Differential Probe: DIY or buy? - Page 1
Forum section: electronics / testgear

author: reyntjensm
author_rank: Regular Contributor
timestamp: October 04, 2020, 04:04:21 pm
content: Hello everyone,I've already been looking around for a differential probe for a long time now.Since i have no experience with such probe i would like to have some feedback before i decide on what to do.I have a RTB2004 oscilloscope and i'm very happy with it! Since my budget isn't that big i don't have the money to buy some R&S differential probes.So i've been looking around for other brands/types and also came along some DIY projects.First of all i want to measure current(switching at 1kHz) with my oscilloscope, since i already have some pretty good shunt resistors i thought it was a good idea to buy a differential probe since i can use this with my shunts but it's also very handy for other things. So it would be 2 bangs for the same buck i guess?Does anyone have any input/feedback on a good probe?Or is this a stupid idea and should i just buy a current probe?Thank you all!

author: Noy
author_rank: Frequent Contributor
timestamp: October 04, 2020, 04:30:13 pm
content: High speed or high voltage diff probe?For high voltage the micsig ones are good starting points <200€.For high speed there are various good diy / already build diy in this forum..

author: oewean
author_rank: Contributor
timestamp: October 04, 2020, 06:46:05 pm
content: Micsig is really affordable and good quality!I don't think you find any diff. or current probe that give more bang for bucks!Disclaimer: We sell Micsig!

author: S. Petrukhin
author_rank: Super Contributor
timestamp: October 04, 2020, 07:05:13 pm
content: Quote from: Noy on October 04, 2020, 04:30:13 pmDo you have a link to a simple DIY project?

author: Noy
author_rank: Frequent Contributor
timestamp: October 04, 2020, 07:13:38 pm
content: https://www.eevblog.com/forum/testgear/gt-1-ghz-diy-differential-probes/150/125€ + tax from the forum member or :https://xellers.wordpress.com/electronics/1ghz-active-differential-probe/Orhttps://hackaday.io/project/169390-a-10x-100mhz-differential-probe

author: reyntjensm
author_rank: Regular Contributor
timestamp: October 04, 2020, 09:20:54 pm
content: Quote from: S. Petrukhin on October 04, 2020, 07:05:13 pmhttps://hackaday.io/project/169390-a-10x-100mhz-differential-probeThis looked fairly good. There are several projects online

author: tautech
author_rank: Super Contributor
timestamp: October 04, 2020, 10:25:01 pm
content: BUY !If used for elevated voltages or on a mains low impedance source for the sake of safety for your instrument and yourself just buy one. $300-400 can get you a CAT rated one that's safe to use on most anything.Decide on the frequency, voltage and attenuation specifications that suit your needs and base your selection on them.

author: Noy
author_rank: Frequent Contributor
timestamp: October 04, 2020, 10:59:25 pm
content: Of course for high voltage stuff buy... Go for the micsig...For low voltage / "high speed" stuff for example lvds or other differential things go for diy..

author: 2N3055
author_rank: Super Contributor
timestamp: October 04, 2020, 11:13:10 pm
content: You need current probe for that...

author: reyntjensm
author_rank: Regular Contributor
timestamp: October 04, 2020, 11:24:16 pm
content: Quote from: 2N3055 on October 04, 2020, 11:13:10 pmWhat's wrong with a shunt resistor? Can a differential probe pickup a difference as small as 10mV? like the micsig ones?

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2020, 12:47:58 am
content: Quote from: reyntjensm on October 04, 2020, 11:24:16 pmResolution.QuoteSure they can but it's right down in the channel noise if the diff probe has limited low attenuation settings.2N3055 is certainly correct as some high resolution/accuracy measurements can only be done with a current probe and they are a specialist bit of gear with their very own deep rabbit hole.OTOH a differential probe is more versatile and even a comparatively low frequency one like the 25 MHz DP-25 can meet most users rudimentary requirements.

author: reyntjensm
author_rank: Regular Contributor
timestamp: October 05, 2020, 12:55:54 am
content: Quote from: tautech on October 05, 2020, 12:47:58 amI have a shunt that gives a 100mV output with a 10A load, this is just the area i want to measure( between 1-10amps). I guess i would have more bang for my bucks if i buy a differential probe? i Can still use another shunt... On the other hand i know a current probe is more suited for the job but i don't think i can find any current probe for the same price as the micsig differential probes? Current probes are always more expensive?

