Low resistance measuring circuit - Page 1
Forum section: electronics / projects

author: Brage
author_rank: Contributor
timestamp: May 02, 2022, 06:58:54 am
content: I'm working on a handheld test instrument to go out in the field for testing existing products. Nothing extreme weather or high precision.One of the features the tester has to have is the ability to measure low ohms range. Think short cables of ~2 meters.The required resolution is a single decimal place measurement from 0-5 ohms. Ideally it would be able to measure a tenth of an ohm (0.X ohms). Anything higher precision is a welcomed bonus.Ultimately the test is a pass/fail, but I would like to have some useful data on what is typical for the product.The reason for not using a multimeter for this job is that the connectors are RJ45 and all four pairs have to be tested. Doing that a hundred times is tedious and error-prone.I sketched up the attached circuit. It sends out a 0-5V signal based on the resistance of the DUT. The circuit is passing a 10mA current trough the DUT and amplifies the voltage over the DUT by 100. A 4 ohm load on the DUT will then give a 4V signal out.The problem I see with this circuit is that I need to have a negative power rail to feed the opamp if I want to measure near-zero ohms.This is nothing new, but it's not something I've done before. I looked around online, but must of the material I found focused on general resistance measurements with a wide measurement range. So I thought it would be a good idea to ask you guys if you had any ideas.

author: Brage
author_rank: Contributor
timestamp: May 02, 2022, 08:25:33 am
content: Thanks for the input, evb149!The resistance measurement is not the only test this instrument will do, so the instrument will have a MCU of some sort anyhow.I particularly liked the INA4180https://www.ti.com/product/INA180. I think I'll order a few and spin up some breakouts so I can test some of these. If I run short on ADC inputs on the MCU I could always use an analog switch.Quote from: evb149 on May 02, 2022, 07:16:49 amI drew up the attached picture. Are you talking about setting it up as a voltage divider circuit? Did I understand you correctly?How much resolution would I get from this? A 10bit ADC would give me Aref/1024. Say Aref = 5V, that would give me roughly 5mV resolution. If the series resistor is 500ohms, that would put the max divison at 500/5. That would give me a span of 0V to (5V/505ohm)*5ohm = ~50mV. That would give me a measurable resolution of halv an ohm.Even 500ohms is pushing it, and I would most likely need a buffer amplifier like you said.Kind regards

author: kripton2035
author_rank: Super Contributor
timestamp: May 02, 2022, 10:36:57 am
content: please take a look at my adaptation of the shorty meter, in the shorty-with-display.http://kripton2035.free.fr/Projects/shorty-display.htmleasy to build one yourself on a protoboard, and low ohm friendly.(with 4-wires measurement)

author: Brage
author_rank: Contributor
timestamp: May 02, 2022, 12:49:34 pm
content: That definitely looks overkill for what I'm trying to do. I love it!How high of a resistance can I expect to measure with that setup?Only downside to using that ADC is that the address is not configurable. All the address bits are set at the factory, and then microchip supplies the different types/addresses. That results in only a couple of them being available, limiting the number of ADCs per I²C line. One option is to have one I²C line for each ADCAnother is to have some switching on the I²C line to switch between the devices, which is also equally dumb. The whole point of having an addressable bus is to deal with this exact problem.I could switch out the ADC, thought.Thanks kripton!Kind regards

author: kripton2035
author_rank: Super Contributor
timestamp: May 02, 2022, 04:42:08 pm
content: Quote from: Brage on May 02, 2022, 12:49:34 pmfrom 0.1mΩ to around 10Ω (but more somewhat around 2-3Ω max)

author: free_electron
author_rank: Super Contributor
timestamp: May 02, 2022, 04:46:24 pm
content: why do you think you need a negative voltage ?simply lift that current source 1 volt above ground and feed that same 1 volt to the negative input of the opamp. principle of biassing.and there are opamps that can work down to ground and even below.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 02, 2022, 04:52:00 pm
content: There are ready made network testers for RJ45 cables.Good with very long wires, too (when each end connector is in a different room).https://duckduckgo.com/?q=RJ45+cable+tester

author: strawberry
author_rank: Super Contributor
timestamp: May 02, 2022, 06:01:21 pm
content: copper oxide thermal EMF ~1mV/°C, could potentially be a problem for ~1...100mΩ range for measuring cable and contact resistancesensitive current sense amplifier is good for finished product but may not be great for random wires and alloyschopper amplifier...

