Please Review - DIY Micro-Ohm Meter Circuit Schematic?  - Page 1
Forum section: electronics / general

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 25, 2025, 06:23:57 pm
content: The original schematic was pulled from this YouTube video (@19:28):https://youtu.be/atynxUcdRxY?feature=shared&t=1157My Falstad Simulation of my sightly-modified circuitis here.My Falstad Simulation of the modified circuitis here, but split-up for easier reading/understanding.How does this schematic look, of my DIY4-wire micro-ohm meter? The goal is so that every 1 ohm = 10v, and 0.00001 ohm (10 microohms) = 0.0001v (0.1mV). Any suggestions and/or fixes? I do know there's a ~50mV error/offset on the output, but I'm not sure where it's coming from. I was thinking of just using the OP07 to nul the output offset (after first calibrating the constant-current load to exactly 100mA), and use some low drift & low-offset op-amps for the other three.

author: Victorman222
author_rank: Regular Contributor
timestamp: April 26, 2025, 04:12:27 pm
content: Hello LooseJunkHater, looks like a fun project. Here are some nitpicksIn the current source you are loading your opamp by putting that 1k from the mosfet gate, also that 1k resistor with the 1k opamp output resistor forms a divider. If mosfet has a high enough Vgs requirement, opamp might not be able to provide it because of that 50% divider.What is the reasoning for the zener and capacitor on the mosfet gate? You might want a small capacitor from the opamp output to the inverting input for stability.You probably also want an isolation resistor from the inverting input to the shunt resistor.I see why you put the opamp buffers, since the 1 ohm (max) output impedance and the 1k? loading resistance of the difference amplifier do make a divider. But there might be a better way.A simpler(read-cheaper) way could be to make the difference amplifier stage have larger input resistors and less gain, after then have a simple gain stage.Like this.(if this is fine depends on what your goals are)If you don't mind having 3 opamps you can go the 3 opamp instrumentation amplifier approach. Even better proabably is to buy an instrumentation amp IC, because it will be more compact, cheaper and you don't have to match those diff amp resistors, INA827 looks ok for this app (if its ok depends on your requirements).Also the trimpot for the 100mA is not optimal setup for precision. Check how i did it in the example link. The (free) book Analog Seekrets by Leslie Green has a nice explanation at page 53. This fancy pot configuration costs 1 more resistor, but the pot TC is greatly attenuated.The current source config is interesting too, with the load resistor below the series pass mosfet, as bigger resistor is put in, voltage on it rises = mosfet source voltage rises, to keep the same Vgs opamp output rises. For small resistors that will have a small voltage drop this is fine probably.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: April 26, 2025, 05:24:04 pm
content: What is the supply voltage?You should use a low voltage to minimize the Mosfet’s heating.A while back I also built a portable milliohmeter, powered from 3AA cells.At such low voltage levels there won’t be enough voltage to properly enhance the Mosfet. I used therefore a bipolar transistor, in a configuration picked up directly from an AD8276 datasheet.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 26, 2025, 07:08:39 pm
content: Quote from: Victorman222 on April 26, 2025, 04:12:27 pmWoops, didn't notice that nor think of that.Quote from: Victorman222 on April 26, 2025, 04:12:27 pmThe zener was only present because it was in the original schematic; I assume it was to protect the FET-gate in-case of over-voltage, but it indeed is probably unnecessary.The resistor between the FET's-gate and the sense line was only placed there because I've seen it done in many other constant-current loads; if I recall, it's to improve stability, but maybe I'm wrong?Quote from: Victorman222 on April 26, 2025, 04:12:27 pmThis is beautiful, I didn't even think of this! Probably better to have 1 less op-amp, to reduce any noise and/or failure points (I'd think? I'm a noob). I'll need to look more into instrumentation amplifiers and see if I have any in my stock.Quote from: Victorman222 on April 26, 2025, 04:12:27 pmI've used my-style of trimpot in the past for other constant current setups without any problem, but I really love the increased granular-control that your trimpot design provides, so I'll be using it!Quote from: UnijunctionTransistor on April 26, 2025, 05:24:04 pmThis is a good idea; no point loading the FET with a high voltage, as it'll simply waste that energy as heat.I'll look into my stock of BJT's and see if I have any that'll work for this build. Kinda like the idea of using a FET for the (slight) improvement in efficiency vs a BJT, plus I have dozens of types of FET's, but I'll look into the BJT option, as it may be simpler.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 26, 2025, 07:14:50 pm
content: Quote from: Victorman222 on April 26, 2025, 04:12:27 pmLooking into your circuit a bit more, I noticed you left the 50nF capacitors (on the center op-amp) disconnected; was this intentional or an accident? I was thinking the 50nF capacitors would likely help with stability (see: yellow arrows, in the image below)?Additionally, I'm not entirely sure what the purpose is of placing the voltage rails + diodes on the sensing lines (see: blue arrows, in the image below). Is this a replacement for the zener diode protection?

