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constant current TC


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#1 jan middelink

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Posted 04 September 2012 - 03:49 PM

I'm a bit puzzled by the the graphs on page 8, the Y axis reads ppm/degc. What is the typical TC say from 20degc to 40degc? is it -30 ppm or 20x-30ppm or -600ppm. I would like to connect PT100 elements using the 200uA current source and achieve accuracies better then 0.1 degc. Typical temperatures I'm looking at are 135 degc. With the suggested settup of Figure 2.5-4 I could do this using the 100mV range for 3 sensors getting more than sufficient resolution. The TC could however cause problems here

#2 LabJack Support

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Posted 04 September 2012 - 04:22 PM

The x-axis is the temperature of the current source. That is, the temperature of the U6. So if you keep the U6 at around 30 degrees C, you can expect the tempco of the current source to be about -20 PPM/degC. This gives you an idea of how much a few degrees of temperature change to the U6 will affect the output of the current source. If you use the precision resistor mentioned earlier in Section 2.5, you don't have to worry about tempco because you will be measuring the actual current in real time. If the U6 temperature is changing greatly, though, you do have to worry about tempco effects on the analog inputs themselves.

#3 jan middelink

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Posted 04 September 2012 - 10:44 PM

The x-axis is the temperature of the current source. That is, the temperature of the U6. So if you keep the U6 at around 30 degrees C, you can expect the tempco of the current source to be about -20 PPM/degC. This gives you an idea of how much a few degrees of temperature change to the U6 will affect the output of the current source.

If you use the precision resistor mentioned earlier in Section 2.5, you don't have to worry about tempco because you will be measuring the actual current in real time. If the U6 temperature is changing greatly, though, you do have to worry about tempco effects on the analog inputs themselves.


I expect the temperature to rise to 40 degc, any idea what the tc is for the analog inputs?

#4 LabJack Support

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Posted 05 September 2012 - 07:36 AM

Appendix A of the user's guide states 15 ppm:

http://labjack.com/s...uide/appendix-a

If this sort of tempco is significant in your system, I suggest you determine the actual tempco of your U6 in the desired region of operation, and then include that in your calculations. One good way to measure tempco is to have a fixed voltage source at steady temperature, and connect that to an analog input on a U6 where temperature is varied.


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