lmc6034-mda National Semiconductor Corporation, lmc6034-mda Datasheet - Page 9

• Manufacturer Part Number: lmc6034-mda
• Description: Cmos Quad Operational Amplifier
• Manufacturer: National Semiconductor Corporation

Applications Hint
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 con-
struction, but the advantages are sometimes well worth the
effort of using point-to-point up-in-the-air wiring. See Figure
10.
(Input pins are lifted out of PC board and soldered directly to components.
All other pins connected to PC board.)
FIGURE 8. Guard Ring Connections
FIGURE 9. Guard Ring Connections
Howland Current Pump
FIGURE 10. Air Wiring
Follower
(Continued)
01113409
01113410
01113411
9
BIAS CURRENT TESTING
The test method of Figure 11 is appropriate for bench-testing
bias current with reasonable accuracy. To understand its
operation, 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
capacitor C2 could cause errors.
Similarly, if S1 is shorted momentarily (while leaving S2
shorted)
where C
FIGURE 11. Simple Input Bias Current Test Circuit
x
is the stray capacitance at the + input.
b
−, the leakage of the capacitor and socket
01113412
www.national.com