Looking for a Probe to Use for 5V DC Power Supply Ripple Measurement - Page 1
Forum section: electronics / testgear

author: bendras
author_rank: Contributor
timestamp: May 11, 2018, 09:33:20 pm
content: I would like to measure voltage ripple of a5VDC-DC buck converter with at least1mVPeak-to-Peak precision. As far as I understand the best way to do it is with some kind of high impedance differential probe (please correct me if I am wrong). Could someone recommend a differential probe suitable for this type of measurement?

author: Keysight DanielBogdanoff
author_rank: Unknown
timestamp: May 11, 2018, 09:43:56 pm
content: Depending on budget and oscilloscope, a lot of people are using power rail probes for this type of measurement. Something like the our N7020A:https://www.keysight.com/en/pd-2471132-pn-N7020A/power-rail-probe?cc=US&lc=engIt was designed to handle offsets higher than 5V, but the power rail probe has been the go-to for a couple years (at least on the enterprise side).Not sure how helpful it will be for your specific application, but this blog post also talks about making single ended measurements with a differential probe:https://community.keysight.com/community/keysight-blogs/oscilloscopes/blog/2018/04/17/use-your-differential-probe-where-you-never-thought-possible

author: nctnico
author_rank: Super Contributor
timestamp: May 11, 2018, 10:14:25 pm
content: The easiest way is to solder a piece of coax to the board and use a DC blocker and 50 Ohm termination at the oscilloscope.

author: bendras
author_rank: Contributor
timestamp: May 12, 2018, 11:34:13 am
content: Quote from: Keysight DanielBogdanoff on May 11, 2018, 09:43:56 pmThe suggested probe seems to be suitable for the task but as far as I can tell it is only compatible with Keysight oscilloscopes. Does anybody know of a similar probe compatible with bog standard oscilloscopes or at least the ones with the "TekProbe Level II" interface?Quote from: nctnico on May 11, 2018, 10:14:25 pmI have investigated this technique but at 5V it seems to be loading the circuit with a few tens of mA of current draw. That is the reason why I am looking for a way to do differential measurements.

author: precaud
author_rank: Frequent Contributor
timestamp: May 12, 2018, 12:30:58 pm
content: "Ripple" suggests low-freq AC, so why not just use a 1:1 probe into a 1M Ohm AC coupled scope input? The probe's capacitive loading won't be a problem here.

author: _Wim_
author_rank: Super Contributor
timestamp: May 12, 2018, 12:41:30 pm
content: You could also go for a preamp like a stanford resaerch SR560 (http://www.thinksrs.com/products/sr560.html) These have a differential input, high input impedance, ac coupling together with very high gain...These pop-up regurarly on Ebay for about 750€, not cheap, but very versatile in many applications, and also universal usable on any scope...

author: Cerebus
author_rank: Super Contributor
timestamp: May 12, 2018, 12:44:26 pm
content: Quote from: precaud on May 12, 2018, 12:30:58 pmYou need a differential probe or all the common mode noise that's generally slopping about will completely swamp the noise that's actually being generated by the power supply and that's all you'll see.

author: Cerebus
author_rank: Super Contributor
timestamp: May 12, 2018, 12:57:28 pm
content: Quote from: Keysight DanielBogdanoff on May 11, 2018, 09:43:56 pmThe N2792A (200MHz ±20V differential ±60V common mode) is probably a better fit than the 2GHz N7020A (which at a guess is probably $2k a pop). That or the N2791A (25 MHz ±70V differential @ 10:1 or ±700V differential @ 100:1, ±700V common mode).Trying to get him to spend more money Daniel?

