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[on:]

Hello Arduino forum!

I've got a sensor that gives me a small voltage between 2 and 10 millivolts.
I'd like to use the Arduino to sample this sensor.

Since the voltage provided is very small, I was wondering what would be the better approach, amplifying the voltage (how?) or decreasing the reference voltage of the ADC?

Thanks a lot for any advice!
Tom

[Reply #1 on:]

The ADC cannot operate with a reference voltage below about 1V, BTW - the built-in analog reference is 1.1V, but that's still not going to help you much.

You need to amplify the signal.  This is a classic case of needing an instrumentation amplifier since your signal is so small.  Look at the specs for the MAX4208 for example for a 5V single-rail device with rail-to-rail capability. http://www.maxim-ic.com/datasheet/index.mvp/id/4925 - other manufacturers make instrumentation opamps too!

Alternatively a single-op-amp circuit can be used but won't be as accurate due to time and temperature drift.  Depends on your precision requirements (you'll definitely need a 'precision low voltage' opamp that works down to the negative rail.  If your sensor has a high output impedance the instrumentation amp is the best route I think.

[Reply #2 on:]

Hello Mark, thanks for your advice!

I had an LM358 here and tried the non inverting dc gain application from the datasheet and it is already a nice improvement. Don't know yet if it will be precise enough, so I will keep your advice on the MAX4208 and instrumentation amplifier in mind, thanks a lot!

Have a great day!
Tom