How to measure ripple using DVM

[Cimteker]

Hi,

What's the best way to measure ripple using DVM (PXI-4072)? My customer's limits are Vp-p so I think digitizing it would be the best way, but I also think that measuring true RMS would also be a good way to measure it. With digitizing the signal there's some post-processing required. Would calculating just V peak to peak in acquired signal be sufficient?

Thanks for any comments,

CT.

[Henrik_Volkers]

I would use AC measurement and if SMPS are involved measure with highes possible bandwidth to get the spikes. Vpp ripple can be a very nasty value, a well defined setup is needed to get reproduceable results.

(A nice soucre: AN118 from www.linear.com) 

[jschwartz]

Hi CT,

To measure ripple, you should be able to use the digitzer mode of the 4072 to measure signals up to 1.8 MS/s.  As long as your ripple voltage is not a higher frequency than that you will be able measure it.  The Developer Zone article linked here gives some more information about DC power  supplies and some considerations for measureing ripple.

[Cimteker]

Thank you Henrik and Jim for your help.

One more question. Should I use AC coupling for waveform acquisition, to filter out any DC that will be present (as it is a DC power supply). Or using AC coupling can alter my signal so I need to use DC coupling since the calculation Vp-p=V(Amp Max)-V(Amp  Min) will automatically remove DC component. The only concern is using 100 VDC range (for 12V PS) but to really measure mV.

Thanks again,

CT

[jschwartz]

Hi CT,

Using AC coupling does nothing more than filter out DC components of the signal with what amounts to a capacitor.  You should be fine using AC coupling for your ripple measurement.