the user must not ignore the surface leakage of the PC
board, even though it may at times appear acceptably low.
Under conditions of high humidity, dust or contamination, the
surface leakage will be appreciable.
1.0 Application Notes (Continued)
To minimize the effect of any surface leakage, lay out a ring
of foil completely surrounding the LMC6035 or LMC6036 in-
puts and the terminals of capacitors, diodes, conductors, re-
sistors, relay terminals, etc. connected to the op amp’s in-
puts. See Figure 8. 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 po-
tential. For example, a PC board trace-to-pad resistance of
1012Ω, 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 amplifiers actual performance. However, if a guard ring is
held within 5 mV of the inputs, then even a resistance of
1011Ω would cause only 0.05 pA of leakage current, or per-
haps a minor (2:1) degradation of the amplifier’s perfor-
mance. See Figure 9a, b, c 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 9 d.
DS012830-50
FIGURE 7. 2 Pole, 1 kHz Active, Bandpass Filter
1.2.3.1 DABP Component Selection Procedure
Component selection for the DABP filter is performed as fol-
lows:
1. First choose a center frequency (fc). Figure 7 represents
component values that were obtained from the following
computation for a center frequency of 1 kHz.
=
=
R2 R3 1/(2 πf C)
c
=
=
6.8 nF
Given: fc 1 kHz and C
(chosen)
=
=
=
R2 R3 1/(2π x 3 kHz x 6.8 nF) 23.4 kΩ
(Chosen standard value is 23.7 kΩ )
2. Then compute R1 for a desired Q (fc/BW) as follows:
=
R1 Q x R2.
Choosing a Q of 100,
=
=
R1 100 x 23.7 kΩ 2.37 MΩ.
1.3 PRINTED-CIRCUIT-BOARD LAYOUT
FOR HIGH-IMPEDANCE WORK
It is generally recognized that any circuit which must operate
<
with 1000 pA of leakage current requires special layout of
the PC board. If one wishes to take advantage of the
DS012830-7
FIGURE 8. Example, using the LMC6036
of Guard Ring in P.C. Board Layout
<
ultra-low bias current of the LMC6035/6, typically 0.04 pA,
it is essential to have an excellent layout. Fortunately, the
techniques for obtaining low leakages are quite simple. First,
13
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