Hi all,
I'm designing/simulating a precision pt100 frontend for temperature measurement between +22C and +42C:
http://www.anderswallin.net/wp-content/uploads/2013/06/pt100_piiri.pngThe main ideas are:
- 4-wire connection for obvious reasons
- ratiometric output, i.e. measure both sensing resistor voltage and reference resistor voltage - the ratio of these two should be insensitive to drift in the sensing current
- switchable +/-500uA "AC"-excitation. Using the sum signal observed with +500uA and - 500uA should get rid of asymmetric error sources (thermovoltages etc.)
- output centered around +22C by way of the 112 Ohm reference resistor
Some of the details:
- By simulation I found that an integrated instrumentation amp has quite high input offset voltage (which results in asymmetric output when we switch the sensing current direction) as well as input bias current (which sources or sinks a variable amount of current into the amp inputs). Hence I've assembled the instrumentation amps from zero-drift op-amps (AD8551) with <10pA input bias currents and low input offset voltage.
Any ideas for how to switch the +5V and -5V voltages to change the direction of the current source? An analog switch with low resistance?
I have not run a sensitivity or temperature-drift simulation yet, but the design goal is stability of 1 mK (0.001 K or C) over long time periods (weeks).
The +/-10V output will be AD-converted at a rate of about 1 SPS, so the analog bandwidth can be < 1Hz. I should probably add low pass-filters into the instr-amps or between them.
Any other improvement ideas or suggestions?
Anders