[EMAIL PROTECTED] wrote: > Hi Bruce, > > would you have pointers to good temperature sensing circuits with sub > millidegree resolution? > > thanks, > Said > _______________________________________________ > time-nuts mailing list > [email protected] > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > Said
This was originally sent at 3am local time but doesn't seem to have made it. The traditional method uses an AC transformer bridge with a platinum resistance sensor and a stable low temperature coefficient reference resistor. However this is probably too bulky and expensive for your application. As I understand it, you don't really need to measure the temperature but some temperature dependent quantity that is a monotonic function of temperature with a resolution and and stability equivalent to better than 0.001K. Measuring the ratio of the resistance of a platinum resistor with to a stable reference resistor with a low temperature coefficient will suffice as long as they both have the same temperature. A reference resistor with a small non zero tempco will not affect the monotonicity however the sensitivity will be affected slightly. It is not even necessary to use a platinum resistance sensor, a copper one will work just as well, and may even be convenient as a length of enamelled copper wire can be would around an object (metal container?) whose temperature one wishes to sense. Indeed the NBS (now NIST) used copper resistance thermometers to sense the temperature of its 10pF fused silica standard capacitors. One could replace the variable variable ratio cascaded tapped transformers used in the the AC bridge by a pair of multiplying DACs with an AC reference, however the DACs would need to be 20 bit DACs that are monotonic to better than 1 lsb. The DACs being adjusted to null the output at the junction of the 2 resistors. Another possibility is to use a 24 bit sigma delta ADC to measure the ratio of the voltages across the RTD and the reference resistor. Off course it would be prudent to measure this ratio also when the current through the 2 resistors is reversed in direction so that residual offsets due to thermoelectric and other causes cancel out. Since the ratio of the voltages is being measured the current flowing through the 2 resistors need only have good short term stability as must the ADC gain. As long as it is monotonic the ADC need not have an integral nonlinearity as small as 1 lsb. An LTC2412 or similar ADC should suffice. The trick is to reverse the current flowing through the series connected RTD and reference resistor without affecting its absolute value. A current source feeding the pair of series connected resistors via a suitably connected set of switches will suffice. Details will follow later its 3am here. An alternative technique is to use a quartz crystal cut with a high tempco in an oscillator and just measure the oscillator frequency using the OCXO as a timebase. The HP2804 quartz crystal thermometer used to do this. Bruce _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
