Measure Ripple of a DC voltage with Scope - Page 1
Forum section: electronics / projects

author: raresvintea
author_rank: Contributor
timestamp: March 27, 2023, 03:25:31 pm
content: 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?

author: pqass
author_rank: Super Contributor
timestamp: March 27, 2023, 03:54:18 pm
content: Use AC coupling.Use 1x probe setting.Limit bandwidth to 20MHzKeep wires short; use the probe ground spring instead of the long ground lead, nor the spring-loaded hook probe cap.

author: srb1954
author_rank: Super Contributor
timestamp: March 27, 2023, 07:56:39 pm
content: Quote from: pqass on March 27, 2023, 03:54:18 pmDon'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.

author: 2N3055
author_rank: Super Contributor
timestamp: March 27, 2023, 08:42:12 pm
content: Quote from: raresvintea on March 27, 2023, 03:25:31 pmYou 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.

author: ogden
author_rank: Super Contributor
timestamp: March 27, 2023, 09:01:09 pm
content: Quote from: srb1954 on March 27, 2023, 07:56:39 pmRight. 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.pdfhttps://www.ti.com/lit/ml/slyy136/slyy136.pdf?ts=1679908498178

author: pqass
author_rank: Super Contributor
timestamp: March 27, 2023, 10:02:37 pm
content: Dave gets into it in video #594.The 20MHz bandwidth limit is so the value is comparable.Also can use a "poor man's differential probe"; subtracting 1 'scope channel from another channel and not using the ground leads.

author: jonpaul
author_rank: Super Contributor
timestamp: March 27, 2023, 10:13:46 pm
content: in decades of SMPS work,1. LINE trig to see 50/60 and mains harmonics eg 120 Hz ripple2. INT trig on SMPS switching rate eg 25 kHz....500 kHz to see HF ripple.3. Take care of return CM noise, and probe DIRECTLY across output bypass ELKO or HF bypass cap.4. We used 10x probes no issues as the noise level is generally 5..500 mv p-p5. Instead of 1x, we just take a RG174/U coax and BNC at one end, termination of coax direct to the PSU output via a 100 nF series cap.6. rare to have a lot of noise > 1 MHz thus probe and scope BW is seldom at issue.Jon

author: metebalci
author_rank: Frequent Contributor
timestamp: March 28, 2023, 01:04:32 pm
content: I checked the InfiniiVision (6000 series) Output Ripple Analysis, it sets the probe (I have 10x) coupling to AC, bandwidth to 200MHz, trigger to the same channel rising and trigger level to a higher than mean level. I dont mean it is the way it should be done, I wondered how the application sets it up.

author: David Hess
author_rank: Super Contributor
timestamp: March 29, 2023, 11:14:07 pm
content: The duration of the record determines the low frequency cutoff, so for instance if you wanted to measure noise down to 0.1 Hz, then a 10 second record is required.

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 30, 2023, 05:35:38 am
content: Don'tforget that typically digital scopes (at least cheap ones) aren't fine for low-ripple or low-noise measurements because of a very low (8-10 bit) ADC resolution.

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2023, 01:17:31 am
content: Quote from: Vovk_Z on March 30, 2023, 05:35:38 amAnd modern scopes have high flicker noise making low frequency noise measurements questionable. Old oscilloscopes with JFET inputs are not good for this either, but modern MOSFET inputs are worse.

author: jonpaul
author_rank: Super Contributor
timestamp: April 01, 2023, 02:02:22 am
content: in DC PS PARD measurement, flicker and low F noise is seldom in questionbesides the mains frequency and harmonic of the PFC/ rectifiér on the raw bus, it's Line / load variations, référence noise, loop feedback noise gain, etc.Suggest you référence, the excellent Hewlett Packard 1980s application note,on power supply measurements and the Tektronix Circuits Concept books, on power supply.Bon SoiréeJon

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 01, 2023, 02:34:49 am
content: Quote from: raresvintea on March 27, 2023, 03:25:31 pmQuote from: ogden on March 27, 2023, 09:01:09 pmNot a good idea to switch your scope to 50ohm input when measuring a DC supply with a 1:1 probe unless the DC voltage is low enough.

author: jonpaul
author_rank: Super Contributor
timestamp: April 01, 2023, 10:21:17 pm
content: a few more notes on probes and scope setup1. 10x probe scope 1 M Zin SET GRD >>connect probe to DC bus of DUTSwitch to AC coupling (precharge scope AC coupling capacitor)1X probe :scope input 1 MegSet scope at max V/dig, same sequence: COUP set to GRD, attach to DC busSet coupling to AC50 Ohm scope : Do not use on PSU. For Zo probe and wide band use onlyhope this clarification assists,Jon

author: David Hess
author_rank: Super Contributor
timestamp: April 07, 2023, 01:23:24 am
content: Quote from: jonpaul on April 01, 2023, 10:21:17 pmThey are rare however some old oscilloscopes had 50 ohm inputs which supported AC coupling and could be used to make this sort of measurement, but a high impedance input is still better.

author: ogden
author_rank: Super Contributor
timestamp: April 12, 2023, 08:07:13 pm
content: Quote from: David Hess on April 07, 2023, 01:23:24 amAnalog Devices experts strongly disagree. Keyword: "blocking capacitor":https://www.analog.com/media/en/technical-documentation/application-notes/an101f.pdfhttps://www.analog.com/media/en/technical-documentation/application-notes/an-1144.pdfEMC [conducted emissions] compliance of power supplies usually is measured using instrument having 50 Ohm input as well.one of many:https://www.minicircuits.com/WebStore/dashboard.html?model=BLK-89-S%2B

author: Andree Henkel
author_rank: Regular Contributor
timestamp: April 13, 2023, 12:20:33 pm
content: 1:1 Probe with 38MHz bandwidth; from Rohhde & Schwarz, however not cheap:https://www.batronix.com/versand/messtechnik/tastkoepfe/Rohde-Schwarz-RT-ZP1X.html