5.0 Printed-Circuit-Board Layout

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.

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

or

R

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:

•

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

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.

DS012041-15

Inverting Amplifier

DS012041-16

Non-Inverting Amplifier

DS012041-17

Follower

FIGURE 12. Typical Connections of Guard Rings

DS012041-18

FIGURE 13. Air Wiring

DS012041-13

FIGURE 14. Canceling the Effect of Input Capacitance

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