LMC66 datasheet(15/24 Pages) NSC

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4.0 Capacitive Load Tolerance (Continued)

The pulse response of the circuit shown in

Figure 9 is shown in Figure 10.

Application Hints

5.0 Printed-Circuit-Board Layout for High-Impedance Work

It is generally recognized that any circuit which must operate

with less than 1000 pA of leakage current requires special

layout of the PC board. When one wishes to take advantage

of the ultra-low input current of the LMC6681/2/4, typically

less than 80 fA, it is essential to have an excellent layout.

Fortunately, the techniques of obtaining low leakages are

quite simple. First, the user must not ignore the surface leak-

age of the PC board, even though it may sometimes appear

acceptably low, because under conditions of high humidity or

dust or contamination, the surface leakage will be appre-

ciable.

To minimize the effect of any surface leakage, lay out a ring

of foil completely surrounding the LMC6681/2/4’s inputs and

the terminals of capacitors, diodes, conductors, resistors, re-

lay terminals, etc. connected to the op-amp’s inputs, as in

Figure 11. To have a significant effect, guard rings should be

placed in 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

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 60 times degradation from the LMC6681/2/

4’s actual performance. However, if a guard ring is held

within 5 mV of the inputs, then even a resistance of 10

Ω

would cause only 0.05 pA of leakage current. See

Figure 12

for typical connections of guard rings for standard op-amp

configurations.

DS012042-14

FIGURE 10. Pulse Response of the

LMC6682 Circuit Shown in

Figure 9

DS012042-18

FIGURE 11. Example of Guard Ring in PC Board

Layout

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