Author Topic: Nice little Milliohm Meter over at Circuit Cellar  (Read 10097 times)

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Offline MisterDiodes

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Nice little Milliohm Meter over at Circuit Cellar
« on: September 06, 2016, 10:36:58 pm »
For those of you who  you haven't seen it yet, there is a nice DIY milliohm meter article in the Sept 2016 issue Circuit Cellar.  Worst case error budget is around ~0.5%, which really isn't too bad for an inexpensive milliohm meter.  Accuracy drops off at the low end of range, as is usual - but at a few mOhm's it becomes difficult to attach leads to just about anything for a repeatable measure.

My suggestion when using this circuit on lowest end of range is to add a known accurate 50 or 100 mOhm into your circuit and then take your measure from that known offset. 

Handy for tracing out shorts and bad bypass caps too.

Code: [Select]
http://circuitcellar.com/cc-blog/build-an-accurate-milliohm-meter/
« Last Edit: September 07, 2016, 12:10:43 am by MisterDiodes »
 
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Offline Vgkid

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Re: Nice little Milliohm Meter over at Ciruit Cellar
« Reply #1 on: September 06, 2016, 10:54:45 pm »
Thanks for that link, I will look into it tonight.
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Offline EmmanuelFaure

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #2 on: September 08, 2016, 09:30:43 pm »
Very nice. The idea of driving the "force" leads by sharing current between several IOs is clever.

It might be built with a greater accuracy ADC (MCP3553, LTC2440...), requiring only some minor adjustments in software, to achieve even lower noise.
 

Offline casinada

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Offline VintageNut

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #4 on: September 30, 2016, 12:47:34 pm »
Offset compensation? If not, source(s) of error are large when measuring low ohms.
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Offline Kleinstein

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #5 on: September 30, 2016, 01:42:08 pm »
They are using 4 wire measurement and test current of both polarity: so there should be not much problem with offsets. The circuit has other problems:
1) it may burn small resistors, as it might provide up to about 80 mA to a 30 Ohms resistor, thus close to 200 mW worst case. Good software or 2 additional diodes could ease on this.

2) The ADC and µC have little ESD protection
 

Offline RoGeorge

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #6 on: September 30, 2016, 02:18:44 pm »
It reminds me of a µOhm-meter Application Note from Analog Devices:
http://www.analog.com/media/en/technical-documentation/application-notes/AN-306.pdf
 
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Offline Kleinstein

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #7 on: September 30, 2016, 03:18:18 pm »
There is some similarity in using both polarities, but the µC version uses a bipolar ADC and does demodulation in software, that is just measure the difference for both polarities. The main trick of the circuit is to have the essentials (switchable amplification and differential input) inside the ADC chip.

Details depend a little on the software, in how to compensate for slightly different currents in both polarities and how to wait for settling of the RC filters and maybe thermal drift of the driver side. The short description notes using each polarity only once. For better drift compensation it should be better to do at least 3 parts, like positive, negative and than positive again. Using only a few of the drivers might also save low power resistors in the 30 Ohms range from to much current.
 

Offline mycroft

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #8 on: October 01, 2016, 02:59:00 pm »
There is also a milliohmmeter by the late Jim Williams at http://cds.linear.com/docs/en/application-note/an98f.pdf pages 10 and 11.
 
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Offline mycroft

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #9 on: October 03, 2016, 04:17:27 pm »
I found an older thread on this interesting subject: A mcu-based milliohm meter https://www.eevblog.com/forum/projects/a-mcu-based-milliohm-meter/ using simple resistors and a MCU.
 
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Offline Dr.Krieger

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #10 on: June 16, 2022, 08:13:43 am »
For those of you who  you haven't seen it yet, there is a nice DIY milliohm meter article in the Sept 2016 issue Circuit Cellar.  Worst case error budget is around ~0.5%, which really isn't too bad for an inexpensive milliohm meter.  Accuracy drops off at the low end of range, as is usual - but at a few mOhm's it becomes difficult to attach leads to just about anything for a repeatable measure.

My suggestion when using this circuit on lowest end of range is to add a known accurate 50 or 100 mOhm into your circuit and then take your measure from that known offset. 

Handy for tracing out shorts and bad bypass caps too.

Code: [Select]
http://circuitcellar.com/cc-blog/build-an-accurate-milliohm-meter/

Does anyone have the code from the original article?

There is slightly modified version of it here but i can't find the original Circuit Cellar code anywhere...  :(
 

Offline kripton2035

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #11 on: June 16, 2022, 09:47:59 am »


Quote>Does anyone have the code from the original article?

this should be the one attached.
 
