I cooked up the following and would like to know if its appropriate or workable. I've not used a digipot before as its almost never the right tool for the job. But this time?
This is supposed to be a circuit to measure temperature using an RTD. The SAMPLE_PULSE node is connected to an Arduino digital output which goes high before every sample; SAMPLE_READ is connected to an analog input. The digipot is configured in rheostat mode which means the wipers are tied to A. The purpose of the 1k and 220 ohm resistors are to maximize the digipot resolution right in the neighborhood of where I calculate my trim values will be set. Channel 0 scales the signal and channel 1 shifts the signal. What I'd like is to be able to calibrate such that the temperature range i care about falls right between 0.5 and 4.5 volts.
Your first opamp's feedback is distorting the bridge - no significant current should be taken from the bridge,
which is why an instrumentation amplifier topology is used with such bridges.
You have very little gain in the amplifiers, the second stage is a redudant unity follower.
Is the temperature sensor a platinum resistance thermometer (PT100 suggests this). You need
precision resistors and a higher gain amplifier.
If using a digipot it must be in potentiometer configuration, not rheostat, otherwise you'll find
high levels of thermal drift.
The normal way would be to use the pot as the middle of one arm of the bridge, using series resistors
to restrict the voltage range the pot covers to the minimum necessary.
Thanks for the helpful info and schematic. I should have known better when I was playing in spice.
The RTD is indeed a PT100. As for using the pot in rheostat mode, shouldn't I be able to avoid that by sampling only during a short excitation pulse through the bridge?
I would like to retain the shift and scale functionality. How about something like this with an AIN as per your advise?
The whole point of a bridge is to match ratios, and ratios are insensitive to variations with temperature, except
when you have a temperature probe as one arm - you want to measure just the variation of the probe, so its
important its companion resistor is very stable with temperature, and the other arm of the bridge uses
matched resistors of low and identical tempco.
The reference is after the gain, its not the place to make shifts like this, the digipot as I've configured it
is a reasonable approach for that, you can have quite big shifts there.
Also consider that you can optimize the overall accuracy and resolution by matching the center of your operating range to the bridge arm resistors. That is, don’t just make make the Rstd side high side 100 ohms as that forces the center of your range at 0 C. That Is okay if your range is -200 to 200 C. But, if your device will measure 0 to 500 C, you’d want Rstd to be 181 ohms.
