LMC660
Application Hints
(Continued)
DS008767-21
(Input pins are lifted out of PC board and soldered directly to components.
All other pins connected to PC board.)
DS008767-17
(a) Inverting Amplifier
FIGURE 7. Air Wiring
BIAS CURRENT TESTING
The test method of
Figure 8
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
DS008767-18
(b) Non-Inverting Amplifier
DS008767-19
(c) Follower
DS008767-22
FIGURE 8. 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)
DS008767-20
(d) Howland Current Pump
FIGURE 6. Guard Ring Connections
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
7.
where C
x
is the stray capacitance at the + input.
www.national.com
8
NSC [ NATIONAL SEMICONDUCTOR ]