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2020, 01:12:28 am
content: Quote from: reyntjensm on October 05, 2020, 12:55:54 amUnderstood however if any diff probe you might select only offers 20x lowest attenuation that equates to just 5mV across your shunt @10A whereas a current probe can commonly offer 1mV/mA sensitivity from a straight through wire connection without the need for a shunt.For a 1A load across your shunt just 500uV is available to the scope inputs from a 20x diff probe, see the resolution problem with shunts and differential probes now ?At such low output levels from a shunt a 2ch connection and A-B Math is probably the only cheap solution.QuoteJust one word.....yep !However they can do stuff nothing else can do if you get good ones but like I mentioned earlier they open another deep rabbit hole of cost vs capability.DC stuff is easy but the pulses that might ride on top of DC current and a whole other story.

author: reyntjensm
author_rank: Regular Contributor
timestamp: October 05, 2020, 01:22:53 am
content: Quote from: tautech on October 05, 2020, 01:12:28 amI see the problem.... So a differential probe with with a low attenuation is necessary, but with this the cost is also rising... As my main target is to measure currents i'm doubting if i would go for the differential probe. They are very handy for other applications but this is not what i want right now. The current i want to measure is a rather slow PWM signal. A current probe would indeed be the preferable option. But they almost cost as many as my scope does... Any ideas on a "cheap" current probe? I already tried to do it with 2 channels and a math function but this didn't result in a good measurement. I also connected my shunt to the high side... I will try it again tomorrow with the shunt connected to the ground side. This would result in a much better accuracy with the math function. I hope... I'm almost always working with digital channels and the RTB was a great scope for all the things i wanted to do with it. This is the first time i want to measure current with my scope. I still have a long way to go i guess....

author: Noy
author_rank: Frequent Contributor
timestamp: October 05, 2020, 06:20:05 am
content: Micsig has also "cheap" <250$ current probes with AC+DC .Or maybe the Hantek CC65 already is enough for you (<50$ from china) and for simple stuff useable, no "high-end" equipment but useable...But if you only need to measure the AC current you can get solutions much cheaper.

author: 2N3055
author_rank: Super Contributor
timestamp: October 05, 2020, 06:52:11 am
content: You still didn't saywhat current and what voltages are you talking about and what max frequency should be.Electronics is bitch like that, you need to know that stuffbeforeyou connect something to something, so there isno smoke....There are cheap current sensors with built in electronics, that go to 100ish kHz, decent accuracy (after calibration), and decent isolation. LEM makes them and few other manufacturers.They basically behave as an AC/DC current transformer.Those are best bet for you, if you can break the circuit (which you do, because you want to add in shunts)Shunt amplifiers need to be precision and low noise, and that is not very compatible with high common voltage and robust...

author: BravoV
author_rank: Super Contributor
timestamp: October 05, 2020, 07:09:38 am
content: Quote from: 2N3055 on October 05, 2020, 06:52:11 amI guess the OP doesn't even know what to look for ?

author: 2N3055
author_rank: Super Contributor
timestamp: October 05, 2020, 07:14:45 am
content: Quote from: BravoV on October 05, 2020, 07:09:38 amWell, in that case, he's looking forsmoke....

author: BravoV
author_rank: Super Contributor
timestamp: October 05, 2020, 07:31:36 am
content: Quote from: reyntjensm on October 04, 2020, 04:04:21 pmMy 2 1/4 watt carbon resistors worth.If only 1kHz, and also assuming you will be working at mains voltage ... AND ... you have "adequate" shunt to work with mains level voltage which needs beefy terminals for securing and measuring it, and also to pass the current relatively safe as its lethal voltage.Sort of like these, but doesn't have to be exact. PS: Black wires are the sense lines.... then as many suggested above, just buy HV diff. probe, for example below the probe was measuring mains voltage at 220VAC that was powering the scope it self. And with "decent" shunt resistor that relatively safe for mains voltage, all you need is just to move the probes to measure the voltage drop across it, or at the resistor sense lines if they're available.

author: 2N3055
author_rank: Super Contributor
timestamp: October 05, 2020, 07:55:26 am
content: HV diff probes have noise that is order of magnitude of usable signal on the shunt.. If OP is measuring up to, say, 12V common voltage, you can go shunt route. Not to mention that noise pickup from high sensitivity diff amp in a EM hostile envronment might be more than signal itself...

author: reyntjensm
author_rank: Regular Contributor
timestamp: October 05, 2020, 09:10:20 pm
content: Quote from: 2N3055 on October 05, 2020, 06:52:11 amThe voltage is 13VDC and max 10amps. The frequency of interest is max 1kHz. I'm building a light controller to use in specialty vehicles. Since my circuit is working on battery's i want to measure the power consumption. I would also like to just measure the current going to my controller without any load, this should be around 10mA. I guess this would be impossible with a shunt. The most important is the total current i want to measure. I'm certainly not looking for a smokey oscilloscope.

author: 2N3055
author_rank: Super Contributor
timestamp: October 05, 2020, 09:23:52 pm
content: Quote from: reyntjensm on October 05, 2020, 09:10:20 pmThank you for the details. At those voltages and frequencies and battery source you can use low side shunt and measure voltage directly with normal passive probe. You can also measure high side by using high side current sensing, you can even get specialised current sense amplifiers (those are made for battery, motor measurements and such). Those current sense amplifiers are specialised differential amplifiers specialized for that use...

