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

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

current required to trigger the SCR gate lead. The LMC6084

is designed to withstand 100 mA surge current 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

Applications

(V

The extremely high input impedance, and low power con-

sumption, of the LMC6084 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

CMRR with 1 k

Ω imbalance in bridge source resistance.

Input 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.

01146707

FIGURE 4. Example of Guard Ring in P.C. Board

Layout

01146708

Inverting Amplifier

01146709

Non-Inverting Amplifier

01146710

Follower

FIGURE 5. Typical Connections of Guard Rings

01146711

(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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