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Power supply ripple measurement

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santoshgurral:
Hi

I am trying to capture the power supply ripple on my PCB with a 50ohm co-axial cable(just coax cable only no additional DC block). I have an R&S RTO2024 oscilloscope. My doubt is, in order to capture the ripple should I set the oscilloscope coupling to
50ohm DC or 1Meg DC or AC coupling?

Thanks
Santosh

Kleinstein:
To capture the ripple one likely needs to use AC coupling, unless the ripply is really high (e.g. more than some 5%).
Using the 50 Ohms DC mode can even be dangerous to the scope (the 50 Ohms termination) if the voltage is too high.

santoshgurral:
Thanks for the reply...but when I use an active differential probe RT-ZD10 from R&S to measure ripple scope mask out AC coupling option. By default, only 50ohm DC coupling mode is set when a differential probe is connected. Why is it so ? Differential probes can not be AC coupled? If not then how to measure ripple (very small ripple in range of 10mVs) using a differential probe?

Thanks
Santosh

Kleinstein:
The differential probe seems to have a 50 Ohms output. At this low impedance AC coupling would only work for high frequencies. Anyway with a differential probe AC coupling would be at the probe input. So there is no more need for AC coupling behind the differential probe.

Zero999:

--- Quote from: santoshgurral on October 22, 2019, 10:41:43 am ---Thanks for the reply...but when I use an active differential probe RT-ZD10 from R&S to measure ripple scope mask out AC coupling option. By default, only 50ohm DC coupling mode is set when a differential probe is connected. Why is it so ? Differential probes can not be AC coupled? If not then how to measure ripple (very small ripple in range of 10mVs) using a differential probe?

Thanks
Santosh

--- End quote ---
Because a differential probe is really an amplifier, with a high input impedance and will be designed to drive a 50Ohm load. As far as your circuit is concerned, it will only see the high impedance of the differential probe's inputs, not the 50 Ohm oscilloscope input.

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