REV. E

OP297

–7–

UNITY-GAIN FOLLOWER

INVERTING AMPLIFIER

NONINVERTING AMPLIFIER

MINI-DIP

BOTTOM VIEW

1

8

B

A

1/2

OP297

1/2

OP297

1/2

OP297

Figure 7. Guard Ring Layout and Connections

GUARDING AND SHIELDING

To maintain the extremely high input impedances of the OP297,

care must be taken in circuit board layout and manufacturing.

Board surfaces must be kept scrupulously clean and free of mois-

ture. Conformal coating is recommended to provide 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 voltage close to

that on the inputs, as shown in Figure 7, so that leakage currents

become minimal. In noninverting applications, the guard ring

should be connected to the common-mode voltage 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 precision

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

loop gain applications. Figure 8 illustrates the typical open-loop

gain linearity of the OP297 over the military temperature range.

OUTPUT VOLTAGE (V)

0

–15

–10

–5

0

5

15

10

Figure 8. Open-Loop Linearity of the OP297

APPLICATIONS

PRECISION ABSOLUTE VALUE AMPLIFIER

The circuit of Figure 9 is a precision absolute value amplifier with

an input impedance of 30 M

of the OP297 ensure accurate operation with microvolt 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.

1/2

OP297

1/2

OP297

+15V

C2

0.1 F

R1

1k

C1

30pF

D1

1N4148

D2

1N4148

R2

2k

0V

10V

R3

1k

–15V

C3

0.1 F

5

6

7

1

28

3

4

Figure 9. Precision Absolute Value Amplifier

PRECISION CURRENT PUMP

Maximum output current of the precision current pump shown

in Figure 10 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.

1/2

OP297

+15V

R3

10k

5

6

7

1

2

8

3

1/2

OP297

10mA

R1

10k

R2

10k

R4

10k

R6

10k

–15V

=

= 10mA/V

R5

100

Figure 10. Precision Current Pump