Capacitive load driving capability is enhanced by using a pull

up resistor to V

ducting 50 µA or more will significantly improve capacitive

load responses. The value of the pull up resistor must be de-

termined based on the current sinking capability of the ampli-

fier 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).

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 LPC661, typically less

than 0.04 pA, it is essential to have an excellent layout. For-

tunately, 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 accept-

ably 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 LPC661’s inputs and the

terminals of capacitors, diodes, conductors, resistors, relay

terminals, etc. connected to the op-amp’s inputs. See

Figure

4. 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 10

12

Ω, which is nor-

mally 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 ac-

tual performance. However, if a guard ring is held within

5 mV of the inputs, then even a resistance of 10

11

Ω would

cause only 0.05 pA of leakage current, or perhaps a minor

(2:1) degradation of the amplifier’s performance. See

Fig-

ures 5, 6, 7 for typical connections of guard rings for stan-

dard 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 8.

DS011227-7

FIGURE 2. Rx, Cx Improve Capacitive Load Tolerance

DS011227-24

FIGURE 3. Compensating for Large

Capacitive Loads with A Pull Up Resistor

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