The designer should be aware that when it is inappropriate

to lay out a PC board for the sake of just a few circuits, there

is another technique which is even better than a guard ring

on a PC board: Don’t insert the amplifier’s input pin into the

board at all, but bend it up in the air and use only air as an in-

sulator. Air is an excellent insulator. In this case you may

have to forego some of the advantages of PC board con-

struction, but the advantages are sometimes well worth the

effort of using point-to-point up-in-the-air wiring. See

Figure

10.

BIAS CURRENT TESTING

The test method of

Figure 11 is appropriate for bench-testing

bias current with reasonable accuracy. To understand its op-

eration, first close switch S2 momentarily. When S2 is

opened, then

A suitable capacitor for C2 would be a 5 pF or 10 pF silver

mica, NPO ceramic, or air-dielectric. When determining the

magnitude of I

must be taken into account. Switch S2 should be left shorted

most of the time, or else the dielectric absorption of the ca-

pacitor C2 could cause errors.

Similarly, if S1 is shorted momentarily (while leaving S2

shorted)

where C

DS011134-7

FIGURE 6. Guard Ring Connections

Inverting Amplifier

DS011134-8

FIGURE 7. Guard Ring Connections

Non-Inverting Amplifier

DS011134-9

FIGURE 8. Guard Ring Connections

Follower

DS011134-10

FIGURE 9. Guard Ring Connections

Howland Current Pump

DS011134-11

(Input pins are lifted out of PC board and soldered directly to components.

All other pins connected to PC board.)

FIGURE 10. Air Wiring

DS011134-12

FIGURE 11. Simple Input Bias Current Test Circuit

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