How to determine the value of the ac couple capacitor?

[slchen]

AC couple capacitor

Dear all:

How to determine the value of the ac couple capacitor?

For example:
The ac couple capacitor is used in the input of OPAMP with Rin=1M ohm.
The amp. and freq. of input signal are 1uV and 1kHz, respectively.
What is the best value of the ac couple capacitor?

Thanks for your help!!!

slchen

 

[IanP]

Re: AC couple capacitor

Without going inot details and assuming that the output impedance of the signal source is close to 0Ω and that you don't want to have a voltage divider of significant ratio build of Zc(coupling capacitor impedance) and Ri, you can easily say that:

Zc=1/ωC « Ri

Here you need to make an assumption on how much smaller you need the Zc to be with relation to Ri .. for example, Zc = 0.1%Ri --> C≈ 1/(0.001 *Ri*2*Π*f) ..

Regards,
IanP

 

[eldo]

AC couple capacitor

The main consideration on chosing the coupling cap is the frequency response.

 

[pfd001]

AC couple capacitor

u must make sure that the amplitude is acceptable after the ac coupling. Ianp's equation is useful