author: reyntjensm
author_rank: Regular Contributor
timestamp: October 05, 2020, 09:47:03 pm
content: Quote from: 2N3055 on October 05, 2020, 09:23:52 pmMy shunt is a good quality shunt. I've bought a differential amp dev kit (https://www.ti.com/lit/ug/sbou177a/sbou177a.pdf?ts=1601934222554&ref_url=https%253A%252F%252Fwww.google.com%252F). But i don't get it to work properly. The output voltage has a very small range, i guess i'm doing something wrong with the reference voltage. I'm pretty sure this circuit is perfect for the things i want to do with it. What would you suggest the for the small current range?

author: S. Petrukhin
author_rank: Super Contributor
timestamp: October 05, 2020, 10:00:20 pm
content: Quote from: reyntjensm on October 05, 2020, 09:10:20 pmI'm sorry, isn't an ordinary ammeter enough for this?

author: reyntjensm
author_rank: Regular Contributor
timestamp: October 05, 2020, 10:20:13 pm
content: Quote from: S. Petrukhin on October 05, 2020, 10:00:20 pmMy goal is to calculate the power consumption over a long period( the battery voltage will also change when i drain the battery). Since the load is PWM'd i can't use a regular Ampmeter

author: 2N3055
author_rank: Super Contributor
timestamp: October 05, 2020, 10:28:33 pm
content: Which version did you get? There are 4 gain versions. They are usually used with smaller burden on shunts..even 20x version will have to swing 2V around 2.5V ref with 100 mV input. And that is pretty much all dynamic range you have with 5V power...

author: reyntjensm
author_rank: Regular Contributor
timestamp: October 05, 2020, 11:06:16 pm
content: Quote from: 2N3055 on October 05, 2020, 10:28:33 pmI've ordered a kit with all 4 different versions. I've attached a 5V supply and my output was only going between 2,6-2,7volt. I couldn't figure out why this was the output. I don't understand how the reference voltage is used to generate the output voltage. I've tested it with the 20V/V gain module. My shunt produces 100mV at 10 amps. So there was nothing wrong with my dynamic range? I guess a very good voltage source is very important for these measurements to output good results. This is the shunt i'm using right nowhttps://www.digikey.be/product-detail/nl/riedon/RSA-10-100/696-1598-ND/4967075

author: 2N3055
author_rank: Super Contributor
timestamp: October 06, 2020, 10:21:49 am
content: Quote from: reyntjensm on October 05, 2020, 11:06:16 pmThat module has 20x gain. SO with 100mV input it has to swing 2 V on the output. You need to connect Ref1 and Ref2 together and to an external 2.5V ref. In that case, you will get 2.5 V on output with zero current, 0.5V output at -10A and 4.5V output at +10A.You could also supply chip with +-2.5V symmetrical around ground, ground Ref1 and Ref2, and get +- 2V for +-10A..Check datasheet again and check again if all is connected right.If it is, maybe something got damaged......Also check voltage on the shunt with meter at 10A just to be sure...

author: nctnico
author_rank: Super Contributor
timestamp: October 06, 2020, 06:42:34 pm
content: Quote from: reyntjensm on October 05, 2020, 09:10:20 pmA cheap clamp-on current probe (like the CP-05A) will do just fine for that purpose. I have one myself and it works just fine. Don't buy the CP-05+ though; according to the measurements of forum member Martin72 this one is way too noisy. Another option is to buy a reasonable accurate PSU to read the current (or a slightly more fancy one which also shows power).

author: David Hess
author_rank: Super Contributor
timestamp: October 06, 2020, 07:54:05 pm
content: Quote from: reyntjensm on October 04, 2020, 11:24:16 pmThe input dividers of a high voltage differential probe divide the high common mode input voltage to something the probe's difference amplifier can handle, but also multiply the input noise, so the input noise of a high voltage differential probe can easily be high enough to hide a 10 milivolt signal.Old differential amplifiers like the Tektronix 7A13 use noisy cascode input stages to increase the input voltage range to 10 volts or more just so that the input divider can be 10 to 100 times less and input noise can be 5 to 50 times better. Modern input amplifiers tend to have an input range of 1 volt and older but higher performance input amplifiers might be only 100 millivolts.Quote from: reyntjensm on October 05, 2020, 09:10:20 pmSo you have a 13 volt common mode voltage and 100 millivolt signal at 1 kHz. The low cost and relatively easy way would be an integrated difference amplifier but you could build one from scratch easy enough. Check out Linear Technology design note 25 for how one can be built from an operational amplifier. The Analog Devices AD626 is a good example of an integrated one which would world well.

