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Assuming I need to measure and amplify a voltage drop over a resistor that is connected to ground on one side (no negative voltages, since current flows always in one direction), is there any real, practical advantage of using differential op amp over a simpler non inverting one?

Non-inverting config:

Non-inverting

Differential config:

operational

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2 Answers 2

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If the distances between measurement point and the measurement circuit are short (i.e. there is no distance/cable involved that could be threatened by an external noise source) AND there is a shared ground between the measurement point and the measurement circuit, then sure, you can use a single ended amplifier.

The problem arises when you have noise threats on the connection between source and measuring circuit OR when there is ground bounce between measurement point and measuring circuit. For instance, you might think that this circuit is fine: -

enter image description here

However, if you analysed how noise might react with the cable and the unbalanced impedances of both wires then you would get this: -

enter image description here

With a single-ended receiving amplifier, one input is firmly tied to local ground and will be largely unaffected by the interference on the cable. Sure, there'll be an injection of energy but, that energy will be "soaked-up" by the local ground producing very little surge artefacts.

However, the other input has an impedance to local ground governed by the input resistor, \$R\$ and, it will develop a surge voltage that will be amplified by the single-ended amplifier.

This can only be solved by a differential amplifier with a balanced input impedance: -

enter image description here

Because both inputs into the differential receiving amplifier have the same impedance to local ground, the interference waveform is seen equally at both input nodes and, the effect of the differential amplifier is to cancel those signals out.

If the measurement point were locally grounded it creates even bigger problems for the single-ended amplifier but can even create problems for the differential/balanced amplifier if the source has output impedance: -

enter image description here

This can only be solved by balancing the input source with respect to its impedances: -

enter image description here

Images and some text from here

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  • \$\begingroup\$ Thank you for the extended answer. While it seems obvious on how differential amp can benefit the design in case we have long input wires, how about the case in which the "sensor" is on the very same PCB, not too far away from the op-amp? \$\endgroup\$ Dec 16, 2020 at 12:14
  • \$\begingroup\$ @ŁukaszPrzeniosło Read the very first paragraph in my answer please. \$\endgroup\$
    – Andy aka
    Dec 16, 2020 at 12:15
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The answer depends on your requirements for such things as SNR, measurement fidelity and linearity. If these are important to you, say, because you have very small currents to measure, or that you require extreme linearity in your measurements, then the differential solution has an advantage. Otherwise if there's nothing particular about your requirements it looks like the non-inverting configuration will suffice. Are you looking at measuring a few mA to 100mA or so, or more like in the range of pA to nA ?? You see, this does make a difference in your final choice ...

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  • \$\begingroup\$ Thank you for the answer. I want to measure current of around 5A, that would yield voltage drops on the shunt resistor of around 50-60 mV. \$\endgroup\$ Dec 16, 2020 at 11:43

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