author: bendras
author_rank: Contributor
timestamp: May 12, 2018, 01:46:46 pm
content: Quote from: _Wim_ on May 12, 2018, 12:41:30 pmThis particular preamp seems to be limited to 3Vpp inputs signals, hence measuring 5V DC rail would be out of spec. (please correct my if I am wrong). Also as far as I understand its 1MHz bandwidth would not allow measurement of high frequency ripple.Quote from: Cerebus on May 12, 2018, 12:44:26 pmThis is exactly the reason why I am looking for a way to do a differential measurement.Quote from: Cerebus on May 12, 2018, 12:57:28 pmThe mentioned N2792A probe has 10:1 attenuation (as far as I understand this would make 1mVpp look like 0.1mVpp on the scope, hence waste scope's vertical resolution). Also this probe seems to have 6mVrms noise floor, which seems far too high for making 1mVpp ripple measurements.

author: precaud
author_rank: Frequent Contributor
timestamp: May 12, 2018, 02:11:57 pm
content: Quote from: bendras on May 12, 2018, 01:46:46 pmOK. How about a Tek AM502 diff preamp? PAR made a similar standalone unit, the 113. Signal Recovery has the 5113, that will be in the same price range used as the SR560, though.

author: bendras
author_rank: Contributor
timestamp: May 12, 2018, 04:14:52 pm
content: Quote from: precaud on May 12, 2018, 02:11:57 pmAll of these preamplifiers seem to have maximum bandwidth of 1MHz. As far as I know the standard for measuring power supplies is 20MHz. Do you know of any models with 20MHz or higher bandwidth?

author: bendras
author_rank: Contributor
timestamp: May 12, 2018, 05:09:51 pm
content: The HP Hewlett Packard 1141A seems like a good candidate, but I am a bit confused about the specification. The datasheethttps://www.testequipmentdepot.com/agilent/pdf/1141a.pdfon page 54 says that without attenuators (i.e. in the 1:1 mode) it can handle up to +/-20V in "Common-mode Operating Range DC" and up to +/-300mV peak in "Differential Input Range". Does this mean that it is suitable for measuring 20V DC rails with 300mV peak-to-peak ripple on them?

author: nctnico
author_rank: Super Contributor
timestamp: May 12, 2018, 05:12:59 pm
content: Quote from: bendras on May 12, 2018, 11:34:13 amNo it doesn't. The DC block (just a capacitor in series) is placed before the 50 Ohm termination so no DC current flows into the oscilloscope.

author: David Hess
author_rank: Super Contributor
timestamp: May 12, 2018, 05:45:45 pm
content: Quote from: bendras on May 12, 2018, 04:14:52 pmThe Tektronix 1MHz AM502 was the standalone implementation of the Tektronix 5A22/7A22 for the 5000/7000 series mainframe oscilloscopes. It is great for these kinds of measurements but of course only up to 1MHz.For higher bandwidth in the 7000 series, Tektronix had the 100MHz 7A13 which provides 1mV/div sensitivity over a common mode range of +/-10 volts and can be used as a differential comparator similar to Keysight's power rail probes or as a differential input amplifier. The problem with the 7A13 (besides being old) is that its switchable bandwidth limit is 5 MHz (1) although this could be modified by changing a capacitor. My solution is to connect my DSO to the vertical output from the 7000 mainframe and implement the 20MHz bandwidth limit there; so effectively the 7A13 and 7000 mainframe become a 100MHz 1mV/div differential front end for my DSO.Preamble made a standalone version of the 7A13. They were bought be LeCroy who still producestheir productsbut that would be an expensive way to go although cheap if you design power supplies for a living. You might be able to find a used Preamble instrument on Ebay. Some external differential probes might be suitable also. ThePintek DP-60HScould be used with external AC coupling capacitors but it only has a 15 MHz bandwidth.The easiest way and what I would try first, which also happens to be recommended for ATX power supply testing, is using a pair of x1 probes and a 2 channel oscilloscope configured for differential (invert and add) input operation. The problem is gain matching between the input channels which some DSOs do not handle well and increased quantization noise. Analog oscilloscopes and some very rare old DSOs can use their analog "variable" vertical controls to match the gain of the channels for pretty good performance when used like this while avoiding an increase in quantization noise.(1) I think the reason for a 5MHz bandwidth limit instead of 20MHz on the Tektronix 7A13 was that 20MHz had not be standardized for power supply noise measurement when it was designed and the 7A13 is pretty noisy due to its complexity so 5MHz was a good choice for operation at its maximum sensitivity. The specified noise of the 7A13 is less than 200uVrms and I measured mine at about 100uVrms which is about 10 times what a simple but good single ended 1mV/div input with a common mode range of +/-10mV has. The difference here is that the 7A13 is differential which doubles the input noise and has a cascode to support operation over +/-10V or 1000 times what a normal oscilloscope input can handle. When used as a differential comparator, it is like having an oscilloscope position control which operates over 20,000 divisions.