author: Victorman222
author_rank: Regular Contributor
timestamp: April 26, 2025, 07:46:43 pm
content: Quote from: LooseJunkHater on April 26, 2025, 07:14:50 pmSorry, was writing that in a hurry and missed some points. The 50nF capacitors here make a low pass filter, it's a good idea to have them in real life, as they will filter some noise. But for DC simulation they aren't necessary (They slow simulation down in falstad, i got tired of waiting). In a fancier simulation program you could make an AC simulation to see the frequency response of it.Quote from: LooseJunkHater on April 26, 2025, 07:14:50 pmThis is input protection, the idea is if the input is a diode drop above the highest supply or a diode drop below the lowest supply it will get clamped by those diodes. The chips have the same thing internally, but the internal diodes are small and can only handle some 10mA of current.Comparing this to the zener protection from original schematic, this better protects the inputs if the circuit is off. If positive and negative supply are 0V, then input can only go +- a diode drop before it starts "powering" the circuit through those diodes.But on second look, this protection won't work if a stiff voltage source is connected to the sense lines (there is no resistance to limit current thru diodes). And since the sense IC inputs are behind large resistors, so current to their internal diodes will be small, it is probably fine to not put any protection. The inverting input of the current regulator opamp is however has a 1k resistor from input, so probably want to increase that to 10k. This way it can tolerate some +- 15V at the input terminals when powered off. This is schematic with the changed resistor and removed protection diodesLink

author: Victorman222
author_rank: Regular Contributor
timestamp: April 26, 2025, 08:19:02 pm
content: What are your design targets for the MicroOhm meter?This will help figure out if the sketchy two opamp circuit is fine or if proper amplifier is necessary.What you drew with 3 opamps is similar to the proper 3 opamp instrumentation amp, just that in the 3 opamp in-amp the input "buffers" also have gain.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 26, 2025, 09:04:03 pm
content: Quote from: Victorman222 on April 26, 2025, 08:19:02 pmI'm kinda loose/flexible on the design targets; at the very least, I'd like to be able to measure in the 5ohm to 1 milli-ohm range, but the lower the better (I'd bevery happyto be able to measure 0.5mOhm resistors). When I was running your earlier circuit, it looked fine down to around 25mOhm (with ~50mV offset), when changing your ideal op-amps to the LM324/LM358. I'm not sure at what point (10mOhm and less, maybe?) I'll start to need to worry about temperature drift effecting the repeatability and reliability of measurement, but this has been on my mind since the start, hence avoiding resistors above 100k ohm.To be honest, I was kinda debating on just creating this circuit today using crappy LM358/NE5532, 1% TL431, 5% carbon resistors, and trying to nul out any offset using trimpots on In+/In-, just to see "how good" this circuit can be as-is.

