 10 kΩ

offset, drift, and bandwidth (Figure 28) when selecting the

+GAIN

SENSE

–GAIN

SENSE

+INPUT

–INPUT

RTI NULL

RTI NULL

RTO

NULL

RTO

NULL

+GAIN DRIVE

–GAIN DRIVE

NC

REF

G =

+1

A1

A2

AD625

10k

10k

10k

10k

A3

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

Figure 27. AD625 in Fixed Gain Configuration

A list of standard resistors which can be used to set some com-

mon gains is shown in Table I.

For single gain applications, only one offset null adjust is neces-

sary; in these cases the RTI null should be used.

RTO OFFSET VOLTAGE DRIFT

6

5

4

3

2

1

60k

50k

40k

30k

20k

10k

BANDWIDTH

1M

100k

10k

1

10

100

1k

10k

20k

50k

FEEDBACK RESISTANCE –

FEEDBACK RESISTANCE –

RTO NOISE

RTO OFFSET VOLTAGE

300

200

100

3

2

10k

20k

30k

40k

50k

60k

10k

20k

30k

40k

50k

60k

FEEDBACK RESISTANCE –

FEEDBACK RESISTANCE –

Figure 28. RTO Noise, Offset, Drift and Bandwidth vs.

Feedback Resistance Normalized to 20 k

Ω

Table I. Common Gains Nominally Within

0.5% Error

Using Standard 1% Resistors

GAIN

1

20 k

Ω∞

2

19.6 k

Ω

39.2 k

Ω

5

20 k

Ω

10 k

Ω

10

20 k

Ω

4.42 k

Ω

20

20 k

Ω

2.1 k

Ω

50

19.6 k

Ω

806

Ω

100

20 k

Ω

402

Ω

200

20.5 k

Ω

205

Ω

500

19.6 k

Ω

78.7

Ω

1000

19.6 k

Ω

39.2

Ω

4

20 k

Ω

13.3 k

Ω

8

19.6 k

Ω

5.62 k

Ω

16

20 k

Ω

2.67 k

Ω

32

19.6 k

Ω

1.27 k

Ω

64

20 k

Ω

634

Ω

128

20 k

Ω

316

Ω

256

19.6 k

Ω

154

Ω

512

19.6 k

Ω

76.8

Ω

1024

19.6 k

Ω

38.3

Ω

SENSE TERMINAL

The sense terminal is the feedback point for the AD625 output

amplifier. Normally it is connected directly to the output. If

heavy load currents are to be drawn through long leads, voltage

drops through lead resistance can cause errors. In these in-

stances the sense terminal can be wired to the load thus putting