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I want to measure the current in a 5V circuit. The current is between 50uA and 2.5mA. The current is from a NTC and it changes slowly (10Hz measurement is fast enough).

Here is the typical application circuit from the MCP6N11 datasheet. My circuit is very similar.

enter image description here

I intend to use the Instrumentation Amplifier MCP6N11 (because it does the job and I can buy it locally) with the output connected to a 12 bit ADC MCP3204.

I plan two identical circuits (to measure similar sensors). For the same current the circuits should produce the same digital value in the ADC.

I am thinking how I can make sure to see identical values. Because even if I use 0.5% resistors they are likely different in each circuit. I could have i.e. a nominal 100 Ohm resistor which is one time 99.5 Ohm and the other one 100.5 Ohm.

I see the following options:

  1. Add a trim pot parallel to Rsense or Rf or Rg and adjust it so that the circuits match.
  2. Like 1 but with a fixed parallel resistor, i.e. 100 Ohm with a parallel 47k Ohm resistor
  3. Don't change the resistors. Adjust the digital output of the ADCs in the microprocessor (i.e. a * 1.013 = b)

What is the best approach to do this? Initially this has to work with a temperature of about 20 to 50 degrees Celsius. But in the future maybe it should work for automotive temperature range.

I don't really know how accurate I have to build this and how accurate I can build this with of the shelf parts. I try to make the accuracy better than 1% - if possible without a huge amount of work.

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    \$\begingroup\$ "For the same current the circuits should produce the same digital value in the ADC." over the full measurement range? That will not happen. \$\endgroup\$ – Bruce Abbott Jan 9 at 6:21
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    \$\begingroup\$ @Edgar If we knew your business, in great detail, then maybe there could be a one or two paragraph answer for you. We could say, "Do this, then do that." But we don't know it and you only write a little, anyway. So how can you expect one or two paragraphs? Doesn't make sense to ask for it. None of us (none that I know, anyway) have a working crystal ball. We are "just folks," like you, trying to get by. \$\endgroup\$ – jonk Jan 9 at 7:07
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    \$\begingroup\$ @Edgar Accuracy requires calibration against a standard. Usually, these standards are found at places like NIST (US) or DIN (Germany.) You will need traceability to them. Directly calibrated by them is best. But often, you cannot afford to hire them to do that work. Next would be to maintain a standard that was directly calibrated by them and to maintain it in some fashion that it is stable for a while (before re-calibration by them.) Etc. But you can't know accuracy for an entire system. Not a priori. You need traceability. Are you looking to make instruments for sale that read the same? \$\endgroup\$ – jonk Jan 9 at 7:08
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    \$\begingroup\$ @Edgar Okay. Hobbyist. That's like me. Have a look at this answer I gave, some time back. Walk through those terms and tell me which of those are important to you and which are not important to you. \$\endgroup\$ – jonk Jan 9 at 7:53
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    \$\begingroup\$ @jonk: Wow, what an answer! Important for me is the repeatability and the detectability. If I use my circuit for an hour then I want that for the same current the two ADC values are the same with little error. I don't care if it is accurate. I.e. if it shows 1.400mA for both currents but in reality it is 1.500mA that is not a big problem. It is also not a big problem if on the next day, maybe 20 degrees warmer, it will show 1.300mA. Important for me is that the two circuits, both in the same case, show the same value if the current is the same. \$\endgroup\$ – Edgar Jan 9 at 9:45
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Specify a realistic accuracy, then work to that.

Hint, to find out what's realistic, do an error analysis including drift over time and temperature, as well as initial tolerance of the components. Initial tolerance can be calibrated out, drift between calibration and measurement cannot be.

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I found an answer by watching one of Dave's videos. He needs similar 10k resistors and his solution is to check a couple of the resistors which he has with the multi meter to find resistors which match as good as possible. Sounds like a great low tech solution to me. That's what I will do with the resistors which I need.

https://youtu.be/8-qar5vgnbc?t=173

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  • \$\begingroup\$ Same, Paul (Mr Carlson) and Dave (EEVblog) both just hand grade what resistors they have and go from there. Having done that it probably makes the most sense to hobbyists as a good solid starting point. Get a decent multimeter :) \$\endgroup\$ – twobob Mar 8 at 11:23

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