author: Victorman222
author_rank: Regular Contributor
timestamp: April 26, 2025, 10:08:29 pm
content: What is the lowest range of your multimeter? My handheld 3.5 digit DMM has a 60mV range. With a current source of 100mA and the DMM having a resolution of 100uV, resistance measurement with current source and DMM will have a resolution of 1 mOhmSo maybe it would be reasonable to split this to two projects, a current source and a microvoltmeter? Having both in one case has its benefits too probably..Quote from: LooseJunkHater on April 26, 2025, 09:04:03 pmTemperature drift will affect all your measurements. All the resistors have TCR - because of this gain of amplifiers will drift, the current shunt is also sensitive, as its resistance changes output current will change.Opamps have offset voltages, and those offset voltages drift with temperature too.Of course the reference drifts with temperature too.When measuring smaller voltages your readings might start to get drowned out by the noise, that might be what you were referring to. For example OP07 has a 0.1-10Hz voltage noise of 0.6uV p-p and 0.1-10Hz current noise of 30pA p-p.If we are using it as an amplifier by 100 with an input impedance of 100kOhm, then the resulting voltage noise is sum of voltage noise and voltage drop from the current noise on the input resistor. Voltage noise is modeled as a voltage source in series with the non inverting input. So it (and the voltage on resistor from the input current noise) gets amplified by the gain.Thus: voltage noise effect will be 0.6uV*100=60uV p-p and current noise effect will be (30pA*100kOhm)*100=3uV*100=300uV p-p. Resulting noise at the output is 360uV p-p in 0.1-10Hz band.Sometimes it's easier to work with input referred noise, as its more useful to see what kind of input signal will be "hidden in the noise" or not. Input referred noise is calculated the same as output referred noise, just without amplifying by gain.We can see that 100k source resistance is a bit high for OP07, noise from the input current starts to dominate. So here would be better another opamp with smaller input current noise, lower the input resistance or choose a different circuit.Lets compare the amplifiers in this thread assuming OP07 opamps and using referred to input noise:Your original 3 opamp one: two buffers add 0.6uV p-p each. Then 100x differential amplifier with 1k input resistance per input makes 0.6uV+(30pA*2kOhm)=0.6uV+0.06uV=0.66uV. And total is 1.86 uV p-p RTI noise.Sketchy 2 opamp one i proposed: the unity gain diff amp with 100k input resistors - 0.6uV+(30pA*200kOhm)=6.6uV and the 100x inverting amplifier with 1k input resistor 0.6uV+(30pA*1kOhm)=0.63uV, in total we have 6.6+0.63=7.23 uV p-p RTI noise.The original circuit has lower noiseAlso with small voltages thermalEMF becomes a problem. Basically - all dissimilar metal contacts are thermocouples and produce some uV voltage if there is even the slightest temperature difference.

author: croma641
author_rank: Regular Contributor
timestamp: April 26, 2025, 11:19:08 pm
content: The most correct way to measure micro-ohms, even for low power shunts, is to use a lock-in amplifier. Try to study carefully this circuit, published by EDN.https://www.edn.com/complete-low-cost-software-programmable-ohmmeter-measures-micro-ohms/Another way to build a micro ohmmeter is to create a software lock-in. Here is a complete, tested and accurate circuit that achieves this purpose.https://www.pittnerovi.com/jiri/hobby/electronics/milliohmmeter/index.htmlBoth circuits are worth making. The first one is easier to make, just a few lines of program with any Arduino and you're done.

author: Overspeed
author_rank: Frequent Contributor
timestamp: April 27, 2025, 09:12:48 am
content: Quote from: Victorman222 on April 26, 2025, 10:08:29 pmHellothat why an acceptable current level is important with very low resistor to be able to make the difference between thermalEMF and the real voltage measured from the resistor DUTRegardsOS

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 29, 2025, 03:09:53 pm
content: Quote from: croma641 on April 26, 2025, 11:19:08 pmHad no idea these projects existed, thanks for the sources!

