OP97

Rev. G | Page 13 of 16

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 PCB can have 100 pA of leakage

currents between adjacent traces; therefore, use guard rings

around the inputs. Guard traces are operated at a voltage close

to that on the inputs, so that leakage currents are minimal. In

noninverting applications, connect the guard ring to the common-

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

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

should be grounded. Make guard traces on both sides of the

circuit board.

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 aid in rejection of line frequency hum.

VOUT

OP97

2

3

6

IO

IO

30pF

AD7548

RFB

DIGITAL

INPUTS

Figure 35. DAC Output Amplifier

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 linearity, even over wide temperature ranges.

Figure 36 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 contribution of the amplifier to common-

mode error negligible, leaving only the error due to the resistor

ratio inequality. Ideally, R2/R4 = R3/R5.

VOUT

RL

IL

2

3

6

7

4

R3

10kΩ

R2

10kΩ

R4

10kΩ

R5

10kΩ

R1

10kΩ

V1

OP97

+15V

–15V

UNITY-GAIN FOLLOWE

Figure 36. Current Monitor

R

NONINVERTING AMPLIFIER

INVERTING AMPLIFIER

OP97

2

3

6

OP97

2

3

6

OP97

2

3

6

PDIP

BOTTOM VIEW

81

Figure 37. Guard Ring Layout and Connections