# Measuring difference of potential between two inputs

I know potential is always about difference so you can't just measure a potential but it's difference with some other potential.

Let's say I have a GND and two inputs each one 0-5v and I have a ADC measuring 0-1V being 0.0 v == GND. I can't change the reference.

I want to amplify small differences between the inputs and measure the difference with the maximum resolution. Is there an easy way to subtract V1 - V2 and supply the result to the ADC?

• I want to amplify small differences between the inputs and measure the difference with the maximum resolution. Maybe with an external ADC. If I subtract the measured values I lose resolution. Nov 29 '17 at 12:19
• Then, what you need is a Differential Amplifier.
– rrz0
Nov 29 '17 at 12:20
• Note: Added subset of a differential amplifier since you mentioned you want to amplify small differences between inputs.
– rrz0
Nov 29 '17 at 12:45

## 2 Answers

I think what you are looking for is an op-amp set up as a subtractor:

Edit:

The above is the simplest form of a differential op-amp, however, it is best to use instrumentation amplifier (wikipedia) setup as mentioned in the comments. It is basically the same concept, but with the use of two extra op-amps (that works as buffers) you can prevent the input signals to be interferred by the load, giving you a much more acurate reading.

Here is the instrumentation amplifier setup:

Do a little reading on the link above, it is fascinating how they work.

• Good start, but I'd also mention instrumentation amps here due to input Z and common mode voltage range issues with the one-op-amp circuit. Nov 29 '17 at 12:36

What you are looking for is a differential amplifier.

Differential amplifiers amplify the difference between two voltages making this type of operational amplifier circuit a Subtractor unlike a summing amplifier which adds or sums together the input voltages. This type of operational amplifier circuit is commonly known as a Differential Amplifier.

All op-amps are “Differential Amplifiers” due to their input configuration.

However, by connecting one voltage signal to one input terminal and another voltage signal to the other input terminal the resultant output voltage will be proportional to the “Difference” between the two input voltage signals of V1 and V2.

EDIT: (Thanks for pointing out)

When resistors, R1 = R2 and R3 = R4, the transfer function for a differential amplifier can be simplified to the following:

Differential Amplifier Equation

You may also want to take a look at an Instrumentation Amplifier.

An Instrumentation Amplifier is a very high gain differential amplifier which has a very high input impedance and a single ended output. Such an amplifier is mainly used to amplify very small signals.

The instrumentation amplifier has a very good CMRR.

Circuitry for a high input impedance instrumentation Amplifier:

• The simpler circuit above will have uneven input resistance across both inputs - that could lead to interesting problems... Nov 29 '17 at 17:51