ure 4. To have a significant effect, guard rings should be

placed on both the top and bottom of the PC board. This PC

foil must then be connected to a voltage which is at the same

voltage as the amplifier inputs, since no leakage current can

flow between two points at the same potential. For example,

a PC board trace-to-pad resistance of 10

12

Ω, which is nor-

mally considered a very large resistance, could leak 5 pA if

the trace were a 5V bus adjacent to the pad of the input.

This would cause a 500 times degradation from the

LMC6001’s actual performance. If a guard ring is used and

held within 1 mV of the inputs, then the same resistance of

10

12

Ω will only cause 10 fA of leakage current. Even this

small amount of leakage will degrade the extremely low input

current performance of the LMC6001. See

Figure 5 for typi-

cal connections of guard rings for standard op-amp

configurations.

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.

Another potential source of leakage that might be over-

looked is the device package. When the LMC6001 is manu-

factured, the device is always handled with conductive finger

cots. This is to assure that salts and skin oils do not cause

leakage paths on the surface of the package. We recom-

mend that these same precautions be adhered to, during all

phases of inspection, test and assembly.

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 LMC6001 is

designed to withstand 100 mA surge current on the I/O pins.

Some resistive method should be used to isolate any capaci-

tance from supplying excess current to the I/O pins. In addi-

tion, like an SCR, there is a minimum holding current for any

latchup mode. Limiting current to the supply pins will also in-

hibit latchup susceptibility.

Typical Applications

The extremely high input resistance, and low power con-

sumption, of the LMC6001 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, electrostatic field de-

tectors and gas chromotographs.

Two Opamp, Temperature

Compensated pH Probe Amplifier

The signal from a pH probe has a typical resistance between

10 M

Ω and 1000 MΩ. Because of this high value, it is very

important that the amplifier input currents be as small as

possible. The LMC6001 with less than 25 fA input current is

an ideal choice for this application.

The theoretical output of the standard Ag/AgCl pH probe is

59.16 mV/pH at 25˚C with 0V out at a pH of 7.00. This output

is proportional to absolute temperature. To compensate for

this, a temperature compensating resistor, R1, is placed in

DS011887-8

FIGURE 4. Examples of Guard

Ring in PC Board Layout

DS011887-9

Inverting Amplifier

DS011887-10

Non-Inverting Amplifier

DS011887-11

Follower

FIGURE 5. Typical Connections of Guard Rings

DS011887-12

(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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