author: precaud
author_rank: Frequent Contributor
timestamp: May 12, 2018, 05:56:51 pm
content: Quote from: bendras on May 12, 2018, 04:14:52 pmNot offhand. I think you're in diff probe territory to get useful CMRR at that bandwidth.

author: _Wim_
author_rank: Super Contributor
timestamp: May 12, 2018, 06:07:38 pm
content: Quote from: bendras on May 12, 2018, 01:46:46 pmAC coupled these can withstand 100Vdc according to the user manual. Yes, bandwidth is limited, but high bandwidth AND low noise is not possible. For me "ripple" seemed like a low frequency thing, I was not aware you wanted to measure at least up to 20Mhz.As suggested, the probes designed for the job are off course ideal, but expensive and typically dedicated to a specific scope brand.Edit: quote formatting

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 12, 2018, 06:24:59 pm
content: Since only AC is important, salvage the coupling transformer from an Ethernet device or DSL modem, connect the primary to the voltage rail with a DC blocking capacitor, and connect the secondary to the scope with a parallel resistor selected to get as flat a frequency response as possible.

author: _Wim_
author_rank: Super Contributor
timestamp: May 12, 2018, 06:57:02 pm
content: Quote from: Cerebus on May 12, 2018, 12:57:28 pmThe N2792A and N2791A have much higher noise (6mVrms and 50mVrms) making them not usefull for this kind of application. The N7020A suggested by Daniel achieves 90uVrms combined with an S-series scope, a top solution if money is not an issue...

author: Kosmic
author_rank: Super Contributor
timestamp: May 12, 2018, 07:11:03 pm
content: There's a interesting application note from linear talking about DC DC converter noise and how to measure it.http://www.analog.com/media/en/technical-documentation/application-notes/an101f.pdfAt page 10 you will find : APPENDIX C Probing Technique for Sub-Millivolt, Wideband SignalIntegrityThere is also a short YouTube videohttps://youtu.be/WxhjLIu-vPg

author: David Hess
author_rank: Super Contributor
timestamp: May 12, 2018, 07:59:18 pm
content: Quote from: precaud on May 12, 2018, 05:56:51 pmIn my experience only a moderate amount of common mode rejection is required for these measurements. The old Tektronix 7A13 achieve a minimum of 300:1 or 50dB at 20MHz from 1mV/div to 20mV/div where despite no input dividers being used, it still allows for a +/-10 volt DC input range. Since it supports AC coupled inputs, higher voltage measurements are feasible and indeed, switching between AC and DC coupling when measuring the ripple on a 5 volt supply makes no difference unless something is broken.I have made the same measurement using analog and analog input DSOs in add and invert mode as long as the common mode rejection is adjusted with the variable volts/div control. The reason I do not do this all the time is simply because none of my oscilloscopes have the native 1mV/div sensitivity that the 7A13 provides although the 7A13 is pretty noisy if its full bandwidth is used.Ideally what is needed is a low voltage 20MHz differential probe with a wide input common mode range or AC coupling but offhand I do not know of any. The old Tektronix P6046 sort of meets these requirements since it supports AC coupling.