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 29, 2025, 03:34:25 pm
content: Quote from: Victorman222 on April 26, 2025, 10:08:29 pmIf I recall, my HoldPeak HP-770D can somewhat-accurately measure down to 1uV. While I could just use my multimeter, I think it would be more fun to have an amplification of 10x or 100x, so that my cheaper multimeters will also be able to easily read low resistances, hence this project.Quote from: Victorman222 on April 26, 2025, 10:08:29 pmI really, really appreciate you doing the math for this. It got me thinking that maybe I should go back to the original design from the YouTube video, which incorporated simply a 10x gain. Last night before I fell asleep, I was messing around withyour earlier Falstad circuit, as well asmy earlier Falstad circuit. I noticed some significant non-linearity which occured from 1 Ohm - 1m Ohm from my circuit (which appeared unrelated to input voltage offset), but I couldn't figure out how to remove/fix the significant non-linearity. It looked like your circuit instead didn't experience it because of the use of ideal op-amps.I began thinking about your previous comment of going with aninstrumentation amplifier. While I could theoretically make one myself using standard op-amps (ex. NE5532's or TL072's), reading theAD620 datasheet, on page 13, it showed how I could make my own instrumentation amplfifier...... If I had 0.02% matching resistors.I don't think I have the ability to actually find resistors with such matching tolerances in my parts drawer, buying those resistors would be unrealistic, andI'd imagineusing potentiometers to make such resistances would introduce too much noise/temperature drift.I've sorta given-up on trying to use discrete op-amps and will now start to look into some discrete instrumentation amplifiers.For those curious or following,here's my most-recent Falstad circuit.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 29, 2025, 04:59:18 pm
content: For the application with a seprate reference for the current and DMM one would not absolutely need a INA. As shown (with a common ground) one would like good common mode suppression to really measure to small resistors in the mOhm an below range. This would want an INA and not the initial circuit with buffers and difference amplifier, that has pretty poor CMRR (set by the resistor matiching).The INA circuit with gain already from the first stage is less critical with the resistor matching, as the initial gain is added before the CMRR.One can get around much of the CMRR problem by having the output ground not the same as the ground of the current source, but from the voltage sense low terminal / wire. In this case a simple 1 OP-amp amplifier could be enough - if wanted with an extra non critical inverter to compensate the current from the gain network. For the overall gain it would be enough to have 1 trimmer. So no need to adjust both the current and the voltage gain.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: July 07, 2025, 03:41:15 pm
content: Been a while since I last replied to my own thread, but I've recently beenlearning about 1/f noise of opamps, which relates to greatly amplifying DC signals.For the DIY Micro-Ohm Meter Circuit, I'm thinking of using theAD8227 instrumentation amplifieror theOP07, as was shown in the YouTube video that I linked earlier (https://youtu.be/atynxUcdRxY?feature=shared&t=1157). HOWEVER, both opamps suffer from some 1/f noise (0.1 - 10 Hz), with the AD8227 having 0.5 μV p-p (nominal; gain of 1000), and the OP07 having 0.65 μV p-p (max) and 0.38 μV p-p (nominal).Question:Will this level of 1/f noise make a significant difference for the measurement of micro-ohms, or will the actual measurement still be above that noise floor?Additionally, I'm thinking of changing the range switching to the method showed byAnalog Devices.Question:Will my method or the method shown by Analog Devices lead to more accurate measurements?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 07, 2025, 06:06:39 pm
content: With a constant signal path (e.g. fixed test current, no extra AZ switching) the 1/f noise does matter for the resolution. It can be the limiting factor, but there can also be other similar low frequency fluctuations from thermal EMF.The range switching as suggested by Analog divices is not providing extra performance advantage, but is quite simple.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: July 07, 2025, 06:17:12 pm
content: Quote from: Kleinstein on July 07, 2025, 06:06:39 pmWhen doing some recent experimenting with constant current circuits (which I got help from inthisthread), I witnessed quite a bit of thermal drift due to the current sense resistors heating up. I had heard of thermal EMF before but I'm not very familiar with it, along with Johnson-Nyquist noise, so I guess I'll need to look more into these forms of electrical noise and how they are similar & different. Additionally, I guess I'll also need to look into whether (and how much) these 3 types of electrical noise will effect this circuit...Thanks!Edit:Just in case anyone (or myself) comes across this thread in the future, it appears that there's a simple resistor calculator online which shows that as frequency increases, Johnson-Nyquist noise in a resistor increases. The way to then decrease that Johnson-Nyquist noise again would be to utilize smaller-value resistors. This is problematic, as smaller value resistors will then produce more heat, which can cause a drift of resistance due to the thermal drift...Temperature changes appears to have a veryminoreffect on the production of Johnson-Nyquist noise.Very, very interesting effect! To me, this also helps me understand why smaller-value resistors are generally preferred in audio applications; the frequency (1Hz-20Khz) is primarily unchanged, therefore to decrease the Johnson-Nyquist noise, lower value resistors are preferred. I primarily see resistors in the 1k-100k range, and rarely higher in audio applications.https://www.daycounter.com/Calculators/Thermal-Noise-Calculator.phtml

