Instrumentation Amplification

Hello,

Is it possible to do instrument amplification with an arduino board? Or is a separate amp required? (in the case of strain gauge voltage output amplification from a wheatstone bridge configuration) If a separate amp is required, any recommendations (preferably cheap) would be appreciated!

Many thanks

Dave

It depends upon your accuracy requirements. The ATmega328 (Uno/Duemilanove) does not have a differential mode on the A/D inputs, while the ATmega1280/2560 (Mega/Mega2560) does.

You could, as a first step, take the outputs of your Wheatstone bridge into two analog inputs on an ATmega328 board and just subtract them out in software. This will be the least precise.

The next step up is to use a Mega board and use the differential A/D input channels with gain. For example, setting the MUX5:0 bits in the ADCSRB register (you'll have to step outside the bounds of the Arduino libraries a bit) to 11 ('b'01011) your A/D readings will be A1-A0 multiplied by a gain of 200. That could work very nicely for a strain gauge.

Another step up is to use an external instrumentation amplifier, voltage reference, and A/D converter, which will provide higher-quality results. The A/D peripheral on an AVR is pretty bad in differential mode, as much as +/-9 LSB's of error (typical...no maximum is guaranteed). So with a 5V reference (which itself is not very accurate) a 10-bit result represents 5V/1024=4.88mV so +/-9 LSB's represents up to 44mV of error in a given reading.

So it's not enough to use an external instrumentation amplifier to get higher quality: you need an external A/D and external voltage reference, and with specifications better than the A/D+reference built-in to the AVR.

-- The Aussie Shield: breakout all 28 pins to quick-connect terminals

I think Im gonna give the external instrument amp way a go,

Thanks for the help!

Dave

@RuggedCircuits: Your first suggestion will not work, unless you do something interesting with AREF. Strain gauges in a wheatstone bridge configuration typically have a full-scale output of less than 50 mV, or about 10 LSB if you're using a 5V reference.

That configuration also needs a good, stable excitation source, since the output is proportional to both the excitation voltage and the strain measured by the gauges. TI makes a very nice integrated amplifier/excitation source device called the INA125. I'm selling an amplifier board that wraps it up with some power supply circuitry at http://www.logos-electro.com/1x3-ana-ina125-1/.