author: reyntjensm
author_rank: Regular Contributor
timestamp: October 06, 2020, 10:22:06 pm
content: Quote from: 2N3055 on October 06, 2020, 10:21:49 amI've connected the module to a 4,5V supply, according to the datasheet the supply voltage could also be used as the reference. But i think i didn't connected the jumpers/solder bridge correctly. I will check this again. The Vref voltage should be very stable i guess? Since i don't have that many power supply's i'm struggling to power up my load and the amplifier. Can an aliexpress RD6006 be used as a reference? If the power supply has a ripple of around 10mV this would also effect the amplifier reference? Since i have all 4 amplifiers, would it be better to use a amplifier with high gain or low gain? Great support here! i will certainly keep you updated when i get it up and running!

author: reyntjensm
author_rank: Regular Contributor
timestamp: October 06, 2020, 10:24:58 pm
content: Quote from: nctnico on October 06, 2020, 06:42:34 pmWhere did you buy the probe? I found one for 130 dollars at aliexpress, for the same price i have a good quality differential probe.... My power supply does show the consumed power. But since my load is using a PWM signal, i don't think my PSU measurements are accurate.

author: reyntjensm
author_rank: Regular Contributor
timestamp: October 06, 2020, 10:29:44 pm
content: Quote from: David Hess on October 06, 2020, 07:54:05 pmGood to know how the voltage is measured with a differential probe. It seems logic now that a 10mV signal will hide in the probe it's own noise. About the differential amp, i already have these modules (https://www.ti.com/lit/ug/sbou177a/sbou177a.pdf?ts=1601934222554&ref_url=https%253A%252F%252Fwww.google.com%252F). I think this would have the same result as the amp you suggest? Only are those modules already mounted on a PCB?

author: nctnico
author_rank: Super Contributor
timestamp: October 07, 2020, 05:31:39 pm
content: Quote from: reyntjensm on October 06, 2020, 10:24:58 pmA differential probe is not the right tool for reading a current shunt. For that you'd need a differential amplifier. With only 13V of common mode you can use a reasonably standard instrumentation amplifier circuit.QuoteIn then end the current measurement on your PSU has to integrate the current and show the average. Unless it is a really crap PSU I doubt the average current is wrong.

author: rstofer
author_rank: Super Contributor
timestamp: October 07, 2020, 06:12:51 pm
content: If you are considering low side sensing, think about how that will impact the control system. For a very simple MOSFET driver type when the source voltage is increased because of current through the shunt, the gate turn-on voltage changes and it is referenced to the real system ground, not the source of the MOSFET which is moving around. Sort of like negative feedback.That's why they invented high side sense amplifiers and I don't know why you aren't just buying the chip and hanging it across the shunt.https://www.analog.com/en/analog-dialogue/articles/high-side-current-sensing.html#https://www.analog.com/media/en/technical-documentation/product-selector-card/Currentsense.pdfAt your low voltages, it would be easy to have the shunt installed on the board (with a jumper to bypass it if necessary) and you could have the sense amplifier either permanently mounted or part of a DIY piece of equipment.

author: 2N3055
author_rank: Super Contributor
timestamp: October 07, 2020, 07:04:52 pm
content: Quote from: reyntjensm on October 06, 2020, 10:29:44 pmListen, you already chose good option. Many advice here is from people who didn't read it all..INA240 is EXACTLY the chip for you. No need to invent current sense amplifiers from the scratch.Shunt you chose is also OK.Problem is that something is broken with your implementation.You need to simply concentrate to make it work.You tried with 20X board. Maybe that board is damaged somehow. Try with other board, one that has INA240A2: 50 V/V.That should give you 5A range with your shunt, and should work nicely in that range. Carefully follow demo board manual and datasheet.Check 3 times before applying power. If it works nicely as 5A board, then probably your INA240A1: 20 V/V board is damaged.If it still doesn't work maybe shunt is not OK? Did you check (as I asked) if you really get 100mV across the shunt if you push 10A trough it? Apply current and measure with simple multimeter.For quick and dirty 2.5V ref for test you can use simple TL431. For production and better accuracy/stability use any of the 2.5 reference chips. There are hundreds of them: low power, high stability, high initial accuracy.. With prices to match. I would use one that can sink and source current and need to work from 5V. LT6657 is expensive, but stable and will work from 3V.. ADR4525, LTC6655, ADR3425 etc. That is from Analog devices/Linear tech alone. Texas instruments (National) will have it's share too. If you are making only one of these, 5-6 USD reference is not a problem..Regards,Sinisa

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2020, 01:37:48 am
content: Quote from: reyntjensm on October 06, 2020, 10:29:44 pmThe INA240 will work fine. What is required in this case is support for an input voltage range which is higher and possibly lower than the supply voltage, which is why an instrumentation amplifier is not suitable.