# Measuring current, instrumentation or differential amplifier?

I want to measure current at the high side of circuit with 0.1 mV accuracy using a 16 bit ADC. for sensing the voltage drop over a 100 mΩ 1% resistor I need to use an op amp which is cheap and easy to implant.

I have found two suitable chips, AD620 in-amp or AD8628 zero drift op amp configured as differential amplifier with hand matched 1% resistors, both from analog devices and are reasonably cheap.

The AD8628 is 3 times cheaper than AD620, so I thought I can get two of AD8628 and measure the voltage with the same accuracy as well. but since I have little to no experience I can't really tell which one is going to give me the result that I'm looking for.

When do we need to choose instrumentation amplifier over differential amplifier and for this specific project which one is more suitable?

Side question; if I match the differential amplifier 1% resistors one by one using DMM, can it be as accurate as 0.1% resistors?

• What is the high-side voltage? Apr 14, 2020 at 6:41
• What is the bandwidth you want to measure? What is hand matched resistor? You need three diff. opamp to make one in-amp. 1% resistors are off by for at least 0.5%, they can't be 0.1%. Apr 14, 2020 at 6:50
• @Mattman944 What do you mean? the shunt is going to be on the positive line. Apr 14, 2020 at 7:04
• @MarkoBuršič I'm going to measure the 1% resistors one by one to match them together. can't a differential amplifier measure the current? why is it have to be in-amp? Apr 14, 2020 at 7:07
• If you are measuring at the high side, be aware that the maximum input voltage for the AD620 is (+Vs-1.2), so for a 5V supply, the maximum input voltage would be 3.8V. It may be worth considering a difference amplifier, something like the AD8202, many of which have maximum input voltages >= supply voltage Apr 14, 2020 at 11:15

## 1 Answer

For this specific project, which is current sense, the best option would be to select a Current Sense Amplifier. Those amplifiers are designed for exactly the desired applications.

For example take a look at LTC6102, page 22 shows an example to drive a 16bit ADC. Calculations can be changed to support 100mOhms resistor.

If you are using LM4040 as ADC reference, make sure you get the "A-Grade" part with 0.1% tolerance which translates to 0.5mV. The parts with more than 0.2% Tolerance will already set your accurarcy 1mV off.

• Thanks but I didn't ask for alternatives and LTC6102 is an expensive chip. my question is: When do we need to choose instrumentation amplifier over differential amplifier and for this specific project which one is more suitable? Apr 14, 2020 at 16:04
• If you want to build this by yourself, the choice would be a op amp. Usually instrumental amplifiers are for very high precision applications where a noisy environment is present and where you have large common mode signals (eg. power line noise). Not meant to be rude, but from reading the question it looks like its going to be a steep learning curve to realize this. Also, a 16bit ADC is not the optimal choice for this circuit since for example 12bit would be more than enough. Apr 14, 2020 at 16:40