We have a space craft application that requires the use of the CX-1080. We need to design a circuit to excite and monitor the RTD. For other temperature sensors we typically provide a constant current excitation source. Will this work with the CX-1080 without creating too much self heating?
Post by Lake Shore Ogi on Apr 16, 2018 15:43:43 GMT -5
Your question is somewhat tricky to answer but I will provide you with some hopefully useful information. Considering that our Model 218 monitor only uses a 10 uA constant current source, you should be able to achieve some kind of temperature monitoring using the same method. The instrument, however, has an input impedance limitation of 7.5 kOhm, which limits the ability to read the Cernox 1080's to at minimum of 50K or greater. Anything lower would not work on that monitor.
I would suggest you testing your circuit and keeping excitation current as small as possible. Going lower on the excitation current has also its limitations and challenges but hopefully you will be able to achieve monitoring without causing too much self-heating of the sensor. I regretfully don't have a chart or some kind of reference I can share with you on self-heating with our sensors.
Post by Lake Shore Ryan on Apr 16, 2018 16:26:15 GMT -5
I can maybe provide a little additional information here. If you use our high-reliability version of the CX-1080 sensors, there is an option where we calibrate the sensor at a fixed 10 µA as many flight programs use this type of simplified circuitry. That way, self-heating is built into the calibration.
The importance of this will depend on your operating temperature, as you would only begin to see self-heating when the sensor goes much higher than 1 kΩ in resistance, resulting in a voltage greater than 10 mV. For CX-1080s, this happens at around 100 K depending on the particular sensor. Either way, our calibrations extend down to 20 K for these sensors.
As Ogi said, your other option would be to pick a custom excitation that will allow you to stay under 10 mV of signal at your lowest expected temperature and then you wouldn't have to worry about self-heating at all. Then it just becomes a matter of determining whether the associated loss of resolution at higher temperatures is acceptable.
If you'd like to know more about our HR Cernox Series, please reach out to us directly, as flight programs have security requirements that we need to adhere to.
Ryan Oliver Product Manager, Sensors and Instruments Lake Shore Cryotronics