OP97

Rev. G | Page 14 of 16

The digitally programmable gain amplifier shown in Figure 38

has 12-bit gain resolution with 10-bit gain linearity over the

range of −1 to −1024. The low bias current of the OP97 main-

tains this linearity, while C1 limits the noise voltage bandwidth,

allowing accurate measurement down to microvolt levels.

Table 5

.

DIGITAL IN

GAIN (Av)

4095

−1.00024

2048

−2

1024

−4

512

−8

256

−16

128

−32

64

−64

32

−128

16

−256

8

−512

4

−1024

2

−2048

1

−4096

0

Open Loop

Many high speed amplifiers suffer from less-than-perfect low

frequency performance. A combination amplifier consisting of

a high precision, slow device like the OP97 and a faster device such

as the AD8610 results in uniformly accurate performance from

dc to the high frequency limit of the AD8610, which has a gain-

bandwidth product of 25 MHz. The circuit shown in Figure 39

accomplishes this, with the AD8610 providing high frequency

amplification and the OP97 operating on low frequency signals

and providing offset correction. Offset voltage and drift of the

circuit are controlled by the OP97.

VOUT

2

3

6

AD7541A

IOUT2

IOUT1

RFB

VREF

18

1

2

3

17

16

+15V

–15V

0.1µF

0.1µF

C1

220pF

VIN

±2.5mV TO ±10V

RANGE DEPENDING

ON GAIN SETTING

+15V

OP97

0.1µF

Figure 38. Precision Programmable Gain Amplifier

2

3

6

2

3

6

5pF

VOUT

AD8610

OP97

0.1µF

0.1µF

10kΩ

10kΩ

1µF

R1

2kΩ

R2

20kΩ

VIN

5

AV = –

R2

R1

Figure 39. Combination High Speed, Precision Amplifier

100

90

10

0%

1V

5V

2µs

Figure 40. Combination Amplifier Transient Response