Author Topic: Power supply ripple measurement  (Read 1578 times)

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Offline santoshgurralTopic starter

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Power supply ripple measurement
« on: October 22, 2019, 09:23:19 am »
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
 

Online Kleinstein

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Re: Power supply ripple measurement
« Reply #1 on: October 22, 2019, 10:31:05 am »
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.
 

Offline santoshgurralTopic starter

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Re: Power supply ripple measurement
« Reply #2 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
 

Online Kleinstein

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Re: Power supply ripple measurement
« Reply #3 on: October 22, 2019, 12:00:20 pm »
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.
 

Online Zero999

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Re: Power supply ripple measurement
« Reply #4 on: October 22, 2019, 12:08:07 pm »
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
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.
 

Offline prasimix

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Re: Power supply ripple measurement
« Reply #5 on: October 22, 2019, 03:57:02 pm »
I'm using the method that is described in some HP/Agilent/Keysight power supply manual (have to find out which one) as described here. Actually, I it's a half way to complete setup since I have just one AC coupling capacitor + inline terminal instead of two (another has to go to Vout-), but that is better then nothing. Also please note that is a huge difference between "$1 per meter" coax and scope probe cable. Therefore scope probe is used (set to x1) and scope input now can be set to DC safely (because of inline capacitor) but gain has to be set to x2 to have accurate reading because of 50 Ohm inline terminator.


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