Accurate small current sensing

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#1
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Hi,

I'm needing to sense small currents (.02 - .6 mA) accurately.

I've been looking for transducers that are designed for this sort of thing, but I haven't been turning anything up in that low range.

I have also considered simply measuring and amplifying the voltage across a resistor. I can get some .1% tolerance 50 ohm resistors from digikey for not so much, but then when I think about amplification, I realize that my feed back resistor networks are going to suffer from considerably worse tolerances given the available resistor tolerances that I see.

I would like to have the entire system accurate to 1uA if possible.

It occurs to me that the I could use some fine variable resistors to tune the circuit in, but then I'm not sure what standard I might measure against?

Any suggestions appreciated.

Thanks,

B

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If the current runs to ground or common and then an Op-amp with a negative voltage supply and a feedback resistor
could function as a virtual ground current-to-voltage converter. With a feedback resistor value of 5K Ohms the Op-amp
output would range from -0.1 to -3V for an input current of +0.02 to +0.6mA. A second Op-amp would be needed to
rescale for +V output if needed.

A wikipedia eaxmple.

Stan

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Search for 'hi side current sensor' from max, lt, analog devices.

Imagecraft compiler user

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Quote:
I can get some .1% tolerance 50 ohm resistors from digikey for not so much, but then when I think about amplification, I realize that my feed back resistor networks are going to suffer from considerably worse tolerances given the available resistor tolerances that I see.

Use .1% resistors for all shunt and gain circuits and you will have .1% overall gain accuracy, which is pretty good, and meets your stated need.

Tom Pappano
Tulsa, Oklahoma

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Another possibility is a hall effect sensor. This measures the magnetic field that the current produces when it runs through a wire. I IMAGINE it's possible to get very precise hall effect sensors but the overall accuracy of your system would depend on where you placed the sensor, ambient magnetic conditions, etc. I have no experience with that. 1uA is a tight spec. Good luck.

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Piece of cake with LTC2484 (this is 24-bit ADC) and current sense resistor. However, precise reference is required.

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If you are going to twiddle pots on each unit anyway, why not use stable resistors of any tolerance and store custom calibrations in firmware, assuming you can tolerate a divide or fixed point multiply.

For a reference, mechanical meters were accurate to a couple percent, 50 and 100 microamp movements were common in VOMs. Scrounge an old 20 Kohms-per-volt Simpson meter (or anything in a bakelite case :))