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Offline Dr.Krieger

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #12 on: June 16, 2022, 11:35:40 am »
this should be the one attached.

Thank you sir, although I'm not 100% sure this is the one, the battery voltage measurement seems to be for the original 9V but the Arduino pins used in the code are not the same with the ones shown on the original schematic:

Code: [Select]
// ---------------------------------------------------------------------------------------------
//  I/O port usage
// ---------------------------------------------------------------------------------------------
//    serial port (debug and s/w download)    0, 1
//    I2C interface to ADC & display          2, 3
//    positive drive                          4, 5, 6, 7
//    push to test input                      8
//    unused                                  9, 10, 11, 12, 13
//    negative drive                          A0, A1, A2, A3
//    battery voltage monitor                 A4
//    debug output                            A5

« Last Edit: June 16, 2022, 11:37:26 am by Dr.Krieger »
 

Offline kripton2035

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #13 on: June 16, 2022, 06:04:29 pm »
I downloaded the article AND the source code that was with it at that time (some years ago)
so I'm pretty sure it is the one that was with it
then I took from it the omega symbol and made the shorty-with-display project
http://kripton2035.free.fr/Projects/shorty-display.html
 
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Offline Dr.Krieger

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #14 on: June 16, 2022, 11:56:27 pm »
I downloaded the article AND the source code that was with it at that time (some years ago)
so I'm pretty sure it is the one that was with it
then I took from it the omega symbol and made the shorty-with-display project
http://kripton2035.free.fr/Projects/shorty-display.html

I think you are right and that is the original code, although the Arduino pins in the code are different from the ones used in the circuit pic.

Schematic shows pins D2,3,4,5 for Positive Drive and D6,7,8,9 for Negative Drive, A3 to measure the battery etc, but original code uses D4,5,6,7 for Positive Drive, Analog pins  A0,1,2,3 for Negative Drive and A4 for battery voltage.

Code: [Select]
// ---------------------------------------------------------------------------------------------
//  I/O port usage
// ---------------------------------------------------------------------------------------------
//    serial port (debug and s/w download)    0, 1
//    I2C interface to ADC & display          2, 3
//    positive drive                          4, 5, 6, 7
//    push to test input                      8
//    unused                                  9, 10, 11, 12, 13
//    negative drive                          A0, A1, A2, A3
//    battery voltage monitor                 A4
//    debug output                            A5
Code: [Select]
//  Drive functions
//   - ports 4-7 and A0-A3 are used to differentially drive resistor under test
//   - the ports are resistively summed to increase current capability
//   - DriveOff() disables the drive, setting the bits to input
//   - DriveOn()  enables the drive,  setting the bits to output
//   - DriveP()   enables drive with positive current flow (from ports 4-7 to ports A0-A3)
//   - DriveN()   enables drive with negative current flow
// ---------------------------------------------------------------------------------------------
void DriveP()
  {
    DriveOff();
    digitalWrite( 4, HIGH);
    digitalWrite( 5, HIGH);   
    digitalWrite( 6, HIGH);   
    digitalWrite( 7, HIGH);
    digitalWrite(A0, LOW);
    digitalWrite(A1, LOW);
    digitalWrite(A2, LOW);
    digitalWrite(A3, LOW); 
    DriveOn();
  }

The modded code seems to use the correct "schematic pins" in the actual code but the comments are presumably left unchanged and there the pins are the same "wrong pins" that the code you uploaded uses, so i guess there is some kind of error in the original article...

Code: [Select]
//  Drive functions
//   - ports 4-7 and A0-A3 are used to differentially drive resistor under test
//   - the ports are resistively summed to increase current capability
//   - DriveOff() disables the drive, setting the bits to input
//   - DriveOn()  enables the drive,  setting the bits to output
//   - DriveP()   enables drive with positive current flow (from ports 4-7 to ports A0-A3)
//   - DriveN()   enables drive with negative current flow
// ---------------------------------------------------------------------------------------------
void DriveP()
  {
    DriveOff();
    digitalWrite( 2, HIGH);
    digitalWrite( 3, HIGH);   
    digitalWrite( 4, HIGH);   
    digitalWrite( 5, HIGH);
    digitalWrite( 6, LOW);
    digitalWrite( 7, LOW);
    digitalWrite( 8, LOW);
    digitalWrite( 9, LOW); 
    DriveOn();
  }

In any case between the two codes i am sure i can make this work, my main issue was that i prefer the 9V battery version and i didn't want to mess with the modded one only to find that there were more changes (he already mentions some changes about how "overload" is displayed) and i would have to troubleshoot the thing (the page uses the original schematic and not the one he actually made).
I think that the original code with the modded pins to match the schematic will work perfectly!


