Blocking on filter out?

[jeth]

Do folks here consider it necessary to put a blocking cap in front of a pot if the source is a Sallen Key 4th order high pass filter?

 

[bcarso]

If you want to avoid d.c. offsets a blocking cap is a good idea. If the amplifier used for realizing the SK filter had guaranteed zero d.c. offset it would not be needed. But that's not usually the case.

 

[jeth]

Thanks.. other query, I see circuits that put a blocking cap pre pot, but others with the cap after the pot output, is there any difference in the function of the cap dependant on the side of the pot it's on?

 

[JohnRoberts]

Cap needs to be pre pot to prevent scratchy noise if any DC error is present at the circuit node. However, bipolar opamps with significant input bias current will also cause a DC shift if that constant bias current is working into a varying pot resistance to ground. So 2 caps might be needed for complete silence.

Several of the modern high performance opamps internally compensate for input bias current so it can be quite small. If the input bias current is specified as +/- , there is a first order compensation. Bifet opamps are typically in the sub picoamp range so not significant.

JR