Datasheet

LMC660

www.ti.com

SNOSBZ3D –APRIL 1998–REVISED MARCH 2013

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 21.

(Input pins are lifted out of PC board and soldered directly to components. All other pins connected to PC board.)

Figure 21. Air Wiring

BIAS CURRENT TESTING

The test method of Figure 21 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 22. 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

−, 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.

Similarly, if S1 is shorted momentarily (while leaving S2 shorted):

(11)

where C

is the stray capacitance at the + input.

Product Folder Links: LMC660