author: ledtester
author_rank: Super Contributor
timestamp: May 02, 2022, 06:51:21 pm
content: Quote from: Brage on May 02, 2022, 12:49:34 pmHow many ADCs were you thinking of?

author: Brage
author_rank: Contributor
timestamp: May 02, 2022, 10:47:26 pm
content: Quote from: free_electron on May 02, 2022, 04:46:24 pmThe opamps I looked at that were rail-to-rail would not truly go all the way to ground, but you're probably right. I did not look hard enoughYes, biasing the current source is a viable option for sure.

author: Brage
author_rank: Contributor
timestamp: May 02, 2022, 10:53:42 pm
content: Quote from: RoGeorge on May 02, 2022, 04:52:00 pmWere it so easy...The instrument is not truly testing a cable. There is a device on the line, and one of the other operations the instrument is going to do is send a bunch of packets to make sure the device operates as it should.It would be possible to have multiple instruments to test the different aspects, but it would be cumbersome. Especially when different techs with different skillsets are testing hundreds of devices.

author: Brage
author_rank: Contributor
timestamp: May 02, 2022, 10:58:25 pm
content: Quote from: strawberry on May 02, 2022, 06:01:21 pmWould this still be an issue when all the DUTs have the same contact materials? If I were to use whatever cabling I'd find in the nearest corner then sure, but all the DUTs are build with the same components from the same suppliers. The only thing that might differ is the actual probes, but that can be addressed.

author: Brage
author_rank: Contributor
timestamp: May 02, 2022, 11:06:50 pm
content: Quote from: ledtester on May 02, 2022, 06:51:21 pmThere are 8 lines to test in total for each DUT. When I first started out I was thinking one for each, just because. My thought was that I'd either have to switch the ADC between all the different lines on the measurement side, potentially effecting the measurement, or I'd use one ADC for each line.It might be a dumb idea, and I'm happy to hear how it could/should be done.Keep in mind that the measurement confidence has to be high. If a measurement is done on a device far out in the field, it might not be possible to have another go at it if we later find out that it's a bogus measurement.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 02, 2022, 11:27:17 pm
content: Increase R102. To compensate for the bias current of the opamp it should be equal to R103 parallel to R104 (So approx 10k in this case).I assume you're measuring with the ADC of your uC. You can further increase the resolution by increasing the measurement current (Duh) But by adding a switch (BJT, MOSfet) you can do the measurement with a low duty cycle. It's quite easy to tun on the switch, wait a few hundred micro seconds, take an ADC reading and then turn the switch off again.Cheap opamps can easily have an offset voltage of 10mV, and if you amplify that with a factor 100, then you already have a volt of error. This can also be callibrated out by your uC though.

author: mawyatt
author_rank: Super Contributor
timestamp: May 02, 2022, 11:45:25 pm
content: @ Barge,Since you are creating the stimulus and measuring the response, have you considered using a synchronous approach?Best,

author: strawberry
author_rank: Super Contributor
timestamp: May 03, 2022, 07:03:17 am
content: Quote from: Brage on May 02, 2022, 10:58:25 pm34401A have 1mΩ sensitivity but cant measure anything reliably below ~50mΩ. 100Ω/0.1V F.S. amplifier sensitivity/gain could be the problemfor 10A current source EMF sensitivity could be less in mΩ rangewhen taking measurement 10A 10..100ms pulse

author: Marco
author_rank: Super Contributor
timestamp: May 03, 2022, 07:43:35 am
content: Just pulse the current and do two measurements to remove offsets. Removes the opamp offset at the same time.

