Best way to choose filter capacitor

Thread Starter


Joined Jun 24, 2009
Hi All,

I'm trying to measure the current drawn by a DC motor using a shunt. This particular motor has onboard power electronics that allow you to control it's speed. When I measure the voltage across the shunt I see it's quite a noisy signal. See Bank 1 pump 8 only running.jpg for a scope trace picture.

To get a more steady signal my plan is to put a filter capacitor across the power lines to the motor. What is the way to choose the most suitable value for the cap? And what type of caps work best in this application (e.g. electrolytics, greencaps, ceramics). Note that I do not have a spectrum analyzer handy so haven't checked what the signal looks like in the freq. domain yet.

Thanks for any help,


Joined Jul 17, 2007
It would be a lot easier to just use a low-pass RC filter across your shunt. The capacitors would be much smaller.

Did you see my reply to your other thread?