Hi, I am trying to replace an analog ammeter with an AVR to measure current (up to 300A). I have an existing 300A/50mV shunt which is used for the analog ammeter. Obviously I don't want to read this directly into the AVR ADC as I will end up with very poor resolution with a maximum of 50mV. I got a couple of AD623 instrument amps to try and boost the voltage to rectify this situation but seem to be unable get it working correctly.
I am initially starting with a planned gain of 100 so that 50mV will become 5.0V, full scale on the AVR input, so I'm using a 1.02kO gain resistor. My problem is that even though the AD623 is supposed to be rail-to-rail it doesn't seem to be working... If I apply a 41.9mV differential across the input of the amp I am only getting 1.217V on the output instead of the 4.19V I am expecting... If I remove the gain resistor and turn it into a unity gain amp I do in fact get 41.9mV on the output as I would expect. Only low gain resistors seem to be giving the amplification I expect. I have tried using Rg=3.89kO and Rg=100 ohms which should be gains of around 26 and around 1000 respectively with actual outputs of 1.107V and 1.229V. So for the gain of 26 things seem to be working about right but for the gain of 1000 which should clearly give the full 5V rail (or close to it) on Vout I only get a max of 1.229V.
Any idea on why this may not be working in full rail-to-rail style? I am feeding it as single supply with 5V if that matters.