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.

Typical Single-Supply Applications

(V

The extremely high input impedance, and low power con-

sumption, of the LMC6041 make it ideal for applications that

require battery-powered instrumentation amplifiers. Ex-

amples of these type of applications are hand-held pH

probes, analytic medical instruments, magnetic field detec-

tors, gas detectors, and silicon based pressure transducers.

The circuit in

Figure 7 is recommended for applications

where the common-mode input range is relatively low and

the differential gain will be in the range of 10 to 1000. This

two op-amp instrumentation amplifier features an indepen-

dent adjustment of the gain and common-mode rejection

trim, and a total quiescent supply current of less than 28 µA.

To maintain ultra-high input impedance, it is advisable to use

ground rings and consider PC board layout an important part

of the overall system design (see Printed-Circuit-Board Lay-

out for High Impedance Work). Referring to

Figure 7, the in-

put voltages are represented as a common-mode input V

CM

plus a differential input V

Rejection of the common-mode component of the input is

accomplished by making the ratio of R1/R2 equal to R3/R4.

So that where,

A suggested design guideline is to minimize the difference of

value between R1 through R4. This will often result in im-

proved resistor tempco, amplifier gain, and CMRR over tem-

perature. If RN = R1 = R2 = R3 = R4 then the gain equation

can be simplified:

Due to the “zero-in, zero-out” performance of the LMC6041,

and output swing rail-rail, the dynamic range is only limited to

the input common-mode range of 0V to V

at room temperature. This feature of the LMC6041 makes it

an ideal choice for low-power instrumentation systems.

A complete instrumentation amplifier designed for a gain of

100 is shown in

Figure 8. Provisions have been made for low

sensitivity trimming of CMRR and gain.

DS011136-8

Inverting Amplifier

DS011136-9

Follower

DS011136-10

Non-Inverting Amplifier

FIGURE 5. Typical Connections of Guard Rings

DS011136-11

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

All other pins connected to PC board.)

FIGURE 6. Air Wiring

DS011136-12

FIGURE 7. Two Op-Amp Instrumentation Amplifier

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