LMC6035 Datasheet, PDF (16/29 Pages) National Semiconductor (TI) – Low Power 2.7V Single Supply CMOS Operational Amplifiers

LMC6035, LMC6035-Q1, LMC6036
SNOS875G – JANUARY 2000 – REVISED APRIL 2013
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Figure 49. 2 Pole, 1kHz Active, Bandpass Filter
DABP Component Selection Procedure
Component selection for the DABP filter is performed as follows:
1. First choose a center frequency (f_c). Figure 49 represents component values that were obtained from the
following computation for a center frequency of 1kHz. R2 = R3 = 1/(2 πf _cC) Given: f_c = 1kHz and C
(chosen) ^{= 6.8nF R2 = R3 = 1/(2π x 3kHz x 6.8nF) = 23.4kΩ}
– (Chosen standard value is 23.7kΩ )
2. Then compute R1 for a desired Q (f_c/BW) as follows: R1 = Q x R2. Choosing a Q of 100, R1 = 100
x 23.7kΩ = 2.37MΩ.
PRINTED-CIRCUIT-BOARD LAYOUT FOR HIGH-IMPEDANCE WORK
It is generally recognized that any circuit which must operate with < 1000pA of leakage current requires special
layout of the PC board. If one wishes to take advantage of the ultra-low bias current of the LMC6035/6, typically
< 0.04pA, 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 at times
appear acceptably low. Under conditions of high humidity, 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 LMC6035 or
LMC6036 inputs and the terminals of capacitors, diodes, conductors, resistors, relay terminals, etc. connected to
the op amp's inputs. See Figure 50. 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 10¹²Î©, which is normally considered a very large resistance, could leak 5pA
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 5mV of the inputs, then even a resistance
of 10¹¹Î© would cause only 0.05pA of leakage current, or perhaps a minor (2:1) degradation of the amplifier's
performance. See Figure 51(a) through Figure 51(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 51(d).
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