LMC662 Datasheet, PDF (8/13 Pages) National Semiconductor (TI) – CMOS Dual Operational Amplifier

Application Hints (Continued)
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
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FIGURE 9. Guard Ring Connections: Howland Current
Pump
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.
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FIGURE 11. Simple Input Bias Current Test Circuit
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_b−, the leakage of the capacitor and socket
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)
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(Input pins are lifted out of PC board and soldered directly to components.
All other pins connected to PC board.)
FIGURE 10. Air Wiring
where C_x is the stray capacitance at the + input.
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