author: mawyatt
author_rank: Super Contributor
timestamp: May 03, 2022, 12:55:26 pm
content: Quote from: Marco on May 03, 2022, 07:43:35 amThis is a subset of the mentioned synchronous approach. This method when implemented properly places a very narrowband filter around the synchronous frequency that very effectively removes any other "noise" sources, where "noise" indicates unwanted signals including offsets, mains, SMPS and so on. This narrow bandpass filter is nothing more than an integration capacitor placed after the synchronous sampling switch. Very effective technique from eons ago, was even used with vacuum tubes, and still effective todayBest,

author: Brage
author_rank: Contributor
timestamp: May 04, 2022, 07:07:29 am
content: Quote from: Doctorandus_P on May 02, 2022, 11:27:17 pmI'll adjust the resistance and add a switch. Thanks!

author: Brage
author_rank: Contributor
timestamp: May 04, 2022, 07:52:03 am
content: @mawyatt @MarcoI don't think I understand the approach you guys are suggesting.So I'll have something similar to the circuit attached, correct?With this circuit I'll create a step response trough Q101 and some known (relatively high) resistance, R102, and lastly the DUT. Having the parallel resistor R104 is optional or a disadvantage? All of this will charge C101 over time X.Then I'm guessing I'll measure the time it takes between the pulse trough Q101 is sent and to when the measurement node reaches a certain voltage.The time it will take is dependent upon how much resistance is between the emitter of Q101 and C101, and that is how I'm getting the measurement.Is this somewhat close to what you guys are suggesting?I can't say I understand what a synchronous approach means. Anywhere I can read more about this? Any good search words?

author: Marco
author_rank: Super Contributor
timestamp: May 04, 2022, 10:14:57 am
content: Send a current pulse through the resistor. ADC the voltage at a sample rate significantly over 1/T-pulse, discard some samples after each edge of the pulse giving the low pass filter output time to stabilise. Average samples for when the current is applied and when not, subtract the latter from the former.

author: mawyatt
author_rank: Super Contributor
timestamp: May 04, 2022, 12:37:27 pm
content: @Brage,If you search here on EEVblog you'll find a thread(s) on measuring low impedances (ESR and such), some use the synchronous method. It's also used in lab grade LCR meters. You can also do a general search (Google) on Synchronous Modulation and Demodulation.Basically the concept creates a stimulus, a pulse or sine wave, then samples the received signal in synch with the stimulus. The output of the synch-phase sample switch is integrated with a simple shunt capacitor which creates a narrowband filter around the stimulus. This can be implemented with a simple MOS switch that is gated by the stimulus signal and thus in "synch". Very simple to implement, requires almost no additional components, and very effective in use. This is exactly why this technique is utilized in lab grade LCR meters, and you can extract the phase of the received signal relative to the stimulus (basically measure complex Z with voltage and current synchronous measurements), another reason for lab LCR use.Best,

author: Marco
author_rank: Super Contributor
timestamp: May 04, 2022, 02:45:09 pm
content: It's possible to sync everything exactly to the pulse and integrate in analogue. Or you can just oversample and throw away a little data near the edges compromised by the low pass filter, for pretty much the same result. The former requires more hardware, the latter only a little software. I'd go for the software.

author: mawyatt
author_rank: Super Contributor
timestamp: May 04, 2022, 06:08:51 pm
content: Actually the synchronous method has the advantage of integrating with the shunt capacitor before the ADC which has benefits with out of band signal rejection before the ADC, not after. This can and does provide significant improvement in dynamic range where the ADC would saturate and produce highly non-linear behavior on large out of band signals, but these will be highly attenuated before the ADC with the synchronous method.Recently discovered is a extension of this technique utilizing multi-phase clocks to effectively sample the RF waveform in the Discrete Time Continuous Amplitude (DTCA) fashion. This works because the wanted RF waveform becomes somewhat synchronous with the effective sample rate of the input mixer or sampler, other signal don't and become significantly attenuated right at the antenna (think ~autocorrelation). This has now made it's way into many SDR and other commercial products. Created quite a bit of activity throughout the research world back when first reveled in ~2009. Here's a thread about this:https://www.eevblog.com/forum/rf-microwave/polyphase-or-n-path-mixer/Best,

author: Terry Bites
author_rank: Super Contributor
timestamp: May 07, 2022, 03:54:57 pm
content: seehttps://www.maximintegrated.com/en/design/technical-documents/app-notes/1/106.htmlWhat could be simpler?