LF355 datasheet(33/47 Pages) TI2

33 / 47 page

LF155, LF156, LF256, LF257

LF355, LF356, LF357

www.ti.com

SNOSBH0D – MAY 2000 – REVISED NOVEMBER 2015

9 Power Supply Recommendations

See the Recommended Operating Conditions for the minimum and maximum values for the supply input voltage

and operating junction temperature.

10 Layout

10.1 Layout Guidelines

10.1.1 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 PCB. When one wishes to take advantage of the low input bias current of the LFx5x,

typically less than 30 pA, 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 PCB, 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 inputs of the

LFx5x and the terminals of capacitors, diodes, conductors, resistors, relay terminals, and so forth, connected to

the inputs of the op amp, as in Figure 62. To have a significant effect, guard rings must be placed 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 10 T

Ω, which is normally considered a very large resistance, could leak 5 pA if

the trace were a 5-V bus adjacent to the pad of the input. If a guard ring is used and held close to the potential of

the amplifier inputs, it will significantly reduce this leakage current.

Figure 58. Inverting Amplifier

Figure 59. Noninverting Amplifier

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LF355 Datasheet(PDF) 33 Page - Texas Instruments