LMC6482 datasheet(28/50 Pages) TI2

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7.3 Power Supply Recommendations

The LMC648x operate over a supply range of 3V to 15.5V. To achieve noise immunity as appropriate to the

application, use good printed circuit board (PCB) layout practices for power-supply rails and planes, as well as

bypass capacitors connected between the power-supply pins and ground.

7.4 Layout

7.4.1 Layout Guidelines

As a general rule, any circuit that must operate with less than 1000pA of leakage current requires special layout

of the PC board. To take advantage of the ultra-low input current of the LMC648x, typically less than 20fA, an

excellent layout is essential. Fortunately, the techniques of obtaining low leakages are quite simple. First, do not

ignore the surface leakage of the PCB even though the leakage current can sometimes appear acceptably low,

because under conditions of high humidity or dust or contamination, the surface leakage can be appreciable.

To minimize the effect of any surface leakage, lay out a ring of foil completely surrounding the LMC648x inputs

and the terminals of capacitors, diodes, conductors, resistors, relay terminals, and so forth connected to the

inputs of the op amp, as in Figure 7-26. To have a significant effect, place guard rings on both the top and

bottom of the PCB. This PC foil must then be connected to a voltage that is at the same voltage as the amplifier

inputs, because no leakage current can flow between two points at the same potential. For example, a PCB

trace-to-pad resistance of 1012Ω, which is normally considered a very large resistance, can leak 5pA if the trace

were a 5V bus adjacent to the pad of the input. This leakage can cause a 250 times degradation from the actual

performance of the LMC648x. However, if a guard ring is held within 5mV of the inputs, then even a resistance

of 1011Ω causes only 0.05pA of leakage current. See Figure 7-27 through Figure 7-29 for typical connections of

guard rings for standard op-amp configurations.

Be aware that when laying out a PCB for the sake of just a few circuits is not practical, another technique is even

better than a guard ring on a PCB: Do not insert the input pin of the amplifier into the PCB at all, but bend the

pin up in the air, and use only air as an insulator. Air is an excellent insulator. In this case you forgo some of the

advantages of PCB construction, but the advantages are sometimes well worth the effort of using point-to-point

up-in-the-air wiring. See Figure 7-30.

7.4.2 Layout Example

Figure 7-26. Example of Guard Ring in PCB Layout Typical Connections of Guard Rings

LMC6482

LMC6484

SNOS674J – OCTOBER 1997 – REVISED SEPTEMBER 2024

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LMC6482 Datasheet(PDF) 28 Page - Texas Instruments