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I have 6 3 wire Pt-100 sensors that i want to connect to my U6 using the 200 uA current source, for measuring temperatures between 20 and 100 °C. Which is the best way for doing so?
A PT-100 will be ~138 Ω at 100 ºC. With a 200 µA current source that's only a 27.6 mV drop. So you can get away with connecting all three PT100s in series and use differential readings, set to 0.1V range, across each sensor. You can perform single ended measurements and subtract the difference in software, but that will be more prone to error.
One more thing, the AINs have ~ 300 pF of capacitance. The current source may need a little extra settling time to adjust to the switched capacitance being added to the circuit. Just turn up the AIN's settling time to compensate.
In Section 12 of the T7 Datasheet it says that the maximum total load resistance that 200UA can drive is 15 kohms, which keeps its drive voltage to 3 volts or less.
Also, if you need the highest accuracy you need to use a precision fixed resistor to measure the actual current, also discussed in Section 12. That will use 1 analog input, so if you use differential inputs for the RTDs your limit is 6 of them, unless you add the Mux80.
Another consideration is that we have found that readings start to get noisy with more than a few loads put in series with the current source. It just seems that it is difficult to get all the wiring done without adding noise as you go. I would consider using the LJTick-Resistance-1k for your RTD measurements. You can at least try them and compare to the current source method.
Another benefit to the LJTR is that you will be doing all single-ended measurements. Each LJTR can handle 2 RTDs and you can connect up to 6 LJTR to the T7 leaving a couple channels open to feedback the excitation voltage.
Also, check out our temperature sensors app note. Of the 4 basic types of temp sensors, RTDs are probably the most difficult to work with and in practical use the average user will get better results from any of the other types of sensors.
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A PT-100 will be ~138 Ω at 100 ºC. With a 200 µA current source that's only a 27.6 mV drop. So you can get away with connecting all three PT100s in series and use differential readings, set to 0.1V range, across each sensor. You can perform single ended measurements and subtract the difference in software, but that will be more prone to error.
One more thing, the AINs have ~ 300 pF of capacitance. The current source may need a little extra settling time to adjust to the switched capacitance being added to the circuit. Just turn up the AIN's settling time to compensate.
Slightly off topic, but if you are not yet set on the U6 the T7 has a build in RTD function that will run the temperature conversion for you. Documented here: https://labjack.com/support/datasheets/t7/ain/extended-features/rtd
Thank you very much.
With this information i will get a T7 then!!! How many PT100 can I connect in series? How can I calculate the maximum number of RTD's?
Thank you
In Section 12 of the T7 Datasheet it says that the maximum total load resistance that 200UA can drive is 15 kohms, which keeps its drive voltage to 3 volts or less.
https://labjack.com/support/datasheets/t7/200ua-and-10ua
Also, if you need the highest accuracy you need to use a precision fixed resistor to measure the actual current, also discussed in Section 12. That will use 1 analog input, so if you use differential inputs for the RTDs your limit is 6 of them, unless you add the Mux80.
Another consideration is that we have found that readings start to get noisy with more than a few loads put in series with the current source. It just seems that it is difficult to get all the wiring done without adding noise as you go. I would consider using the LJTick-Resistance-1k for your RTD measurements. You can at least try them and compare to the current source method.
https://labjack.com/accessories/ljtick-resistance
Another benefit to the LJTR is that you will be doing all single-ended measurements. Each LJTR can handle 2 RTDs and you can connect up to 6 LJTR to the T7 leaving a couple channels open to feedback the excitation voltage.
Also, check out our temperature sensors app note. Of the 4 basic types of temp sensors, RTDs are probably the most difficult to work with and in practical use the average user will get better results from any of the other types of sensors.
https://labjack.com/support/app-notes/temperature-sensors