LMC6035 Datasheet, PDF (14/20 Pages) National Semiconductor (TI) – Low Power 2.7V Single Supply CMOS Operational Amplifiers

1.0 Application Notes (Continued)
1.2.2.1 High-Pass Frequency Scaling Procedure
Choose a standard capacitor value and scale the imped-
ances in the circuit according to the desired cutoff frequency
(300Hz) as follows: C = C1 = C2 Z = 1 Farad/C_(chosen)
x 2π x (desired cutoff freq.) = 1 Farad/6.8nF x 2π x 300
Hz = 78.05k
R1 = Z x R1_(normalized) = 78.05k x (1/0.707) = 110.4kΩ
(Standard value chosen for R1 is 110kΩ )
R2 = Z x R2_(normalized) = 78.05k x (1/1.414) = 55.2kΩ
(Standard value chosen for R1 is 54.9kΩ )
1.2.3 Dual Amplifier Bandpass Filter
The dual amplifier bandpass (DABP) filter features the ability
to independently adjust f_c and Q. In most other bandpass
topologies, the f_c and Q adjustments interact with each other.
The DABP filter also offers both low sensitivity to component
values and high Qs. The following application of Figure 7,
provides a 1kHz center frequency and a Q of 100.
1.3 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 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
potential. For example, a PC board trace-to-pad resistance
of 10¹²â„¦, which is normally considered a very large resis-
tance, could leak 5pA if the trace were a 5V bus adjacent to
the pad of an input. This would cause a 100 times degrada-
tion from the amplifiers actual performance. However, if a
guard ring is held within 5mV of the inputs, then even a
resistance of 10¹¹â„¦ would cause only 0.05pA of leakage
current, or perhaps a minor (2:1) degradation of the amplifi-
er’s performance. 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.
01283050
FIGURE 7. 2 Pole, 1kHz Active, Bandpass Filter
1.2.3.1 DABP Component Selection Procedure
Component selection for the DABP filter is performed as
follows:
1. First choose a center frequency (f_c). Figure 7 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.
01283007
FIGURE 8. Example, using the LMC6036
of Guard Ring in P.C. Board Layout
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