Chopper-Stabilized Op Amps

8

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Input Guarding

Low-leakage, high-impedance CMOS inputs allow the

ICL7650/ICL7653 to measure high-impedance sources.

Stray leakage paths can decrease input resistance and

increase input currents unless inputs are guarded.

Boards must be thoroughly cleaned with TCE or alcohol

and blown dry with compressed air. The board should

be coated with epoxy or silicone after cleaning to pre-

vent contamination.

Leakage currents may cause trouble even with properly

cleaned and coated boards, particularly since the input

pins are adjacent to pins that are at supply potentials.

Leakage can be significantly reduced by using guard-

ing to decrease the voltage difference between inputs

and adjacent metal runs. Use a 10-lead pin circle, with

the leads of the device formed so that the holes adja-

cent to the inputs are empty when it is inserted in the

board to accomplish input guarding of the 8-pin TO-99

package. A conductive ring surrounding the inputs, the

“guard,” is connected to a low-impedance point that is

approximately the same voltage as the inputs. The

guard then absorbs the leakage current from the high-

voltage pins. Typical guard connections are shown in

Figure 3.

OUTPUT

INVERTING AMPLIFIER

FOLLOWER

INPUT

R2

R1

R3*

R3*

OUTPUT

INPUT

USE R3 TO COMPENSATE FOR LARGE

SOURCE RESISTANCES, OR FOR CLAMP

OPERATION (FIGURE 5).

*

NONINVERTING AMPLIFIER

R3*

OUTPUT

INPUT

R2

R1

NOTE:

SHOULD BE LOW IMPEDANCE FOR

OPTIMUM GUARDING.

R1 R2

R1 + R2

BOTTOM VIEW

BOARD LAYOUT FOR INPUT GUARDING

WITH TO-99 PACKAGE.

1

V+

V-

GUARD

OUTPUT

EXTERNAL

CAPACITORS

EXTERNAL

CAPACITORS

8

7

6

5

4 3

2

Figure 3. Input Guard Connection