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I have a DC-DC converter which converts 16V to 23V. I have an output E-Cap at the end of the converter. I need to measure the ripple voltage at the E-Cap. I am using a 300MHz scope.

I connect my probe to the positive terminal of E-cap and ground the scope ground with my board ground. I am using the scope in Auto Mode.

I have sent the horizontal scale at 100mV/div and Vertical scale at 2us/div.

I see the voltage signal continuously running and at certain moments when I see the peak values, I stop the reading. I get different values at different instances. How to measure the ripple voltage now?

And how will I set the trigger point if I dont know the max ripple voltage?

Can you please help me?

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    \$\begingroup\$ Put the input coupling in AC mode. Set the trigger mode to normal and adjust the trigger level until you get a steady sweep on the screen. \$\endgroup\$ – Peter Karlsen Jan 30 '19 at 12:40
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    \$\begingroup\$ The equivalent series resistance (ESR) of your capacitor is likely so small that probing must be done very carefully, with both probe and ground connections exactly at capacitor terminals. \$\endgroup\$ – glen_geek Jan 30 '19 at 15:09
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    \$\begingroup\$ I hope you mean "vertical scale at 100mV/div and horizontal scale at 2us/div." You should probably use a slower sweep rate. \$\endgroup\$ – Peter Bennett Jan 30 '19 at 16:42
  • \$\begingroup\$ When I put the mode in AC coupling and normal trigger(also tried auto mode) and measuring 23V at the output E-Cap, only the ripple voltage around 23V must come,right? But I am getting the DC offset of 23V also. Any mistake I am making? Or is this correct? \$\endgroup\$ – Freshman Jan 31 '19 at 5:25
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Power supply ripple is conventionally measured with the 20MHz bandwidth filter engaged.

As mentioned in the comments, probing technique is critical. You can't use a long ground clip and hope to get a true measurement. You have to keep your ground as short as possible. One of the best ways to do this is to wind bare wire around the barrel of the probe and tip and solder the connections across the output cap as shown below:

enter image description here

Then the ripple should be periodic, so you should be able to trigger on it if you AC couple the scope or add a DC offset so you can get to an appropriate scale. (See Peter K.'s comment above.)

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