LMC6464QML

SNOSAR8A – DECEMBER 2010 – REVISED MARCH 2013

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Figure 40. Inverting Configuration, Offset Voltage Adjustment

Figure 41. Non-Inverting Configuration, Offset Voltage Adjustment

SPICE MACROMODEL

A Spice macromodel is available for the LMC6464. This model includes a simulation of:

•

Input common-mode voltage range

•

Frequency and transient response

•

GBW dependence on loading conditions

•

Quiescent and dynamic supply current

•

Output swing dependence on loading conditions

and many more characteristics as listed on the macromodel disk.

Contact the Texas Instruments Customer Response Center to obtain an operational amplifier Spice model library

disk.

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

LMC6464, typically 150 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 LMC6464's inputs

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

inputs, as in Figure 42. 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

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 30 times degradation from the

LMC6464'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 Typical Connections of Guard Rings for

standard op-amp configurations.

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