AD8224 Low Frequency Noise Issue

In one of our designs we are doing signal conditioning of a load cell (Wheatstone bridge)

We have an issue where the main instrumentation amplifier an AD8224 has an output signal that wanders by around 20mV over the course of 10's of seconds.

We have gone through troubleshooting the signal chain and tracked the issue back to here, the above plot is taken with the inputs to the amplifier shorted out and connected to 0V.

Supply to the op-amp is +-15V, and has 0.1uF bypass directly near the IC, and 10uF about 3cm away. We are using a gain resistor of 49.9ohm which should give a gain of around 991.

We are not worried about the high frequency noise, this gets tidied up latter in the chain.

I was able to find one of our old board revisions that does not show the same issue, these boards are modular, and I was able to slot in the new and old next to each other to compare the results. The old board used a TI INA2126 instead of the AD8224.

Channel 0 red is the old revision and shows what I would expect of changes due to thermal effects.

Channel 1 yellow shows the same board as per above.

In terms of ruling out other factors, this is in a room without much airflow, breathing on the board doesn't seem to have a large effect, nor does insulating it from airflow by wrapping it in rags. My reference pins are tied directly to zero volts. I have confirmed I get a nice clean (1-2 bits of noise) signal recorded if I short the output of the amplfier to 0V (as expected).

I am all out of ideas, is this expected performance of the AD8224 (from the datasheet I think I should be getting at least an order of magnitude better).



Fixed picture
[edited by: Hugoagogo at 2:28 PM (GMT 0) on 31 Jul 2019]