Help! Measuring DC across signal caps (newbie question)

Hi, I was wondering how to measure the DC across a signal cap, in order to determine if it can be removed - is it as simple as connecting a multimeter (set to, say, 200m DC) and touching the terminals of the cap?

In the following schematic, using the above method, the DC across C8 is 34mV and across C7 is 2.5mV - could either of these be removed? The cap values are C8=10uF/50V and C7=47uF/25V. Also, with the +ve multimeter lead touching the +ve cap leg, both of those voltages were reading negative - is that normal?

I'm planning to upgrade the opamps to, say, OPA2134 or 2228, would this change the DC across them significantly? Current opamps are JRC 4558 (yuck).

Thanks in advance for any help!

Connect the multimeter neg probe to ground and touch opamp output with positive probe, multimeter set to DC voltage mode (200mV DC as you said), now you know the DC offset at opamp output.

moley1390 said:

In the following schematic, using the above method, the DC across C8 is 34mV and across C7 is 2.5mV - could either of these be removed? The cap values are C8=10uF/50V and C7=47uF/25V. Also, with the +ve multimeter lead touching the +ve cap leg, both of those voltages were reading negative - is that normal?

I'm planning to upgrade the opamps to, say, OPA2134 or 2228, would this change the DC across them significantly? Current opamps are JRC 4558 (yuck).

Thanks in advance for any help!

Click to expand...

You could probably get away with removing C7, since any offset it's blocking will eventually get blocked by C1 anyway. C8 needs to stay, because it's blocking DC that will otherwise get onto P1b and make it noisy.

It's not unusual for the offset voltage from an opamp to be negqtive.

Changing to OPA2134 opamps will definitely change the DC conditions, probably for the better. You'll still need C8, though.

Peace,
Paul