LMC6034
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SNOS608C –MAY 1998–REVISED MARCH 2013
Figure 21. 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 insulator. Air is an
excellent insulator. In this case you may have to forego some of the advantages of PC board construction, but
the advantages are sometimes well worth the effort of using point-to-point up-in-the-air wiring. See Figure 22.
(Input pins are lifted out of PC board and soldered directly to components. All other pins connected to PC board.)
Figure 22. Air Wiring
BIAS CURRENT TESTING
The test method of Figure 23 is appropriate for bench-testing bias current with reasonable accuracy. To
understand its operation, first close switch S2 momentarily. When S2 is opened, then
(10)
Figure 23. 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 Ib−, 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 capacitor C2 could cause errors.
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