REV. D

OP97

–8–

OP97

10k

2

3

6

Figure 5. Unity-Gain Follower

Figure 6. Small-Signal Transient Response with Overcom-

GUARDING AND SHIELDING

To maintain the extremely high input impedances of the OP97,

care must be taken in circuit board layout and manufacturing.

Board surfaces must be kept scrupulously clean and free of moisture.

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 that guard rings should be used around the

inputs. Guard traces are operated at a voltage close to that on the

inputs, so that leakage currents become minimal. In nonin-

verting applications, the guard ring should be connected to the

common-mode voltage at the inverting input (Pin 2). In inverting

applications, both inputs remain at ground, so that the guard trace

should be grounded. Guard traces should be made on both sides

of the circuit board.

OP97

2

3

6

DIGITAL

INPUTS

30pF

PM7548

Figure 7. DAC Output Amplifier

OP97

R5

10k

2

3

6

V1

R1

10k

R2

10k

R3

10k

R4

10k

–15V

+15V

7

4

Figure 8. Current Monitor

High impedance circuitry is extremely susceptible to RF pickup,

line frequency hum, and radiated noise from switching power

supplies. Enclosing sensitive analog sections within grounded

shields is generally necessary to prevent excessive noise pickup.

Twisted-pair cable will aid in rejection of line frequency hum.

The OP97 is an excellent choice as an output amplifier for higher

resolution CMOS DACs. Its tightly trimmed offset voltage and

minimal bias current result in virtually no degradation of linear-

ity, even over wide temperature ranges.

Figure 8 shows a versatile monitor circuit that can typically

sense current at any point between the

±15 V supplies. This

makes it ideal for sensing current in applications such as full

bridge drivers where bidirectional current is associated with large

common-mode voltage changes. The 114 dB CMRR of the OP97

makes the amplifier’s contribution to common-mode error

negligible, leaving only the error due to the resistor ratio

inequality. Ideally, R2/R4 = R3/R5. This is best trimmed via R4

OP97

2

3

6

UNITY-GAIN FOLLOWER

OP97

2

3

6

NONINVERTING AMPLIFIER

OP97

2

3

6

INVERTING AMPLIFIER

1

8

1

8

TO-99

BOTTOM VIEW

MINI-DIP

BOTTOM VIEW

Figure 9. Guard Ring Layout and Connections