author: precaud
author_rank: Frequent Contributor
timestamp: May 12, 2018, 09:40:53 pm
content: Quote from: David Hess on May 12, 2018, 07:59:18 pmI can't recall ever making a realtime noise diff measurement above 1MHz but what you're saying makes sense.Quote from: NiHaoMike on May 12, 2018, 06:24:59 pmI played around with that once, those xfmrs roll off the low end pretty early, the one I tried was only good down to 1kHz or so.

author: MarkL
author_rank: Unknown
timestamp: May 12, 2018, 09:49:18 pm
content: Another possibility is the Lecroy DA1855A differential amplifier (which is a distant offspring of the 7A13 that David Hess is eluding to).It has a switchable gain of x1 and x10, and a BW of 100MHz. There are selectable filters to reduce the BW to 20MHz, 1MHz, or 100kHz. CM range is +/-15.5V (x1) or +/-155V (x10).They can be found on ebay for exorbitant prices, or you can hang out and wait for a deal. There was one that just sold for USD$330 with a beat up front panel. More commonly they can be had for around $600 - $800. It's a good tool to have if your budget allows.Additional data:http://teledynelecroy.com/probes/differential-amplifiers/da1855a

author: KrudyZ
author_rank: Frequent Contributor
timestamp: May 12, 2018, 10:11:03 pm
content: Quote from: bendras on May 12, 2018, 11:34:13 amYou could use a Tektronix P6247 or P6248 if you're looking For a TekProbe interface.Common mode range of +-7VTo zoom in on the ripple, apply a constant 5V from a reference to the negative input of the probe.Make sure the ground side of the reference is hooked up to the ground of the cap on the DUT supply that you are measuring your ripple across. Add a local bypass cap for the reference and either twist your leads or use coax for the feed.In 1:1 mode these probes have a 850 mV range and they are FAST (> 1GHz). This setup will give you DC readings and you can switch the probe to 10:1 to look at the turn-on and turn-off of the regulator.The P6246 is a slower model at 400 MHz and there is also a P6250 with higher voltage ratings, but it's less common to find and does not have a 1:1 range.

author: bendras
author_rank: Contributor
timestamp: May 12, 2018, 10:33:37 pm
content: Quote from: MarkL on May 12, 2018, 09:49:18 pmHaving looked through the specs. of this device it seems that it would not only meet my current needs but that it would also be a safe bet for the futureQuote from: KrudyZ on May 12, 2018, 10:11:03 pmThe Tektronix P6246 definitely seems suitable. Having said that, since I do not have a compatible scope at the moment it would workout cheaper for me to get the Lecroy DA1855A.

author: nctnico
author_rank: Super Contributor
timestamp: May 12, 2018, 10:50:08 pm
content: Quote from: KrudyZ on May 12, 2018, 10:11:03 pmI'm wondering if this isn't an overly obfustigated way of measuring ripple. One of the problems I see is that the 5V offset must be ripple free as well over a large frequency band. Just look at the Jim Williams appnote linked to above which uses a DC blocking capacitor and a coax cable soldered directly to the point of interest.

author: David Hess
author_rank: Super Contributor
timestamp: May 13, 2018, 02:39:30 am
content: Quote from: nctnico on May 12, 2018, 10:50:08 pmThe advantage of this method, differential comparison, is that the low frequency response extends to DC. Most oscilloscopes support this to a limited extent using their position control and to a greater extent using an offset adjustment if available. The previously mentioned 7A13 and the differential amplifiers from Preamble (now LeCroy) support this to an extreme extent and so do the Keysight power measurement probes.Noise from the comparison voltage is not a problem; the reference it is just too easy to filter and low frequency drift is not an issue at 1mV/div.The problem with this method is that the difference in potential and resulting common mode current between the DUT (device under test) ground and oscilloscope grounds can corrupt the measurement. Jim Williams mentioned this in his application note:GROUND POTENTIAL DIFFERENCES PROMOTE OUTPUT HIGH FREQUENCY CONTENT AND CORRUPT MEASUREMENT.Using a continuous coaxial connection as Jim Williams recommends definitely helps and is sufficient in many applications. I do it by soldering a short coaxial BNC pigtail to the test point and then using a BNC to probe tip adapter to connect a x1 probe. Then the oscilloscope's AC coupling capacitor can remove the DC offset or a differential comparison can be made if the offset range is sufficient.A differential measurement removes the ground imbalance but coaxial connections are still required (or possibly twisted pair as KrudyZ suggested) for best performance. The ATX power supply specifications do not even bother recommending a single ended coaxial test setup.