BTW isn't it a strange coincidence that the reason i am making a milliohm meter is that i saw YOUR "Shorty" build ???
I just ended up Googling for more "Arduino Milliohm meter" circuits after i saw yours and after a couple of candidates i decided on the Circuit Cellar one because i already have the ADS1115 module in my stash (i don't have to buy AD8628/MCP3421) and it seems like a easier build.
I still plan to use your clever "poor man's Kelvin probes", already ordered cheap eBay probes that i hope are the correct type (tip unscrews so i can solder headphones wire).
Only thing that i would like in the Circuit Cellar circuit is a "zero" button you could press with the probes shorted to improve the measurement a bit more, but i can live without it (or even able to add it to the code?).

Anyways, thanks again for the code :)
 

Offline Dr.Krieger

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #15 on: June 26, 2022, 07:57:54 pm »
Well the original code with altered pins (like in the Li-Ion version code) works just fine like i suspected, but there is a big issue:

The Meter is painfully slow, it needs about 5-6sec to measure on the breadboard and I don't believe it will be any faster on a PCB...
Ever with 1/10 of the samples of the original code and "continuous" operation without waiting for a button press, its still not fast and probably inaccurate too in this configuration...



"faster" test:
https://youtu.be/iX1K5Oj_yUk

Maybe it can be improved by ditching the optimal gain finding routine and sacrifice the high-value resistor measuring for speed (switch from single shot to continuous mode), in my breadboard tests i used 8x100Ω resistors for ~67mA max (at short with only the 10.0Ω in the loop) @4.140V, that is enough to measure up to 20-30 or even 50Ω depending on the setup, you could even lose the 10Ω altogether and just use a Constant Current Source (like a two BJT Current Mirror or a simple TL431+BJT setup) so you only need to measure the DUT's Vdrop (still in both current directions) if that helps with speed, i don't know anything about ADCs...

I now kinda regret not going for kripton2035's Shorty, which is the original device i saw and made me want to make a sub-Ohm meter and it seems to work fine in the videos (looks both fast and accurate in the "solder video") but I'm not 100% sure about the device since the circuit seems to not use a CCS and only measures the DUT's Vrop (no current measurement or a 2nd "reference" resistor measurement to compare) so by design it shouldn't be accurate...
I did a few fast calculations and current changes ~0.4mA between a 0.1 and a 1Ω DUT, that is a lot for a 5mV measurement... :/
Also it would only take one more PNP to convert the circuit to a Current Mirror so that schematic seems more like a temporary setup to sort things out and not a "final" project.

Do note that i am only a novice so if i wrote anything wrong, please correct me :)

In conclusion i still don't know in which direction i should go to, trying to improve Circuit Cellar's design seems futile since i don't know much about Arduino code or ADC's, kripton2035's Shorty seems like an unfinished project with at least one major fault that totally undermines the accuracy and i don't want to wait two weeks for parts only to find that the thing "needs work" that i am not able to do, and other designs are either too simple (to be reasonably accurate for <1Ω values) or way too complicated for my taste (like Scullcom's)...

Anyone has any thoughts about the issue?
« Last Edit: June 26, 2022, 08:01:27 pm by Dr.Krieger »
 
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Offline kripton2035

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #16 on: June 27, 2022, 10:02:51 am »
I would try an esp8266 or esp32 in place or the arduino, they will be faster and may be enough for this circuit cellar meter.
then, my shorty-with-display is certainly as you said not accurate as there is no precision resistor for calibration.
but what's matters is relative continuity, so you zero the meter at each powerup and the relative measurment is quite enough
certainly for short circuit finding it's more than enough you need "only" 0.1mΩ detection. 1mΩ is not enough.
 

Offline Dr.Krieger

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #17 on: June 27, 2022, 11:09:40 am »
Thanks for your input, truth is that i don't need 1mΩ accuracy or resolution, but i would like to be able to use this as a low-value resistance meter, like being able to measure somewhat accurately the real value of a 0.1/0.22/0.33Ω 5W resistor like i had to for this project* or a 50mΩ Shunt, something that my cheap UNI-T 61E DMM can't do, so the ideal range would only have to be up to 10-100Ω as my DMM can take it from there.

Like you said for a "shorty" device that finds shorts on a PCB you only need relative accuracy, you could argue that even just the buzzer would be good enough.

