I'm designing an automotive oil temperature sensor based around a 3-wire RTD. This is my analog circuitry:
U1 and U2 provide a 1mA bias on the RTD which is measured by U3 and gained up by U4. R5, R10, D1 and D2 are for input protection on the op amps. Op amps are a quad package MCP604 single supply and are properly bypassed with a 0.1u cap.
I have the whole thing prototyped out and it works perfectly if Vin to the 7805 circuit is powered by my 12V bench supply, but as soon as I connect it to 12V battery power in my car, I get some sort of ripple voltage or oscillation in the RTD that throws the ADC readings way off.
I don't have access to a scope at home to really see what's going on, but there's a small ~10mV ripple voltage on the RTD which is being amplified by the gain stage. I added the 1uF caps in the feedback path to try and remove it, which helped, but the ripple is still bad enough to render the measurements useless.
If I disconnect Vin from the vehicle 12V and power it with my bench supply (with GND remaining through the vehicle chassis) the ripple disappears and the temperature reading is spot on.
Powering it with a 9V battery also works just fine.
This is with the vehicle ignition off, so there shouldn't be any crazy noise present.
Also, if I leave it powered by vehicle 12V and replace the RTD with a 100 ohm resistor, it works fine. So it looks like some sort of interaction with either the platinum resistance element or the sensor leads.
Is there some sort of interaction I'm not accounting for with vehicle power vs. my bench supply or 9V battery? Or is there a potential stability issue with my circuit that is being pushed over the edge when connected to vehicle power?
EDIT: I'm fairly certain this is a ground loop issue. Rewiring the circuit into a star-ground configuration improved the situation, but there are still measurable stray ground currents through the RTD causing unpredictable results.