LMC6042

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SNOS611E – AUGUST 1999 – REVISED MARCH 2013

Figure 30. LMC6042 Noninverting Gain of 10 Amplifier, Compensated to Handle Capacitive Loads

In the circuit of Figure 30, R1 and C1 serve to counteract the loss of phase margin by feeding the high frequency

component of the output signal back to the amplifier's inverting input, thereby preserving phase margin in the

overall feedback loop.

resistor conducting 10

μA or more will significantly improve capacitive load responses. The value of the pull up

resistor must be determined based on the current sinking capability of the amplifier with respect to the desired

output swing. Open loop gain of the amplifier can also be affected by the pull up resistor (see Electrical

Characteristics).

Figure 31. Compensating for Large

Capacitive Loads with a Pull Up Resistor

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 bias current of the

LMC6042, typically less than 2 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 leakage 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 appreciable.

To minimize the effect of any surface leakage, lay out a ring of foil completely surrounding the LMC6042's inputs

and the terminals of capacitors, diodes, conductors, resistors, relay terminals etc. connected to the op-amp's

inputs, as in Figure 32. 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

Ω, which is normally 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 100 times degradation from the

LMC6042's actual performance. However, if a guard ring is held within 5 mV of the inputs, then even a

Ω would cause only 0.05 pA of leakage current. See Figure 36 for typical connections of guard

rings for standard op-amp configurations.

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