author: JohanH
author_rank: Frequent Contributor
timestamp: July 07, 2025, 06:42:36 pm
content: Quote from: LooseJunkHater on April 29, 2025, 03:34:25 pmMaybe unrealistic if you talk about through hole components? But small thin film SMD resistors have become quite affordable. A quick search found some 10k 0.01% 5ppm RNCF0603TKY10K0 1.30 €/each.I used 1k 0.02% 10ppm RG1608N-102-P-T1 in a project of mine recently, just 0.73 € each. No problem soldering 0603 (1608) size components with some magnification.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: July 07, 2025, 06:49:49 pm
content: Quote from: JohanH on July 07, 2025, 06:42:36 pmAt 1.30 €/each resistor, then it would definitely make more-sense to use a low noise instrumentation amplifier for this project (using DC current, and measuring the DC voltage drop, to determine the Milli/Micro-Ohm resistance).If I needed an instrumentation amplifiers precision for high frequency applications, then going with those high-precision and high-cost resistors makes sense.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: July 07, 2025, 06:58:57 pm
content: Quote from: Kleinstein on July 07, 2025, 06:06:39 pmThinking about this more, it seems like there would be tradeoffs between the two implementations.With my implementation, I'd need to use trim pots to get precise op-amp gains (ex. 1% 1k resistor for setting gain, and then a 10-100 ohm trimpot for fine-tuning the gain), but then these trimpots would suffer due to the high thermal coefficient drift of most trimpots (from environmental temp fluctuations; mostly 250ppm and much higher, from what I've seen) + noise.With the Analog Devices implementation, trying to precisely trim multiple current sense resistors myself would be incredibly very difficult. There do exist low PPM (less than 25ppm) current sense resistors, but they can be quite expensive.There's probably also other aspects of the two design implementations that I'm not thinking/aware of.....

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 07, 2025, 07:25:24 pm
content: The way to trim resistors is never in series. Set it up so the resistor to trim is very slightly high. Put a high value resistor and series trim pot in parallel with the main resistor. Slight changes in the pot or even the parallel fixed resistor will have near zero effect.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: July 07, 2025, 07:26:33 pm
content: Quote from: Conrad Hoffman on July 07, 2025, 07:25:24 pmI've never heard of this; why?Quote from: Conrad Hoffman on July 07, 2025, 07:25:24 pmLol nevermind, that makes perfect sense! Thanks!

author: JohanH
author_rank: Frequent Contributor
timestamp: July 07, 2025, 07:40:47 pm
content: Quote from: LooseJunkHater on July 07, 2025, 07:26:33 pmSee for instance Linear Technology, Application Note 42, page 30.

