8/21/97 4:00 PM

OP297

–8–

REV. D

a humidity barrier. Even a clean PC board can have 100 pA of

leakage currents between adjacent traces, so guard rings should

be used around the inputs. Guard traces are operated at a volt-

age close to that on the inputs, as shown in Figure 28, so that

leakage currents become minimal. In noninverting applications,

the guard ring should be connected to the common-mode volt-

age at the inverting input. In inverting applications, both inputs

remain at ground, so the guard trace should be grounded. Guard

traces should be on both sides of the circuit board.

OPEN-LOOP GAIN LINEARITY

The OP297 has both an extremely high gain of 2000 V/mV

minimum and constant gain linearity. This enhances the preci-

sion of the OP297 and provides for very high accuracy in high

closed loop gain applications. Figure 29 illustrates the typical

open-loop gain linearity of the OP297 over the military tempera-

ture range.

OUTPUT VOLTAGE (VOLTS)

–15

–10

–5

0

5

10

15

Figure 29. Open-Loop Linearity of the OP297

APPLICATIONS

PRECISION ABSOLUTE VALUE AMPLIFIER

The circuit of Figure 30 is a precision absolute value amplifier

with an input impedance of 30 M

Ω. The high gain and low

input signals. In this circuit, the input always appears as a

common-mode signal to the op amps. The CMR of the OP297

exceeds 120 dB, yielding an error of less than 2 ppm.

+

–

7

6

5

1k

Ω

1/2

OP-297

2k

Ω

1k

Ω

1N4148

1N4148

+

–

1/2

OP-297

30pF

–15V

0.1

µF

4

1

8

2

3

0.1

µF

+15V

Figure 30. Precision Absolute Value Amplifier

PRECISION CURRENT PUMP

Maximum output current of the precision current pump shown

in Figure 31 is

± 10 mA. Voltage compliance is ± 10 V with

±15 V supplies. Output impedance of the current transmitter

exceeds 3 M

Ω with linearity better than 16 bits.

–

+

UNITY-GAIN FOLLOWER

INVERTING AMPLIFIER

NONINVERTING AMPLIFIER

1/2

OP-297

1/2

OP-297

–

+

MINI-DIP

BOTTOM VIEW

1/2

OP-297

–

+

1

8

B

A

Figure 28. Guard Ring Layout and Connections