AD524

REV. E

–12–

PROGRAMMABLE GAIN

Figure 41 shows the AD524 being used as a software program-

mable gain amplifier. Gain switching can be accomplished with

mechanical switches such as DIP switches or reed relays. It

should be noted that the “on” resistance of the switch in series

with the internal gain resistor becomes part of the gain equation

and will have an effect on gain accuracy.

The AD524 can also be connected for gain in the output stage.

Figure 42 shows an AD711 used as an active attenuator in the

output amplifier’s feedback loop. The active attenuation pre-

sents a very low impedance to the feedback resistors, therefore

minimizing the common-mode rejection ratio degradation.

R2

10k

1 F

35V

OUTPUT

OFFSET

NULL

TO –V

AD524

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

20k

20k

20k

404

4.44k

20k

+V

S

20k

20k

40

PROTECTION

PROTECTION

–IN

+IN

(+INPUT)

(–INPUT)

10k

INPUT

OFFSET

NULL

10pF

20k

AD711

AD7590

39.2k

28.7k

316k

1k

1k

1k

Figure 42. Programmable Output Gain

Y0

Y2

Y1

+5V

INPUT

OFFSET

TRIM

C1

C2

ANALOG

COMMON

A

B

INPUTS

GAIN

RANGE

LOGIC

COMMON

+5V

G = 10

K1

G = 100

K2

G = 1000

K3

RELAY

SHIELDS

–IN

+IN

K1 – K3 =

THERMOSEN DM2C

4.5V COIL

D1 – D3 = IN4148

OUT

K1

K2

K3

D1

D2

D3

10 F

NC

GAIN TABLE

A

B GAIN

0

0

1

1

0

1

0

1

10

1000

100

1

OUTPUT

OFFSET

TRIM

R2

10k

74LS138

DECODER

7407N

BUFFER

DRIVER

1 F

35V

NC = NO CONNECT

R1

10k

A1

AD524

1

2

3

4

5

6

7

8

20k

20k

20k

404

4.44k

20k

+V

S

20k

20k

40

PROTECTION

PROTECTION

16

15

14

13

12

11

10

9

Figure 41. Three Decade Gain Programmable Amplifier