9 Replies Latest reply on Feb 25, 2019 10:30 PM by NiNa_4043086

    Vhi, Vlow for thermistor measurement

    NiNa_4043086

      I would like to measure temperature on my system using a thermistor. Can I connect the Vhi and Vlow to Vdd and '0' internally on PSOC to save two GPIO?

      If yes, how it will affect the measurements accuracy?

       

      Many Thanks!

        • 1. Re: Vhi, Vlow for thermistor measurement
          BoTa_264741

          ninatan,

          Internal connections have undefined resistance of 100-700 oHm, which will affect measurement result. In such connection, when current flows through Rref and Rtherm, voltage potentials on high and low side won't be equal to Vhi / Vlow. Using 3-point measurement this effect is eliminated.

          /odissey1

          • 2. Re: Vhi, Vlow for thermistor measurement
            NiNa_4043086

            Thanks,

            But as I understand there is almost no current flowing through the internal resistance because Rbuf input to ADC is minimum 10MOhm (see below my schematic) so practically high ~ Vhi and Low~gnd. What do you think? I actually don't need more than 0.5 C precision.

            One more question - Is it OK to place the Thermistor at the low side (pulled low as you can see below)?

             

            • 3. Re: Vhi, Vlow for thermistor measurement
              BoTa_264741

              ninatan,

              I would try both to see the difference. Depending on the accuracy requirements it might be OK (probably good for 1 degC, certainly not for 0.01 degC). Keep in mind that Vdd inside is not guaranteed to be exactly same as Vdd outside, same to Gnd.

              /odissey1 

              • 4. Re: Vhi, Vlow for thermistor measurement
                NiNa_4043086

                Thanks!

                Can you explain why in steady state Vhi will not be equale to Vdd and Vlow to GND? What is the variations from these values? I want to calculate the error that I will get.

                 

                regarding the other question -

                I did not sure that I got it. Is it is OK to place the thermistor at the low side (as I mentioned above)? I mean that the thermistor components can handle it?

                • 6. Re: Vhi, Vlow for thermistor measurement
                  MiSt_296941

                  I would not connect the Thermistor to power all the time because it will self heat and introduce an inaccuracy in your measurement.  In my project, I fed it with a VDAC so I could set the voltage to what I wanted and followed that with an Opamp in the PSoC to get enough drive current.  I could then turn the Opamp on only during the time I was going to make the measurements.

                   

                  I tested using an internal 'ground' vs running a pin to the grounded side of the Thermistor and found that indeed there was a slight difference in the reading from the ADC, but it was not very significant.  Once the reading was converted to temperature I could not see a 0.1C change with the two methods.

                   

                  Mike.

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                  • 7. Re: Vhi, Vlow for thermistor measurement
                    NiNa_4043086

                    Thanks Mike,

                    As I mentioned I am trying to measure the thermistor temperature with less GPIOs as I can (preferably one). So I will try to avoid adding a GPIO to fed the thermistor.

                    I will only measure temperature below 0C, at this range the power developed on the thermistor is less than 0.2mW. The thermistor has a dissipation factor of 23mW/C so we get in worst case a self heating of 0.0087C. At accuracy goal of 1C it is negligible.

                     

                    Did you put the thermistor on the low side (pulled down)?

                    • 8. Re: Vhi, Vlow for thermistor measurement
                      MiSt_296941

                      No.  In our circuit the 10K is below the Thermistor.  Not sure it makes any difference as this is a ratiometric measurement.  In other words it's just as important to accurately measure the 10K as it is the Thermistor since both of these results are fed into the function to give you the temperature. 

                       

                      That being said, since you are connecting the Thermistor and 10K to the rails, then how are you measuring beyond the rails with the ADC?  Buffer bypassed?

                       

                      Mike.

                      • 9. Re: Vhi, Vlow for thermistor measurement
                        NiNa_4043086

                        I will not need to use off rail measurements. I have to buffer the ADC because of its relative low resistance which will alter my measurements.

                        I plan to use a pulled up resistor of 150K Ohm. The thermistor resistance vary from 1K to 360K Ohm. Vdd is 3.3V. Therefore Vthermistor will vary from 0.22 to 2.32 V. Than I can use a buffered Rail to Rail  Mode: