Measuring small mV voltages with scope - preamps? - Page 1
Forum section: electronics / testgear

author: rx8pilot
author_rank: Super Contributor
timestamp: February 17, 2015, 07:28:44 pm
content: I need to measure small mV/div or better on my scope that maxes out at 10mV/div. This is to measure current via very small value sense resistors (5mOhm). 1A = 5mV. Hoping to be able to resolve about 100ma steps.I have looked at the Tek ADA400A and the ARI OSP-1http://aricorp.com/OSP-1.htmlBoth are relatively low BW at ~1Mhz but have excellent sensitivity. Does anyone know of other and hopefully better options? Current probes are out of the question because they are far too big and the sense resistors are already on the DUT.

author: plazma
author_rank: Frequent Contributor
timestamp: February 17, 2015, 07:31:42 pm
content: How do you power the DUT? Do you use a lab PSU with sense lines?

author: Howardlong
author_rank: Super Contributor
timestamp: February 17, 2015, 07:35:25 pm
content: Quote from: rx8pilot on February 17, 2015, 07:28:44 pmIn this situation, I think I'd build my own test fixture. A fixed gain differential amplifier, with band limiting as necessary, on a small board located close to the DUT. This way you could generate a single ended low impedance output to probe conventionally.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 17, 2015, 07:46:34 pm
content: Quote from: plazma on February 17, 2015, 07:31:42 pmYes, lab PSU with sense. What I need to capture though are fast current transients so the PSU does not help.Quote from: Howardlong on February 17, 2015, 07:35:25 pmI considered that approach but was concerned with the time it takes to design/build/verify. I had considered the LMP8640 which seemed to be simple enough. I am assuming that the price tag of the models mentioned above indicate the challenge is deeper than a down and dirty circuit built around a $5 chip.

author: plazma
author_rank: Frequent Contributor
timestamp: February 17, 2015, 07:54:12 pm
content: This is how I measure current with an oscilloscope and a lab psu with sense lines: connect lab psu sense+ and out+. Take positive voltage from out+. Connect a 1ohm resistor (or smaller, I=U/R) between sense- and out-. Take gnd from sense-. Measure the voltage over the 1 ohm resistor with the scope. 1mA = 1mV. The sense- compensates the voltage drop over the resistor.

author: owiecc
author_rank: Frequent Contributor
timestamp: February 17, 2015, 08:01:08 pm
content: Quote from: rx8pilot on February 17, 2015, 07:28:44 pmWe use a Pearson current monitor with a secondary small core current transformer (ca. 10mm diameter). We put the secondary core on a leg of a MOSFET we want to measure. This way it is quite easy to measure current rise times of 20ns in tight spaces. What BW do you need?

author: rx8pilot
author_rank: Super Contributor
timestamp: February 17, 2015, 08:56:09 pm
content: Quote from: plazma on February 17, 2015, 07:54:12 pmThat is an interesting technique. Does it not rely on the response time of the sense circuit of the PSU? The downside is that I need to measure the current at a specific points in the DUT where the sense resistors are already part of the design/PCB. There are multiple points that need to be observed.Quote from: owiecc on February 17, 2015, 08:01:08 pmAnything >10Mhz would likely be enough. Don't the Pearson units require a breakout of the circuit to run through the coil? To add more information that may help clarify. The DUT is an electronic fuse circuit (part of a bigger system). There are current sense resistors, amplifiers, and FETs that limit the current. They work well with static loads. The need is to tweak them to handle various capacitive loads with inrush currents well above the trip point. I have a delay built in to allow over-current for a defined period to allow these transients to pass (up to an absolute max preset limit) without a false trip. The goal is to disturb the circuit as little as possible and the that is made more challenging because the PCB is so small and dense. Creating a loop to pass through a transformer type current monitor or probe, is technically possible, but difficult. It would also be nice to be able to measure any PCB without having to specially configure it for current measurements.Since the sense resistors are already part of the PCB/design, it seems logical to use them as a point to measure.

author: splin
author_rank: Frequent Contributor
timestamp: February 17, 2015, 11:16:21 pm
content: These Rogowski coils are only 1.6mm diameter and can be threaded between the legs of a soldered-in TO-220. They have a 30MHz bandwidth, sensitivity up to 200mV/A and they can make custom versions if required. There are several good technical papers on the site discussing the benefuts and limitations of Rogowski coils and the alternatives.http://www.pemuk.com/products/cwt-current-probe/cwt-ultra-mini.aspxThey won't be cheap though - probably around $1000 - but excellent for some types of problems.Splin

author: robrenz
author_rank: Super Contributor
timestamp: February 17, 2015, 11:53:40 pm
content: Preamble 1855 diff amp will give you 10X into 50 ohm or 20X without 50 ohm termination. Lecroy now markets them. 100MHz bw and integral precision offset supply.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 18, 2015, 12:52:56 am
content: Quote from: robrenz on February 17, 2015, 11:53:40 pmNaturally, the one with the best performance is $CHA-$CHING!! That looks like the solution if I can figure out a way to pay for it. One on eBay for $1500 close to me so I can test it. Others seem to be around $4k, which would require a little money dance.Thank you for the link, at least I now have an option that I love.

author: tautech
author_rank: Super Contributor
timestamp: February 18, 2015, 02:09:13 am
content: Quote from: rx8pilot on February 17, 2015, 08:56:09 pmAt the same time?QuoteWould you not have been better to have a prototype PCB just for this purpose, then miniaturize it after development had finished?

author: rx8pilot
author_rank: Super Contributor
timestamp: February 18, 2015, 02:36:59 am
content: Quote from: tautech on February 18, 2015, 02:09:13 amI think that in general, I can get away with looking at one at a time. I would be looking at the current, input bus voltage, FET gate, and output bus voltage at the same time. Part of the timing challenge is to allow big capacitive loads to get started with minimal impact on the bus voltage. I only have a modest amount of bulk capacitance on the bus so I need to cut off a branch circuit just before it can droop the main bus below a threshold. The best current resolution will be at the sense resistor which is exactly what the circuit is reacting to.Once I get past that, I would like to do similar tests with adjacent channels of the circuit active with dynamic loads to see if they may interfere in any significant way. This would require a multi-channel setup.Quote from: tautech on February 18, 2015, 02:09:13 amPossibly, but I am wanting do characterize the final circuit exactly as it will ship. I have proven the fundamentals of the circuit and this is where I am wanting to fine tune the timing of how it will react to various di/dt, current over shoots, etc.I may try a test with a Tek ADA400A since they are dirt cheap on eBay. It seems it will be good enough to test the concept. The Preamble 1855 is the no-excuses tools for the job if I can get over the cost.

author: owiecc
author_rank: Frequent Contributor
timestamp: February 18, 2015, 02:48:50 pm
content: Quote from: rx8pilot on February 18, 2015, 02:36:59 amRemember you will need matched probes for optimal performance.http://teledynelecroy.com/probes/probemodel.aspx?modelid=144&categoryid=3&mseries=433

author: robrenz
author_rank: Super Contributor
timestamp: February 18, 2015, 03:31:04 pm
content: Quote from: rx8pilot on February 18, 2015, 12:52:56 amI got mine for a $300.00 offer when the ask was $900.00Don't be afraid to make some stupid low offers

author: tszaboo
author_rank: Super Contributor
timestamp: February 18, 2015, 04:17:36 pm
content: Quote from: Howardlong on February 17, 2015, 07:35:25 pmI considered that approach but was concerned with the time it takes to design/build/verify. I had considered the LMP8640 which seemed to be simple enough. I am assuming that the price tag of the models mentioned above indicate the challenge is deeper than a down and dirty circuit built around a $5 chip.The LMP8640 will not give you enougth bandwidth, but you already know that. If you only need ~100Khz bandwidth, a simple instrumentation amplifier is all you need. If you need 100x times the bandwidth, it depende on your requirements. If the common mode voltage of the input doesnt change too much, you can make a discrete INA with carefully selscted components, it is not rocket science. It will give you better accuracy(and DC) than any transformer coil or inductive solution. I usually prefer DC. Use multiple gain stages if you need more GBW, also, not equal power supplies (eg. +25V /-5V). I can be done. It will not be universal OFC.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 18, 2015, 07:05:13 pm
content: Quote from: robrenz on February 18, 2015, 03:31:04 pm$300??? That is a bargain! Even $900 seems to be a bargain.

author: dannyf
author_rank: Super Contributor
timestamp: February 18, 2015, 07:06:14 pm
content: Simply amplify it. google current sense amplifier or build one from opamps.

author: dadler
author_rank: Unknown
timestamp: February 18, 2015, 08:34:47 pm
content: Any way the uCurrent could be of use here, or is the bandwidth too low?

author: rx8pilot
author_rank: Super Contributor
timestamp: February 18, 2015, 08:39:15 pm
content: Quote from: dannyf on February 18, 2015, 07:06:14 pmiIf the solution is $thousands, then yes - build. If I get lucky enough to find an instrument at a good price designed by people that designed instruments, I would feel more confident.Quote from: dadler on February 18, 2015, 08:34:47 pmGood question! I have no idea what bandwidth it is? looking.....EDIT: 200Khz, which is too slow for this application but very handy to have. Hope they are back in stock soon.

author: Howardlong
author_rank: Super Contributor
timestamp: February 20, 2015, 05:10:27 pm
content: Quote from: dannyf on February 18, 2015, 07:06:14 pmAgreed, not sure why I didn't suggest a current sense amplifier to begin with rather than a diff amp.They are fixed gain, acccurate, and are available to operate high side or low side. Some even operate a little outside of their supply voltage range. Also some even have integrated ADCs with I2C presentation, but I think you're after direct voltage reading.Only downside may be bandwidth, they are not generally designed for high sampling rates. However other than that they are designed for exactly what you are trying to do.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 20, 2015, 05:34:34 pm
content: I have looked in that direction, but the problem is that they are all relatively slow around 500Khz with a few options at around 1Mhz. In addition, they require me to design, build, and test it while at best I would be getting no better performance than an Tek ADA400A from eBay for $200. For that performance, I would rather forgo the experimental step and get a pre-made instrument. When looking at the cost of the big boy solution, Preamble/LeCroy 1855 - it may be more cost effective to consider rolling my own but only if I can get better performance than the $200-300 Tek ADA400A.The original hope is that I find something approaching 10Mhz with an analog output. I have a digital/i2c circuit based on an INA226 but it is WAY too slow to catch fast edges. The goal is to precisely control the DUT responses to various over-current events which can be very slow or very fast.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 04, 2015, 06:52:07 pm
content: I got a Tek ADA400A differential pre-amp today. It really works great for doing current measurements with a shunt. Great CMRR so the signals are clean and believable, although somewhat bandwidth limited. It does not state the actual bandwidth, it just says >1Mhz. Kinda strange. 100x amplification is very sensitive and works with very small value current sense resistors.Anyway, I feel like I can now see high-res current traces on my scope and time correlate them to other events. The shunt method has no delay so I don't have to shift anything.

author: Neganur
author_rank: Unknown
timestamp: March 05, 2015, 01:25:15 am
content: One commercially available low current wideband probe I've see so far has 100 MHz bandwidth (the Agilent N2783B) and claims 5 mA as lowest measurable current at DC, does not mention lowest AC reading at 100 MHz but according to performance de-rating graphs, the maximum continuous current at 100 MHz is less than 2 A rms. That's a current clamp though and they're probably a hall effect hybrid (for the DC).Their Series current probes are 3 MHz bandwidth (N2820A) and claim 50 uA min - 2.2 A max with a 100 mOhm sense resistor. The graphs claim that you can achieve an absolute minimum current of 500 nA with a user defined 1 kOhm resistor head (500 nA - 1.2 mA).The manual is a bit ambiguous about the high gain view, since it limits the bandwidth to 500 kHz and has 300x amplification (30V/A with 100 mOhm shunt) while the full bandwidth has a ~2x amplification (0.2V/A with 100 mOhm shunt). Full bandwidth 50 uA with the 100mOh shunt @ 0.2 V/A corresponds to a 10 uV output. I don't think my oscilloscope can do anything with that. With the zoom mode (500 kHz bw) this increases to 1.5 mV.In my eyes, this is a uCurrent with a fancy schmancy Autoprobe interface that costs 3k EUR.

author: EEVblog
author_rank: Unknown
timestamp: March 05, 2015, 01:30:23 am
content: Quote from: rx8pilot on February 18, 2015, 12:52:56 amI've been watching them for a long time, and they never seem to go cheap.

author: dannyf
author_rank: Super Contributor
timestamp: March 05, 2015, 02:08:12 am
content: QuoteCheck out current sense amplifiers - one opamp + one discrete device. You can use off the shelf ICs or build your own.

author: owiecc
author_rank: Frequent Contributor
timestamp: March 06, 2015, 07:15:11 am
content: Quote from: EEVblog on March 05, 2015, 01:30:23 amI found them for $900 few years back. Bought four. I wish I had bought more.

author: MarkL
author_rank: Unknown
timestamp: March 06, 2015, 08:26:56 pm
content: Very similar in spec to the Tek ADA400A is the Tek AM502 differential amp. You can find them for under $100 but they do require a Tek TM series power frame.I picked one up for US$51 last year. Although it was a little flaky at first, all it needed was some switch contact cleaning.

author: dom0
author_rank: Super Contributor
timestamp: March 06, 2015, 10:36:46 pm
content: Quote from: EEVblog on March 05, 2015, 01:30:23 amI think this is the usual pattern of niche measurement devices. A diff-preamp is not something many people need*, is not easily build (on the same level as commercially available units), but saves lot of work and as such have high value for few people, thus the prices are high.* it is often/usually possible to build a specialized amp for one specific measurement, but this is work and takes time. It's a bit like a good function generator: yeees, you can build a new signal source every time you need a specific signal ... or you pay a good amount of cash for a function generator ...

author: rx8pilot
author_rank: Super Contributor
timestamp: March 06, 2015, 10:47:14 pm
content: Yep, I want and need a proven commercial design that I can count on from the first day. The ADA400A that I picked up is a good start for sure, but I will soon run out of bandwidth and the Preamble is WAY more than I would need. I am focusing on power electronics right now so I would use rather often, but I understand that it is a specialized tool that few really need. Priced accordingly of course.

author: Neganur
author_rank: Unknown
timestamp: March 07, 2015, 01:52:23 am
content: What kind of bandwidth? I think it is possible to achieve 5 MHz, beyond that it's not a simple design anymore

author: HighVoltage
author_rank: Super Contributor
timestamp: March 07, 2015, 11:29:41 am
content: Quote from: splin on February 17, 2015, 11:16:21 pmWhat an interesting current probeThanks for the link

author: Marco
author_rank: Super Contributor
timestamp: March 07, 2015, 12:19:02 pm
content: The 1855 is not actually an amplifier at 100 MHz BTW (it only works at 100 MHz with :10 probes, so no overall gain).

author: robrenz
author_rank: Super Contributor
timestamp: March 07, 2015, 12:44:42 pm
content: Quote from: Marco on March 07, 2015, 12:19:02 pmI am no expert but I don't think this is correct. As I read the manual you can have 10X gain at all bandwidths and the 1855 10X is quieter than most scopes so they suggest using it and running the scope at a 10X lesser gain.

author: Marco
author_rank: Super Contributor
timestamp: March 07, 2015, 01:20:37 pm
content: But you have to use the 1x probes to actually have amplification in the chain including the probe ... I underestimated their bandwidth though, the DXC200 probes still have a respectable 50 MHz bandwidth.

author: robrenz
author_rank: Super Contributor
timestamp: March 07, 2015, 01:24:16 pm
content: Like I said, I am no experttotally forgetting the BW limiting of the 1X probes themselves

author: rx8pilot
author_rank: Super Contributor
timestamp: March 07, 2015, 06:15:14 pm
content: 50Mhz is indeed respectable for 1x probes. So the system may be limited by the probes if gain is needed. I would be rather happy with anything over 10Mhz, so maybe that is not such a problem.Does anyone know what these cost new? The used prices are all over the map and I am trying to figure out what a 'deal' would be. Some of them are being sold with the original matched probes and some not. It feels like the using any old 1x probes might skew the performance in a way that might not be obvious.

