I was wondering if someone could help me understand why this instrumentation amp isn't working the way I think it should. The amp is an INA155.
My intention is that at if a voltage (positive or negative) is applied across the input pins, I should see the same value, multiplied by 10 (since this amp has a gain of 10) at the output pin, relative to the ref pin. I have a virtual ground using a 2.5V reference at the ref pin. I have some low pass RC filters on the input. Eventually I want to read the output voltage using an ADC.
I have the circuit in this picture bread-boarded:
I have the ref and output pin connected to a multi meter. I am powering it with a bench top supply. I haven't applied a voltage to the input pins yet, because I thought I would first test the offset voltage by measuring the output when the input pins are shorted together. It should show a voltage close to 0, with a small offset right???
What I'm seeing is the output voltage start at close to the rail (so I measure exactly 2.5V on the meter since the ref pin is tied to 2.5V),then it slowy starts to drift, over the course of minutes, until the voltage on the meter hovers somewhere around 200 to 300mV between the output and the ref pin. The stability increased since I added the low pass filter, but the output doesn't make sense (at least to me).
Can anyone explain why I'm seeing this? There is obviously some gap in my understanding of these amps. So far all I've worked with is a unity gain buffer. I'm just trying to design a simple buffer for measuring a signal, mainly DC, that can be bipolar.
Looking for any tips or help with this, or even a different approach to achieve the same thing. I am forced to work with a single 5V supply, hence the 2.5V virtual ground, but I need to measure both positive and negative voltage. I've tried this with added resistors from the input to 2.5V for a return path, but the output didn't change.