author: mtdoc
author_rank: Super Contributor
timestamp: May 13, 2018, 04:25:58 am
content: If you’re on a tight budget, why not go old school and pick up a Tek p6046?

author: BravoV
author_rank: Super Contributor
timestamp: May 13, 2018, 04:47:42 am
content: Quote from: mtdoc on May 13, 2018, 04:25:58 am+1 on P6046 if the OP only on hobbyist budget.Its also the example probe in the official standard ATX document.

author: David Hess
author_rank: Super Contributor
timestamp: May 13, 2018, 09:45:10 pm
content: Quote from: BravoV on May 13, 2018, 04:47:42 amIf you can find one which is undamaged; the input JFETs are unprotected. It is just barely possible to replace them but calibration is difficult.A modern implementation of the P6046 is one of the projects I have considered but so far I have managed to get by using other means.

author: icpart
author_rank: Regular Contributor
timestamp: May 13, 2018, 10:16:14 pm
content: Another possibility is to use very low noise preamplifier for oscilloscope measurements of noise and ripples of power supplies. Just look these another great AN83 from Jim Williamshttp://www.analog.com/media/en/technical-documentation/application-notes/an83f.pdfAlso I find another great improved version of that amplifier int that ANhttp://www.analog.com/media/en/technical-documentation/application-notes/an159fa.pdfI also have two p6046 probes but I am not tested how they are perform for low noise measurements. I am also very interested in measurements of parameters of DC/DC converters.P.S.WOW That AN159 is great source for power supplies measurements and characterization.Also another possible alternative is that new differential amplifier from Picotest if you have to spend 1500$https://www.picotest.com/products_J2113A.htmlorhttps://www.picotest.com/products_J2180A.htmlPicotest have a great products for power supplies measurements but there instruments are not very cheap.

author: David Hess
author_rank: Super Contributor
timestamp: May 13, 2018, 11:52:32 pm
content: I wonder what the common mode rejection versus frequency and isolation of that J2113A actually is; Picotest does not say.

author: precaud
author_rank: Frequent Contributor
timestamp: May 14, 2018, 04:57:58 am
content: Quote from: David Hess on May 13, 2018, 11:52:32 pmIt'll cost ya a cool fifteen hundred to find out...I wonder why their J2102A transformer isolates so poorly below 1kHz.

author: Cerebus
author_rank: Super Contributor
timestamp: May 14, 2018, 12:10:41 pm
content: Quote from: precaud on May 14, 2018, 04:57:58 amI'd presume that the core permeability was frequency dependent and the losses increase (or at the extreme the core just saturates) at lower frequencies. As I say though, a presumption, as I don't know what material the core is.

author: tggzzz
author_rank: Super Contributor
timestamp: May 14, 2018, 01:04:42 pm
content: If you aren't concerned with DC and are prepared to try a little homebrewing, you might like to experiment with Doug Smith's 1-500MHz balanced probe.http://emcesd.com/pdf/cd94scr.pdfI haven't tried it, so I have no valid opinion as to whether it might be relevant.

author: Kosmic
author_rank: Super Contributor
timestamp: May 14, 2018, 05:29:07 pm
content: Quote from: icpart on May 13, 2018, 10:16:14 pmThe amplifier described in an159 look really interesting. I will definitely try to build one of those!Thanks for the link!