author: dom0
author_rank: Super Contributor
timestamp: March 07, 2015, 06:23:17 pm
content: CMRR might suffer when using probes with poor impedance matching w.r.t. to each other, but this obviously depends a lot on how the amplifier is designed internally.

author: coflynn
author_rank: Regular Contributor
timestamp: March 07, 2015, 11:49:41 pm
content: If it's useful for the OP: I had a similar problem, as was measuring USB in-rush current. I used a cheap differential line-drive part (AD8129/AD8130), you cansee some details here.It's worth nothing I didn't require any sort of high accuracy, anything within 10-20% was fine - thus I never characterized the errors (non-linearity, CMRR), besides vaguely verifying with a few test loads. The output goes to the scope directly, so no issues w.r.t. 1x probe bandwidth. The amplifiers themselves have pretty high bandwidth.Here's the type of graph I got (you can ignore the annotations):I can zoom in if you want a closer detailed look in the time domain. As I mentioned this was for a specific use I had, so it might not be appropriate for what you need!

author: Marco
author_rank: Super Contributor
timestamp: March 08, 2015, 12:17:58 am
content: Nice chip/board, 26.4 volt range is pretty amazing for a chip that fast ... what's the default amplification on that chipwisperer board?PS. 5x I see, but that's just a one resistor to change.

author: coflynn
author_rank: Regular Contributor
timestamp: March 08, 2015, 12:31:53 am
content: I think it was set to 10x... it's just two resistors that set the gain (might be 5x, I'd have to double-check whats on the probe). The datasheet specs up to 100x so could easily go that high I think.

author: owiecc
author_rank: Frequent Contributor
timestamp: March 08, 2015, 05:29:16 pm
content: Quote from: rx8pilot on March 07, 2015, 06:15:14 pmhttp://www.digikey.com/product-detail/en/DXC200/DXC200-ND/3961339

author: robrenz
author_rank: Super Contributor
timestamp: March 08, 2015, 08:06:53 pm
content: Quote from: rx8pilot on March 07, 2015, 06:15:14 pmRoughly $5200.00 with no probes

author: Marco
author_rank: Super Contributor
timestamp: March 08, 2015, 10:25:28 pm
content: Unless the Chipwhisperer PCB screws it up I really don't see much reason to look any further. The IC has the same noise density as the 1855, harmonic distortion negligible at 10 MHz. The 1855 is not worth the 2 orders of magnitude higher cost.

author: coflynn
author_rank: Regular Contributor
timestamp: March 09, 2015, 12:48:36 am
content: QuoteThe way I see it, my design can be 500x worse design and *still* be a better value ;-)

author: Marco
author_rank: Super Contributor
timestamp: March 09, 2015, 05:40:45 am
content: Quote from: coflynn on March 09, 2015, 12:48:36 amTalking about the design ... AFAICS D2 creates two grounds in the circuit, the level of these grounds will shift with IC supply current (except output current). With one of the grounds being used for the IC and REF divider and the other being used for the SMA ground this seems a recipe for trouble to me. Why did you not simply make the anode of D2 AGND with a single ground for everything? (REF would still shift around a bit though, a shunt regulator to give a stable voltage for the R3 pot would be nice.)PS. the R3 pot is strange, you want the lower leg of the divider to be in the 10s of Ohm, the DC impedance to REF has to be low, otherwise it's going to affect the feedback divider. It should be buffered with an opamp so the interaction with the feedback divider becomes irrelevant.

author: coflynn
author_rank: Regular Contributor
timestamp: March 09, 2015, 12:23:51 pm
content: D2 is just reverse polarity protection on the negative supply, so the other 'GND' is normally -7V, not GND. The diode was setup that way to allow one to place a shunt between pin 5 & 6 of the 6-pin connector to use the device with a single supply if you don't need common-mode voltages near 0V. In which case AGND becomes the GND you were mentioning.The lack of precision regulators for the offset trimmer and buffers are due to the original use-case of this. Basically when measuring current variations for attacks w/ the ChipWhisperer project, you don't care about small changes in gain, non-linearity, or offset. The device this plugs into is always AC-coupled, and data is frequently normalized anyway.So these aspects were omitted to make the design easier for someone to build dead-bug style (as my prototype was). Since the final boards had the same use-case I never bothered to change anything (and the design worked so didn't want to change it any more).As an interested side-note theeval board for this device uses the same approach (just an unbuffered resistor), even though the datasheet had called out that the source impedance of the offset voltage should be low.Perhaps there would be a market for a slightly better version for people wanting to use this as a generic scope probe....

