LM6035IBP datasheet(14/20 Pages) NSC

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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)

π x (desired cutoff freq.)

= 1 Farad/6.8nF x2

π x 300

Hz = 78.05k

R1 = ZxR1

(normalized) = 78.05k x (1/0.707) = 110.4k

Ω

(Standard value chosen for R1 is 110k

Ω )

R2 = ZxR2

(normalized) = 78.05k x (1/1.414) = 55.2k

Ω

(Standard value chosen for R1 is 54.9k

Ω )

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 andaQof 100.

1.2.3.1 DABP Component Selection Procedure

Component selection for the DABP filter is performed as

follows:

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/(2

π x 3kHz x 6.8nF) = 23.4kΩ

(Chosen standard value is 23.7k

Ω )

Then compute R1 for a desired Q (f

c/BW) as follows:

R1 = Q x R2.

ChoosingaQof 100,

R1 = 100 x

23.7k

Ω = 2.37MΩ.

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 10

Ω, 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 10

Ω 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

01283050

FIGURE 7. 2 Pole, 1kHz Active, Bandpass Filter

01283007

FIGURE 8. Example, using the LMC6036

of Guard Ring in P.C. Board Layout

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