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DIY Milliohmmeter - how to solve grounding issue?

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Avelino Sampaio:
Hi

I'm completing this project and would like a tip on how I can solve a grounding issue. Note in the image that the current of 50mA, passing through the DUT, should completely cover the AEBF path. However, this current is divided by the AEDF path, adding the resistance of the reading cable to the final value. What would be the best technique used to resolve this issue? This question seems to be fundamental so that I can read a very low resistance value (1mR).

Doctorandus_P:
For the measurement side, do not use a grounded connection, but use a differential amplifier.

Avelino Sampaio:

--- Quote ---Para o lado da medição, não use uma conexão aterrada, mas use um amplificador diferencial.
--- End quote ---

I can't change the project this way. These are the best opamps I have available. There has to be another output without changing the design too much.

Gyro:
That seems a very complicated implementation for what you are trying to achieve (it would be a different matter if you were doing AC measurement and synchronous detection). Try the attached app note (substituting one of your own opamp and reference). As long as 1A maximum measurement current isn't an issue you will achieve 1mV/mR, so at least one decimal place on a typical DMM.

Completely breaking any internal links between force and sense leads is essential when Kelvin (4 wire) sensing.

Doctorandus_P:
I really like the simplicity of the circuit described in Circuit Cellar of September 2016, issue 314.

How it works:
the microcontroller has 2x 4 pins parallel with a 30 Ohm resistor on each of its pins, and this creates a bidirectional test current.
This test current goes through a 10 Ohm reference resistor and your DUT (Device Under Test).

Then the ADS1115 is used to measure both the voltage over the reference resistor and over the DUT, then swaps the direction of the current though the DUT with the microcontroller, repeats the measurement in the other direction. It also repeats this whole process a bunch of times and does some averaging.

It also uses 4 wire kelvin sensing. (Two "drive" wires, and two "sense" wires), as shown in the schematic.

Go buy that back issue if you like it. Unlike Elektor, circuit cellar projects always come with all source code.. In this case it's a quite simple "arduino" contraption, but it is something to get you started and to improve upon.

With the 1uF and 1k resistors there is some protection for the ADS1115, but I would add a bunch of extra diodes to short any voltages over 3Volt or so over the test leads.

Also, because there are no extra opamps in this circuit, they can also not generate any errors, and it makes the circuit cheaper.

I quite liked CC. They did not have many (interesting) projects, but they had the source code available for all of them and a subscription was relatively inexpensive. Then they had some kind of collaboration with elektor, the subscription doubled in price and I canceled (or better, did not renew) my subscription.

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