Now that you mention it i vaguely remember about faster Arduino-compatible clones like Teensy and it sounds like a good idea, if the Arduino's speed is the issue and not the ADS1115 chip and it's being run at single-shot mode, i am not exactly sure what is the issue as i haven't really messed around with the thing besides that very quick "1 cycle @ 10 samples and no wait for button press" test, i have to try more things with it since i still got in on the breadboard i guess.

I think that seeing how fast it runs on videos, your design would be a better place to start though, just replace the single PNP with a true Constant Current Source current mirror and maybe change the amount of current through the resistors since in your design the current constantly flows though the 82/2/1 resistors even if the Kelvin clips are not shorted and see how that works or strait-up call it "good enough" and be done with it.

For now i think i will mess around with the Circuit Cellar circuit since i have it working on the breadboard, but i will order a couple of AD8628/ MCP3421 or even an Int Amp instead of the OpAmp that works with single supply (INA213 for example has a fixed 50 gain) so i don't have to match two sets of precision resistors.


*I had everything except the 10Ω precision resistor and didn't feel like paying 15€ postage for just that so i used a typical +-10% 10Ω/5W white cement resistor that i measured as accurately as i could using a 50-100mA CCS (measured both current and voltage drop on the 10Ω at ~55mA and then again at 100mA to verify) and replaced the "Rr = 10.0" in code with "Rr = 10.747" which was good enough for breadboard testing...
« Last Edit: June 27, 2022, 11:34:26 am by Dr.Krieger »
 

Offline Dr.Krieger

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #18 on: June 28, 2022, 05:40:42 am »
Update:

Actually with 3 Cycles / 2 samples per half and no waiting for a button press its not that bad, i measured a few resistors like 5mΩ/3W +-1% (BIG measurement error here), 40mΩ/50W 0.1%, a few 0.1Ω/5W 5% from the same batch etc up to a few KΩ and at around one measurement per secon its not lighting fast but its something i can work with (as a "resistor meter" and not a "shorty").

I tried to calibrate it as best as i can but i don't have a good desktop multi-digit meter to compare to so i am not sure how good of a job i can do. At the moment i am reading slightly higher in really low values and a tiiiny bit under on higher than a few Ohms values, but my setup on the breadboard with CAT5 wires is not ideal, will keep working on it... Not sure how linear the ADC measurements should be with the forward-reverse current flow and all, i could easily add a different final calibrate factor for every Gain but i don't know if i should do it, i really need a couple or resistor standards to compare to...

Another thing i will try is a lower value fixed res like maybe 1Ω instead of 10, to see if that helps with low-end accuracy and if it also affects high-value measuring.

Also a "zero-out" button you could press with the probes shorted to cancel the leads resistance would be a useful addition if you want to use this circuit as a meter.
« Last Edit: June 28, 2022, 06:01:04 am by Dr.Krieger »
 

Offline imo

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #19 on: June 28, 2022, 06:04:55 am »
..
Now that you mention it i vaguely remember about faster Arduino-compatible clones like Teensy and it sounds like a good idea, if the Arduino's speed is the issue and not the ADS1115 chip and it's being run at single-shot mode, i am not exactly sure what is the issue as i haven't really messed around with the thing ..

A quick look into the source shows the arduino itself is not the limiting factor, imho.
The show takes so long because of rather large number of ADS1115 conversions needed for a single resistance measurement - something like 2x(50+50)*XXms + overhead, where XX is the time for a single ADS conversion..

You may speed up the show by changing the M and N values here:
Code: [Select]
// ---------------------------------------------------------------------------------------------
//    main measurement loop
// ---------------------------------------------------------------------------------------------
  {
    const float      Rr = 10.0;             // reference resistor value, ohms
    const float      Rcal = 1.002419;       // calibration factor
    const int        N = 2;                 // number of cycles to average
    const int        M = 50;                // samples per half cycle
« Last Edit: June 28, 2022, 06:20:32 am by imo »
 

Online Avelino Sampaio

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #20 on: June 28, 2022, 08:57:21 am »
Hi

Would this project be limited to measuring only resistors? What can happen if I want to measure the windings of a transformer? Would you measure an RDson from a mosfet?
 

Offline Dr.Krieger

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #21 on: June 28, 2022, 10:30:29 am »
A quick look into the source shows the arduino itself is not the limiting factor, imho.
The show takes so long because of rather large number of ADS1115 conversions needed for a single resistance measurement - something like 2x(50+50)*XXms + overhead, where XX is the time for a single ADS conversion..