author: JohanH
author_rank: Frequent Contributor
timestamp: July 07, 2025, 07:45:59 pm
content: Actual link to the document:https://www.analog.com/media/en/technical-documentation/app-notes/an42.pdfLots of good info about resistors on page 24 and forward and trimming techniques, page 29.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: July 07, 2025, 09:00:44 pm
content: Quote from: JohanH on July 07, 2025, 07:45:59 pmTook a bit of a peak at that Analog Devices document, and wow, that's an amazing document that I'll try to read later today. I've been reading a few Application Note documents recently and they're super interesting. It's leading me down a rabbit hole and wanting to make so many more circuits....Edit:Reading the AN42 document, LT1029 datasheet, messing withthis simple Falstad circuitto create a positive + negative voltage rail from a single battery, and also readingthis blog post about the TL431has been super interesting. Ended up stumbling upon how "simple" zener diodes can actually be used as pretty damn good voltage references when the temperature coefficients cancel out (around5.6v zener). Combining the zener diode with a precision op-amp can then allow for a quite stable, variable reference voltage.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 09, 2025, 03:48:40 am
content: Quote from: LooseJunkHater on July 07, 2025, 09:00:44 pmZeners are still used, for high end references like TI's REF80https://www.ti.com/product/REF80For more discrete use, Zeners have some issues :The tempco null is quite variable, and using a zener alone the TC null is on a softer part of the curve.Most designs use a Zener and a diode, or better, a Zener and a diode plus tempco gain opamp.The tolerance is not great, whilst you can now get 1% Zeners, they do not come as dual parts.Dual parts like PLVA2668 are spec'd at lower zener bias, for lower power and self heating, and ~ 3% tolerance.The thermal tracking of the dual parts is not well defined, packages are usually poor thermallyA triple Zener in SOT363 allows 2 TC diodes to wrap the zener, but those are 5% tolerance.The manually found null is a difference of two tempcos - so single digit ppm/K is the best you can hope forAll of that means you are better in 2025 to pick a good quality, single VREF part, that can give you0.05% calibration, typical 2.5ppm/K and max 6ppm/K for sub 100uA and ~45c

author: ivo
author_rank: Regular Contributor
timestamp: July 11, 2025, 04:02:27 pm
content: For the difference amplifier and similar circuits, what matters is not absolute matching of resistors, but matching of relative values and ratios. In numbers, this means you do not need to find an absolute 10.0000k resistor, it just means if you use a base resistor of any value X, your other one should be of value (9.000 * X) or (10.000 * X), etc. It's perfectly fine to use a 9.857k resistor, if you found its soulmate in a 98.57k resistor in your batch of 100k's. Most DMMs have pretty excellent linearity of their measurements in this regard. If you have time to spare, then this is quite achievable from buying 20 or 200 cheap resistors, noting down all their nominal values, and simply finding the best integer ratio matches, probably in a spreadsheet or similar. Devoting all this time to selection is imminently viable for the hobbiest, but not viable for automated production of a commercial part (unless very specialised).For a discrete INA, IMHO a flexible design would use a nice modern chopper opamp for the two buffers (OPA2189, ADA4522-2, etc), and then using a single OP77/177/277 at the end, because its offset trim pins can be used to achieve a perfect zero. You already discovered the more flexible resistor arrangement in another post, which allows to change just a single resistor to change the gain (meaning you could switch between 10x and 100x gain if you fancied).Voltage reference zener diodes have a special trick up their sleeves: they are usuallyburiedzeners. This means that the active P/N silicon junctions are "buried" inside the fabricated silicon (other layers are put on top of them), which allows for much quieter 1/f noise from the zener. This is a big reason they are used in most of the best voltage references; and also a reason why replicating them with a discrete 'normal' zener diode will never get you 100% of the way to that performance (apart from all the magic the designers incorporate in tuning the rest of the chip). Although it can still be fun to do so. I have a couple of 1N829A's I need to do something with...

