Which is the most accurate method to measure current?

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treez

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Hello,

I am required to measure current which can be up to 10 Amps.
Which is the most accurate of the following three methods?….

…one is a non-inverting amplifier, and the other is a differential amplifier.

Non inverting amplifier (Gain 101)
https://i44.tinypic.com/2z9d2f7.jpg

Differential amplifier (inverting) (Gain 100)
https://i42.tinypic.com/e25fo.jpg

Differential amplifier (non-inverting) (Gain 100)
https://i40.tinypic.com/24me3vm.jpg


The opamps will feed into microcontroller ADC inputs
Here are the schematics in pdf
 

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  • current measure ..diff-amp.pdf
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  • current measure noninv-diff.pdf
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A differential amp, with high precision resistors and capacitors, will better reject common mode noise. Which could be significant if you are in an industrial environment and the sense resistor is away from the opamp.
 
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OK thanks....which of the above two diff amps would be the most accurate one...the inverting diff amp , or the non inverting diff amp?
 

To invert or not to invert, that is the question.

The advantage of an inverting configuration is that common 100k and 1k resistor could give you the desired gain of 100.....or more accurately -100.

The question is, what are you going to do with that negative output? If fed to -let's say- to a SMPS controller which sees an increasing voltage equal to an increasing current, then a second inverting stage is required...which always adds some small but nonzero errors.
If you are feeding that voltage into an ADC and then a microcontroller, then again it does not accept negative voltages, and a second inverting stage is also required.

For those reasons, I would rather use a noninverting stage..
 
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The schematics are hiding the critical point which determines measurement accuracy, a clear 4-wire connection of the shunt. To reflect the problem in the schematic, you should add wiring resistance elements. Then the amplifier input connection can be corrected to remove the wiring resistance effect.
 
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The schematics are hiding the critical point which determines measurement accuracy, a clear 4-wire connection of the shunt.
Click to expand...

..so even the plain noninverting amp is accurate as long as i get the amp right near the seanse resistor and wiring resistances dont come into it?

To invert or not to invert, that is the question.
Click to expand...
...indeed , but the outputs of all 3 opamps above go more positive, the greater the current......this is because the "inverting" diff amp has an input which is negative going and two negatives make a positive.

I cannot understand why people use diff amps when it appears that the simple noninverting amp will do? I thought diff amps were needed to get rid of common mode noise?...as per page 20 of book "how to use opamps"?
 

..so even the plain noninverting amp is accurate as long as i get the amp right near the seanse resistor and wiring resistances dont come into it?
Click to expand...
Yes, if. The simple amp has it's voltage reference at the shunt sense terminal. If you manage to make it the common ground for the control circuit, wiring voltage drops don't affect the measurement, otherwise they do. But the measurement error may be still small enough to be neglected.
 
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