You may speed up the show by changing the M and N values here:
Code: [Select]
// ---------------------------------------------------------------------------------------------
//    main measurement loop
// ---------------------------------------------------------------------------------------------
  {
    const float      Rr = 10.0;             // reference resistor value, ohms
    const float      Rcal = 1.002419;       // calibration factor
    const int        N = 2;                 // number of cycles to average
    const int        M = 50;                // samples per half cycle

As i mentioned above, i used N=1 and M=10 in my first "fast" video and also removed the wait for button press and the "measuring" message to make it faster already:

Original N=2 / M=50 (button shorted):


N=1 M=10 w/o wait for button:


In my latest test i use N=3 / M=2 (same speed as the "fast" video) and ofc different Rr and Rcal variables since my "10Ω" is actually ~10.747Ω and my test leads are two short CAT5 pieces.
The ADC still measures in "single shot mode" (9ms conversion time?), didn't change that at all. I did try removing the "Calculate optimum PGA setting based on a sample voltage" and set the PGA to 4 (+-1.024V range) to see if that made any difference, it didn't.

Hi

Would this project be limited to measuring only resistors? What can happen if I want to measure the windings of a transformer? Would you measure an RDson from a mosfet?

MOSFETs would be a problem since it flips the polarity of the current (body diode), you can use a Constant Current Source between D-S and measure the voltage drop with a DMM (with power to the gate ofc), then simply calculate R= V / I.
Inductors would also pose an issue since they won't behave like resistors in a short voltage pulse across them, you would need to seriously alter the code for "coil mode" or something or use something else like a cheap "LCR meter"
« Last Edit: June 28, 2022, 10:52:37 am by Dr.Krieger »
 

Offline imo

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #22 on: June 28, 2022, 11:38:07 am »
You may change the conversion rate (also valid for single shot, afaik).
With your solderless breadboard construction the ADC noise (which increases with a shorter conversion) is not extremely relevant, therefore try to set the higher speed..

PS: the default is 128 measurements per second, perhaps set in your source..

Quote
The ADS111x offer programmable output data rates. Use the DR[2:0] bits in the Config register to select output data rates of 8 SPS, 16 SPS, 32 SPS, 64 SPS, 128 SPS, 250 SPS, 475 SPS, or 860 SPS.
Conversions in the ADS111x settle within a single cycle; thus, the conversion time is equal to 1 / DR.
 

Offline imo

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #23 on: June 28, 2022, 11:49:50 am »
Code: [Select]
..
  hiByte = B10000001 | (channel<<4) | (gain<<1);    // conversion start command
  loByte = B10000011;
..

Your loByte has got 100 speed setting (the bits 7 6 5 of it), that is the 128 samples per second..
Try with loByte = B11100011; that will set the highest 850 samples per second. You may then find the optimal setup, changing M, N, and the speed (not on the solderless breadboard, however)..

PS: Do not expect to see some relevant results with your prototype construction. You are chasing XX uV measurements, that is not possible with the solderless breadboard. You can validate "how good the milliohm ohmmeter works" only with a pretty rugged construction, short thick wires well soldered, no mechanical contacts or sockets, good grounding (star like), no ground loops, good decoupling, etc.)..
« Last Edit: June 28, 2022, 12:13:05 pm by imo »
 
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Offline Dr.Krieger

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Re: Nice little Milliohm Meter over at Circuit Cellar
« Reply #24 on: June 28, 2022, 01:00:41 pm »
...
PS: Do not expect to see some relevant results with your prototype construction. You are chasing XX uV measurements, that is not possible with the solderless breadboard. You can validate "how good the milliohm ohmmeter works" only with a pretty rugged construction, short thick wires well soldered, no mechanical contacts or sockets, good grounding (star like), no ground loops, good decoupling, etc.)..
...

You are right about that and i'm taking it into account, i'm trying to make the thing work "perfectly" on the breadboard, just messing around with it to see if should go with this Milliohm Meter design or search for something else. The first test with the original code was a massive disappointment because of its speed, now i think i can make it work good enough (for my taste).
Like i said i am only a novice and need time to figure out the code and stuff, but i will try changing the Samples Per Second like you said and see how that works, thank you for your input! :)

The thing is that i want to use long, multimeter-like leads (using coaxial cable for power/signal that short right at the tip like in Kripton2035's Shorty) in the final design so the breadboard may actually be the "low noise" version and i will have to lower the speed and/or increase the samples in the final setup XD

I should have gone for Kripton's design from the start but i had the ADS1115 module i didn't want to pay for new parts... :/
That one seems to work sooooooooooo much better:

« Last Edit: June 28, 2022, 01:18:06 pm by Dr.Krieger »
 
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