Application Hints
(Continued)
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 6d.
DS008767-5
FIGURE 3. Rx, Cx Improve Capacitive Load Tolerance
Capacitive load driving capability is enhanced by using a pull
up resistor to V
+
(Figure
4).
Typically a pull up resistor con-
ducting 500 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).
DS008767-16
FIGURE 5. Example, using the LMC660,
of Guard Ring in P.C. Board Layout
DS008767-23
FIGURE 4. Compensating for Large Capacitive 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 LMC662, 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 LMC660’s inputs and the
terminals of capacitors, diodes, conductors, resistors, relay
terminals, etc. connected to the op-amp’s inputs. See
Figure
5.
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 LMC660’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
Figure
6a, Figure 6b, Figure 6c
for typical connections of guard
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