ICL7650 Datasheet, PDF (8/12 Pages) Maxim Integrated Products – Chopper-Stabilized Op Amps

Chopper-Stabilized Op Amps
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.
INPUT ^R1
R2
R3*
OUTPUT
R3*
INPUT
OUTPUT
INVERTING AMPLIFIER
*USE _{R3 TO COMPENSATE FOR LARGE}
SOURCE RESISTANCES, OR FOR CLAMP
OPERATION (FIGURE 5).
R2
R3*
OUTPUT
R1
INPUT
NONINVERTING AMPLIFIER
NOTE: ^{R1 R2 SHOULD BE LOW IMPEDANCE FOR}
R1 + R2 OPTIMUM GUARDING.
FOLLOWER
EXTERNAL
CAPACITORS
V+
OUTPUT _{6 7 8 1}
EXTERNAL
CAPACITORS
5
4
2
3
V-
GUARD
BOTTOM VIEW
BOARD LAYOUT FOR INPUT GUARDING
WITH TO-99 PACKAGE.
Figure 3. Input Guard Connection
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