LPC660

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SNOS554D – MAY 1998 – REVISED MARCH 2013

Capacitive load driving capability is enhanced by using a pull up resistor to V+ (Figure 27). Typically a pull up

resistor conducting 50

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 27. Compensating for LargeCapacitive 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 LPC660,

typically less than 0.04 pA, it is essential to have an excellent layout. Fortunately, the techniques for 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 LPC660's inputs

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

inputs. See Figure 28. 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

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

trace were a 5V bus adjacent to the pad of an input. This would cause a 100 times degradation from the

LPC660's actual performance. However, if a guard ring is held within 5 mV of the inputs, then even a resistance

of 1011 ohms would cause only 0.05 pA of leakage current, or perhaps a minor (2:1) degradation of the

amplifier's performance. See Figure 29a, Figure 30b, Figure 31c for typical connections of guard rings for

standard op-amp configurations. If both inputs are active and at high impedance, the guard can be tied to ground

and still provide some protection; see Figure 32d.

Figure 28. Example of Guard Ring in P.C. Board Layout using the LPC660

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