Electronic – Current measurement opamp, is this circuit flawed? Output all over the place

amplifiercurrent measurementoperational-amplifier

This is a circuit to measure a current flow that is 0-10A DC. The output goes to an ADC.

The circuit itself works fine, however, over a dozen cards, the output of this circuit is all over the place.

At rest, I=0, the output offset can vary by a factor 10 (300 to 3'000 count on the ADC) from one board to another; but is stable in a specific board.

So far, it might be related to the input offset voltage, given at 0.11mV typ. but this would account for perhaps 10%.

The manufacturing of the cards was also poor and noticed some pads seem not well soldered, I wonder if that could be a cause as well.

Are there other fundamental flaws in this circuit?

I need to fix an existing batch of boards.

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  • VDD: 3.3V
  • ADC is 0-3V with 0.1% reference. the ADC is not saturating.
  • ADC is 12bits, 16 measurements summed: 16*12bits = 65535 counts.

Two random cards measured at rest I = 0A voltage at the circuit output:

  • A: 150mV / 3445 count
  • B: 34mV / 336 count

enter image description here

Best Answer

Your input signal is 10A, which through your R19 sensing resistor develops a voltage of 10mV. Your opamp is specified with a maximum offset voltage of 7mV, which is 70% of full scale!

Note, calacuating the offset like this, it doesn't matter what your gain is (it's about x250), or your ADC resolution or scale, it's presupposes you have the gain and ADC correct to get a full scale measurement, and gives the offset in terms of full scale.

You have several options:

1) Calibrate the offset. The specified offset drift is fairly small, 1uV/C, or in full scale terms 0.01% FSD per degree C, which also fits with your description of the error being stable on any particular board.

2) Use an amplifier with a bit better offset, down to 1mV should be easily acheivable for not too much money with a bit of research. Offset down to 10% FSD.

3) Use a 'zero offset' amplifier, chopper or AutoZero based, which will be pricey, but will work. There are several options now from the big beasts TI and AD, which will get you 0.01% FSD offset.

4) Can you increase the size of R19? 10mV is quite a small burden voltage for an ammeter, though it could be all you want to budget for an in-circuit meter.

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