DC blocking capacitors

[jgalak]

AC coupling of circuits is pretty common, with a simple series capacitor blocking the DC.  What I've failed to find so far is any guidance on how to pick the right capacitor. 

Mostly I see thing like "just use a 0.1uF" or something.

Is there any sort of formula or design method for determining the correct capacitor?

ETA:  not looking at a specific application, just a general understanding

[hamster_nz]

Yes, there is a formula.  The blocking capacitor and input and output impedance form a high pass network, and can be analyzed as such.

You need enough capacitance to pass the frequencies of interest, but not too much that it costs to much or that high frequency suffers.

See things like https://www.electronics-tutorials.ws/filter/filter_3.html for a full explanation.

[jgalak]

Ah!  That may be what I was missing.  I wasn't thinking in terms of filters, so didn't see where the R would come from.

So if the blocking capacitor is in the middle of a 50 Ohm path, I just treat it as an RC filter with R=50 and pick the appropriate C?  Or do I need to add the two 50 Ohm paths on either side and treat it as R=100 ?

[ferdieCX]

Yes, you have to add the internal resistance of the signal source at the input and the load resistance at the output.