I am using an AD8226 instrumentation amplifier as the pre-amp in a circuit which uses a micro fuel cell oxygen sensor to measure oxygen content in a gas stream. The pre-amp is configured in single ended supply mode, with a 5 V supply. Gain resistor was chosen to give a gain of ~80.
The output of the in-amp is then passed through an active low pass filter and to a microcontroller's ADC, but I don't believe this is relevant to the problem. Everything discussed here was done with the preamp outut (O2_PRE node in the diagram) floating and only connected to a multimeter.
When testing this circuit with the sensor I found that the gain was nice and linear - up until a certain point where the gain dropped off (this point was at about 20mV input/1.6V output).
To eliminate any issues with the sensor I replaced the sensor with a voltage divider made up of a fixed reisistor and a multi-turn linear pot:
With the resistor network I observed the same problem (see further down for a plot of the inamp output).
To further eliminate any issues with the surrounding circuitry I wired directly up to the AD8226, using my bechtop supply as VCC. Giving the circuit seen here:
This showed the same behaviour as seen in this plot:
('mainboard' refers to the output when the voltage divider is used to drive the original circuit, 'breadboard' gain is slightly lower as I used a 630R gain resistor)
The AD8226 data sheet specifies an output voltage swing of 0.1 V to +VS−0.1 V in single ended mode. I am measuring the output into a high quality multimeter (ie. high impedance), although I did observe the same results when I added a load resistor of 20 K. I have repeated the results with multiple AD8226 devices.
Can someone explain why the gain would be falling off above a differential input of ~20 mV given that I seem to be staying well within the output limits?