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: August 09, 2025, 11:16:17 pm
content: Quote from: croma641 on April 26, 2025, 11:19:08 pmBeen exploring various different micro ohm meter designs again and decided to spend some time trying to understand this circuit. Initially I was quite off-putted due to the complexity, but it seems relatively simple. However, it's decently expensive, at around $100USD for just the 5 IC's (ADG436 = CMOS switch, AD8228 = Instrumentation amp, AD8253 = Programmable Gain Instrumentation amp, AD630 = Balanced Modulator/Demodulator, AD820 = Op-amp).I'm wondering if some components could actually be replaced/swapped:- ADG436 --> push-pull driver + FET- AD8228 + AD8253 --> Single instrumentation amplifier (ex. AD8227)- AD630 --> ?? (not familiar with Modulator/Demodulator IC's. Seems to be related to RF?)- AD820 --> TL071 (as it has voltage offset correction support + similar offset + bias currents)I'm also thinking that the low 1 mA current pulse could be easily increased to 10 mA or 100 mA, for better resolution of low-value resistors. If a 1% duty cycle 1KHz current pulse is used, I'd think tempco effects would minimally come into play.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 10, 2025, 02:30:09 pm
content: The circuit indeed uses rather expensive parts - more like application note circuits. The AD630 is more an RF part not so much for precision and low frequency. There are cheaper ways with CMOS switches (like HC4053). It gets a bit more complicated but can also be done with a duty cycle different from 50%.There is now also the alternative of not doing analog demodulation, but digitize the signal and do the difference digitally. This is what some better DMMs do with offset compensated Ohm mode.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: August 10, 2025, 03:26:35 pm
content: Quote from: Kleinstein on August 10, 2025, 02:30:09 pmCould you elaborate a bit more on these two points (CMOS switch to replace the AD630, and the duty cycle), to point me in the right direction for further research?Quote from: Kleinstein on August 10, 2025, 02:30:09 pmMaybe another source for me, please? I'm not sure what most of this means, but would love to learn about it._________________________When I first started researching this "DIY Micro-Ohm Meter Circuit", I wasn't aware of the influence that tempco would have on the circuit, hence why I'mnowtrying to find ways around its influence. Ultra-low tempco current sense resistors are expensive, and DIYing a current sense resistor seems futile (bare copper wire appears to have pretty high tempco, and alloyed metal wires with low tempco appear to be difficult to find, or expensive).

author: Kleinstein
author_rank: Super Contributor
timestamp: August 10, 2025, 07:23:48 pm
content: For the CMOS switch modulator an example circuit is found here:https://ee.iitkgp.ac.in/assets/file/TeachingLabs/Instrumentation/Analog%20Multiplexer.pdfThe circuit is a kind of standard circuit.To get the amplitude of the AC part one needs either one phase at zero or an average zero voltage, like from AC coupling and a 50% PWM ratio. With a non 50% PWM ratio one would still get some DC level feed through. In this case one would more use a circuit like in the Taloe mixer with averaging for the on and off phase separate and than take the difference of the filtered voltages.The digital way is by digitizing the signal and than take the averages and differences in the digital domain. The amplifcation can be AC coupled to suppress DC drift even for a short duty cycle. Part of the ADC resolution may be lost to headroom to allow for some main hum and similar interference signals. It depends on the environment if one needs or wants extra suppresson of mains hum.How critical the current sense resistor or reference resistor the measurement is relative too depends on the wanted resolution. A DIY solution may be possible in 2 ways. There are Manganin wires available hat have a reasonable low TC, but testing and verifying this is extra effort.Another DIY solution could be combining resistors in parallel or series to get compensation in the TC. So a suitable mix of positive and negative TC.With higher current also the cooling part can become relevant.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 10, 2025, 07:52:12 pm
content: Have you seen the Analog Devices AN306 (attached at the end of this post):https://www.eevblog.com/forum/projects/esr-evaluation-and-verification/msg4081948/#msg4081948? They are using synchronous detection.