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2018, 06:14:52 pm
content: Quote from: Kosmic on May 14, 2018, 05:29:07 pmDo you mean the one shown in figure 15? That can work but the specific example was intended for measuring the noise at the output of a linear regulator and has a lot more gain than required.This sort of things works well if it is just built into the original board layout and only populated for testing.

author: Kosmic
author_rank: Super Contributor
timestamp: May 14, 2018, 11:39:35 pm
content: Quote from: David Hess on May 14, 2018, 06:14:52 pmYes page 4 or 15 (page 15 is the differential version). On my side I'm interested in linear regulator noise and/or residual switcher noise after a linear regulator.So I guess if the amplifier is not close enough to the measurement point it will pickup a lot of external noise ?

author: David Hess
author_rank: Super Contributor
timestamp: May 15, 2018, 02:47:18 am
content: Quote from: Kosmic on May 14, 2018, 11:39:35 pmAll of the usual probing rules apply which is why building the test circuit into the layout is advantageous.Maintaining a good common mode rejection up to 20MHz and beyond is not trivial either and unless calibration is performed, external dividers become a problem. I have not tried them but I was thinking parts like the LT1187, LT1189, or LT1193 video difference amplifiers would work well in these applications but like all video amplifiers, they are pretty noisy and too noisy to measure linear regulators although they could be used with a low noise preamplifier. They have the advantage of getting rid of the external feedback networks back to the inputs so common mode rejection should be easier to maintain without calibration and their datasheets show about 40dB CMRR at 20MHz.

author: Kosmic
author_rank: Super Contributor
timestamp: May 15, 2018, 03:40:50 am
content: Quote from: David Hess on May 15, 2018, 02:47:18 amOK interesting, thanks for the clarification.

author: JDW
author_rank: Frequent Contributor
timestamp: January 23, 2024, 06:34:01 am
content: I found this thread while Googling "power rail probe" today, having a similar need as the opening poster. In my case, I want to measure ripple and noise for 5v & 12v switch mode supplies. (Scope: Rigol DHO804)A $4,000 power rail probe from Keysight, Tektronix, etc. is laughably out of the questionfor a hobbyist or casual user, and yet, these specialty probes are superior to other probing solutions out there, including general purpose active probes.The upside is that I came across alow-cost solution, albeit one that I have not personally purchased and tried yet.I first discovered the following power rail probe design byAndrew Levido, which is quite interesting:https://circuitcellar.com/research-design-hub/projects/building-a-power-rail-probe/Googling further led me toPatrick Coleman, who has forked Levido's design and posted all the details on his Github page here:https://github.com/blinken/power-rail-probe/tree/mainColeman even sells a fully assembled device, complete with a matte black enclosure professionally overlaid with white text:https://paradar.co.uk/products/low-noise-oscilloscope-power-rail-probe£249(US$317) fully assembled£45(US$57) for blank PCB & enclosure (must buy components onBOMseparately yourself)

author: blinken
author_rank: Newbie
timestamp: February 03, 2024, 03:15:31 pm
content: Quote from: JDW on January 23, 2024, 06:34:01 amI was sent a link to this thread - thank you for the recommendation, JDW! The power rail probe project is still early days, though the v1.0 model works very well for my purposes (I use it day-to-day, and the frequency response is flat within about 3dB to 500MHz).If anyone is interested in assisting, it would be good to publish some real-world data on the noise levels of the design. If you happen to have access to a calibrated noise source (ie. with well-characterised ENR to 1GHz) and a good spectrum analyser or noise figure meter, and have a little bit of time to help, I would be very grateful if you could reach out.-Patrick

author: Kosmic
author_rank: Super Contributor
timestamp: February 03, 2024, 03:30:34 pm
content: User blackdog has been posting recently about an amplifier he is designing to measure power supply noise:https://www.eevblog.com/forum/projects/40db-measuring-amplifier-with-filters/Look interesting.