I am measuring the DC voltage ripple at 400V provided from CAEN DT5534EM by connecting the output to the oscilloscope (AC coupling) through a 1000:1 probe. From the picture below, the ripple is around 0.3mV. However, I am unsure should I multiply by 1000 to get the "real" ripple voltage, i.e. \$400\pm0.3V\$? If so, this will contradict the manual as the module is specified to have a typical ripple Vpp of \$\leq15mV\$. Therefore, my question is that will the ripple be scaled according to the fraction of the probe? enter image description here

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    \$\begingroup\$ Your oscilloscope has a probe setting such that it will take care of that display scaling for you. This is should have been among the first things three things you learn when using an oscilloscope. You can find the setting or if you really don't know how, you can figure out what it is set to by setting to DC coupling and see if it measures 400VDC, or if it measures 40V, 4V, or 0.4V. \$\endgroup\$
    – DKNguyen
    Jul 7, 2021 at 15:51
  • \$\begingroup\$ There is a probe setting for each channel (since you can use different probes for each channel) so the setting is probably hidden behind the channel selection buttons. Well, you have Tek scope so I know it is. \$\endgroup\$
    – DKNguyen
    Jul 7, 2021 at 15:59
  • \$\begingroup\$ Ripple differs from the noise that appears in your photo. Ripple shows up at fixed frequencies (usually harmonically related). The item you're measuring is likely a switcher-type whose ripple is 10's or 100's of kilohertz. Furthermore, your 1000:1 probe may have a limited bandwidth that is lower than your oscilloscope bandwidth. \$\endgroup\$
    – glen_geek
    Jul 7, 2021 at 16:38

1 Answer 1


You have the time base set to 10ns/division. That is not the ripple you are seeing, just high-frequency noise.

Ripple would normally be interpreted as being at low frequencies, 50Hz-120Hz for mains AC linear supplies or maybe a few hundred kHz for switching power supplies.

Also, you won't be able to meaningfully measure that ripple with a 1000:1 probe. The noise and sensitivity of the input stage of the scope will be in many 10's of microvolts, multiply that by 1000 and the noise of the scope will be effectively tens of millivolts.

The usual way to measure ripple on a supply is to select AC coupling at the input of the scope (as you seem to have done) and select a bandwidth limit for the input (most scopes can limit their input bandwidth to something like 20MHz).

The AC coupling will reject the large DC voltage of the power supply so the sensitivity of the scope can be set to something that can measure a few millivolts.

With lower voltage supplies I would use a 1:1 scope probe but with a 400V supply you need to careful about damaging the scope itself with excessive input voltage. I would expect that a 10:1 probe would be safe.


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