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

6.

Latchup

CMOS devices tend to be susceptible to latchup due to their

internal parasitic SCR effects. The (I/O) input and output pins

look similar to the gate of the SCR. There is a minimum cur-

rent required to trigger the SCR gate lead. The LMC6061

and LMC6081 are designed to withstand 100 mA surge cur-

rent on the I/O pins. Some resistive method should be used

to isolate any capacitance from supplying excess current to

the I/O pins. In addition, like an SCR, there is a minimum

holding current for any latchup mode. Limiting current to the

supply pins will also inhibit latchup susceptibility.

Typical Single-Supply

The extremely high input impedance, and low power con-

sumption, of the LMC6061 make it ideal for applications that

require battery-powered instrumentation amplifiers. Ex-

amples of these types of applications are hand-held pH

probes, analytic medical instruments, magnetic field detec-

tors, gas detectors, and silicon based pressure transducers.

Figure 7 shows an instrumentation amplifier that features

high differential and common mode input resistance

14

Ω), 0.01% gain accuracy at A

V

= 100, excellent

CMRR with 1 k

Ω imbalance in bridge source resistance. In-

put current is less than 100 fA and offset drift is less than

2.5 µV/˚C. R

over a wide range without degrading CMRR. R

trim used to maximize CMRR without using super precision

matched resistors. For good CMRR over temperature, low

drift resistors should be used.

DS011422-7

Inverting Amplifier

DS011422-8

Non-Inverting Amplifier

DS011422-9

Follower

FIGURE 5. Typical Connections of Guard Rings

DS011422-10

(Input pins are lifted out of PC board and soldered directly to components.

All other pins connected to PC board).

FIGURE 6. Air Wiring

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