author: Marco
author_rank: Super Contributor
timestamp: March 09, 2015, 03:13:56 pm
content: Ohh I completely misread that.The evaluation board seems AC coupled by default, the DC impedance to REF doesn't really matter then because of the cap. I'm getting a bit of a headache trying to think through how the circuit would behave at DC with the pot in the center ... the amplification purely from the feedback divider to ground becomes 10/3.25, but at the same time the REF voltage shifts providing additional amplification. Do those effects effectively cancel out to still provide 2x amplification overall?

author: Lunasix
author_rank: Regular Contributor
timestamp: March 09, 2015, 03:38:56 pm
content: The evaluation board is only the PCB whithout any component !

author: Marco
author_rank: Super Contributor
timestamp: March 09, 2015, 04:25:07 pm
content: A quick sim seems to show that if you use the pot the circuit simply does not work correctly at DC, the amplification is not the same as at higher frequency.PS. with the values from the diff probe the amplification goes from 6.6 at DC to 6 at high frequencies, not a small error.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 09, 2015, 04:53:50 pm
content: Quote from: coflynn on March 07, 2015, 11:49:41 pmThat is really slick how it plugs on to the header pins like that. What you are measuring is very similar to what I am after, only I will be dealing with a wider dynamic range from 1ma to about 50A (in a few ranges). The concept being the same though. I am hoping for much better precision. Because of that, building one is not really practical for a couple of reasons. The first is that I don't have amplifier design experience suitable for building a lab grade instrument that I need to count on for precision. The second is that even if I did have the experience, the time needed for the design, layout, testing, characterization, re-layout, and re-testing, etc is very expensive and slow (compared to buying a commercial unit). With this USB scenario explained, high precision is not needed so it makes sense. I don't even have the tools (or the talent) to characterize and verify the performance of an amplifier like this so I have to choose my battles wisely.Something like the Preamble is a great value if your design is stalled and the business is counting on the new PCB to be validated for production ASAP. If it is just another handy little gadget on the bench that would be neat to have - it looks very expensive. To me, the value is in the time savings of it just working from day one and being able to count on it. Of course, I am the business owner so I am very sensitive to time efficiency. ;-)

author: coflynn
author_rank: Regular Contributor
timestamp: March 10, 2015, 11:13:44 am
content: QuoteI'm travelling right now but will check my physical unit when I'm back out of interest. But this does sound about right - again for the original use-case DC response isn't a problem. In fact when I was originally testing the design I was using a VNA from ~200kHz - 100MHz I think, so never even looked at lower frequencies.This does throw my reuse of the probe for the USB measurements off a bit, so will double-check them! In my case I had a 3x safety factor so I wasn't too worried about small measurement errors... even 50% off is possibly acceptable for me, but I doubt most people will appreciate that level of inaccuracy...Thanks for running the sim! I guess it must have been driving you crazy ;-)QuoteYup - I very much appreciate that for your application DIY would end up costing a lot more.

author: coflynn
author_rank: Regular Contributor
timestamp: March 19, 2015, 10:15:42 pm
content: Just as a follow-up, finally had a chance to test the hardware. Attached shows linear frequency response plot of input and output while sweeping frequency. Red is input, Blue is output (red should be flat obviously, I had graphed it just to confirm no oddness in signal generator at very low frequencies). The probe has slightly different values than the original schematic (and thus your sim), but the basic results are the same!On the plus side I already had some solder jumpers which can be used to ground that node (instead of going to the pot). For my current measurements that will improve the results a lot, since it effectively flattens the gain curve. For these measurements you don't need the offset nulling that the pot provides anyway.Thanks again for the note about that!EDIT: As a note this was done with a FFT and sweeping the signal gen frequency, so there is some spikes in the measurement due to incomplete coverage of all frequencies, and large errors at the start/end when either not enough samples for a complete waveform or insufficient sample rate.