Hi all,
I'm designing a precision (I hope!) temperature measurement circuit. I'm simulating/designing with NI Multisim.
The idea is to use AC excitation for a pt100 RTD sensor.
With 500uA through a 116 Ohm pt100 resistor I get a signal of 58 mV. With the same current reversed I obviously get a -58mV signal (these are the same to 6 digit precision in the simulator).
I've connected the resistor to an instrumentation amp, currently the AD8429, with a gain of about 76.
With a positive current I get +4,39930 V out, with the current reversed -4,41314 V out.
So the gain for the +signal is 75.85 and for the -signal 76.09
That's a difference in gain of about 0.3% (if I calculated correctly)
This is not because the current through the resistor would be unequal, the simulator shows +/-500uA with 6 digits precision.
Also not because the actual input to the instr-amp would change, probing the inputs I get exactly +58mV and -58mV on the input.
So I have to conclude that the instrumentation amp has unequal (DC-) gains for positive and negative swinging signals?!?
What am I missing?
Is there a better instr-amp I should be using?