I am working with the AD8222 dual package instrumentation amplifier from Analog Devices. I want to build a compact EMG system so I am trying to use this chip since there are not a lot of dual IAs on the market. I need 4 channels eventually.
I have the chip working on a dual supply, and I tested it with a sine wave and the amplitude increases based on the resistor. I tried building the system using a VS- as GND, VS+ as the supply voltage and a voltage divider for the reference voltage. I tested two values of gain resistor (499ohm = 100 gain and 4k which is about 10) and two values for the voltage divider (1k and 10k) to see if the impedance was changing anything.
The output of the amplifier is always the voltage divider value. If I change VS+ to 5V the output jumps to 2.5V. I have tested an input signal of 100 and 200mV with both negative and positive offsets and nothing changes.
The data sheet says the device should work in single supply mode so I am not sure what I am doing wrong. Does anybody have any suggestions of how to change the circuit, or a difference dual package IA that I could test. I really need the single supply since I want to run this system off a battery for a small embedded device.
EDIT: Fixed schematic to reflect the part better.