This bridge is connected to an in-amp with 1000x gain. In order to calibrate the bridge and unbalance it (so I can have, say 1V output at the in-amp output) generally I use a very high resistor (around 500k ohms) in parallel with one of the resistors on the bridge. These 350 resistors are 0.02% tolerance and 0.2ppm/C. The opamp is a AD8571 (extremely low drift and offset).
Unfortunately I have been struggling for months with a drift that shows in several different ways:
- Temperature - Only by blowing some hot air briefly, the output varies +/-10mV
- When the voltage output is increased by any external factor that is removed afterwards, it never goes back to its initial value
- There is a voltage drift without any temperature change, I managed to have a very precise temperature sensor mounted on the top of the circuit to confirm.
All components are very high quality, spec'd for this particular application, low drift, low offset and really expensive. I do not know whatelse I can do to get rid of this annoying drift, unpredictability and lack of repeatability.