LMC6582AIMX NSC [National Semiconductor], LMC6582AIMX Datasheet - Page 13

• Manufacturer Part Number: LMC6582AIMX
• Description: Low Voltage, Rail-To-Rail Input and Output CMOS Operational Amplifier
• Manufacturer: NSC [National Semiconductor]

5.0 Printed-Circuit-Board Layout
for High-Impedance Work
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
13 .
FIGURE 12. Typical Connections of Guard Rings
Non-Inverting Amplifier
FIGURE 13. Air Wiring
Inverting Amplifier
Follower
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(Continued)
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13
6.0 Compensating for Input
Capacitance
It is quite common to use large values of feedback resis-
tance with amplifiers that have ultra-low input current, like
the LMC6582/4. Large feedback resistors can react with
small values of input capacitance due to transducers, photo-
diodes, and circuits board parasitics to reduce phase
margins.
The effect of input capacitance can be compensated for by
adding a feedback capacitor. The feedback capacitor (as in
Figure 14 ), C
which typically provides significant overcompensation.
Printed circuit board stray capacitance may be larger or
smaller than that of a breadboard, so the actual optimum
value for C
checked on the actual circuit. (Refer to the LMC660 quad
CMOS amplifier data sheet for a more detailed discussion.)
7.0 Spice Macromodel
A Spice Macromodel is available for the LMC6582/4. The
model includes a simulation of:
and many more characteristics as listed on the macromodel
disk.
Contact the National Semiconductor Customer Response
Center at 1-800-272-9959 to obtain an operational amplifier
spice model library disk.
• Input common-mode voltage range
• Frequency and transient response
• GBW dependence on loading conditions
• Quiescent and dynamic supply current
• Output swing dependence on loading conditions
FIGURE 14. Canceling the Effect of Input Capacitance
F
F
may be different. The values of C
, is first estimated by:
R
1
C
IN
or
R
2
C
F
F
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should be