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Measure Ripple of a DC voltage with Scope

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raresvintea:
Hello, i have a debate related to Ripple of a DC voltage. My method is to set the probe to AC and select from the scope measurements Vp-p and zoom in as much as i can an then capture several frames and that's it. I observed today that if i go to us and a 10mV/div i see a value, but if i take a larger time base i get a diffrent value.
What's a rule of thumb or how do you measure?

pqass:
Use AC coupling.
Use 1x probe setting.
Limit bandwidth to 20MHz
Keep wires short; use the probe ground spring instead of the long ground lead, nor the spring-loaded hook probe cap.

srb1954:

--- Quote from: pqass on March 27, 2023, 03:54:18 pm ---Use AC coupling.
Use 1x probe setting.
Limit bandwidth to 20MHz
Keep wires short; use the probe ground spring instead of the long ground lead, nor the spring-loaded hook probe cap.



--- End quote ---
Don't use a 1x probe. This will quite often have a bandwidth of less than 10MHz so you won't see so much of the higher frequency components present in switching supply ripple waveforms. It is best to use a 10x probe to preserve the full bandwidth of the scope and get a more sensitive scope to compensate.

Limiting the scope bandwidth to 20MHz will also hide some of the higher speed transient signals that might be on the output of a switching power supply. The 20MHz limit comes from decades-old standards mainly intended for the evaluation of linear supplies and shouldn't be used with modern switching supplies. However, many manufacturers still use this old standard to make the noise levels of their supplies look more favourable so the only time it is valid to use the 20MHz limit is when comparing compliance of a power supply with the manufacturer's published spec.

The advice on scope probe grounding is good for general circuits but with switching power supplies you may get common-mode currents from the switcher operation traversing through the scope ground lead producing aberrations on the displayed waveform. Better results can be obtained with a wide bandwidth differential probe which will minimise interference from common-mode currents.

2N3055:

--- Quote from: raresvintea on March 27, 2023, 03:25:31 pm ---Hello, i have a debate related to Ripple of a DC voltage. My method is to set the probe to AC and select from the scope measurements Vp-p and zoom in as much as i can an then capture several frames and that's it. I observed today that if i go to us and a 10mV/div i see a value, but if i take a larger time base i get a diffrent value.
What's a rule of thumb or how do you measure?

--- End quote ---

You need to go as low as to capture at least several periods of mains frequency (50/60Hz). Like others said you need to engage 20Mhz frequency limit. That should get you covered.

ogden:

--- Quote from: srb1954 on March 27, 2023, 07:56:39 pm ---Don't use a 1x probe. This will quite often have a bandwidth of less than 10MHz so you won't see so much of the higher frequency components present in switching supply ripple waveforms.

--- End quote ---

Right. Use 50 Ohm input instead. Further reading, few of many:

http://www.how2power.com/pdf_view.php?url=/newsletters/1712/articles/H2PToday1712_design_TexasInstruments_Part%201.pdf

https://www.ti.com/lit/ml/slyy136/slyy136.pdf?ts=1679908498178

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