A capacitor in series with the signal path acts as a high pass filter,

blocking any DC in the signal from passing through while letting

frequencies of interest an easy path.

The value of the capacitor depends upon the input impedance of

the circuit that follows it, and the frequencies that you want to

allow to pass. Normally, we consider the "cutoff" frequency to

be that where the output is reduced by 3 dB (0.707). At lower

frequencies, the output is reduced even more, proportional

to the frequency: Every halving of the frequency below cutoff

results in a halving of the output level (-6 dB / octave).

The formula for the cutoff is F3 = 1 / (2 * pi * R * C)

where F3 is the cutoff frequency in Hertz,

R is the following stage input impedance

in ohms, and C is the capacitance in *Farads*.

You can rearrange this to C = 1 / (2 * pi * F3 * C)

when you are trying to pick a C from design

considerations. In your example case, if the

following stage happens to have an input

impedance of 100K the F3 would be

F3 = 1 / (2 * pi * 100000 * 0.02x10^-6) = 79.6 Hz.

Hope this helps!

Bob Masta

dqatechATdaqartaDOTcom

D A Q A R T A

Data AcQuisition And Real-Time Analysis

www.daqarta.com