11 Replies Latest reply: Jul 29, 2015 3:50 PM by rmiddleton RSS
      • 1. Re: CN0287: Isolated 4-Channel, Temperature Measurement Circuit
        Suresha

        Hi,

         

        One of our new designs, requires us to implement RTD & Thermocouple measurement circuit.

        We need to get a accuracy of better than "0.2% of FSR" over the operating temperature range of "-20°C to 60°C".

        We have looked at CN0287 board & planning to use the same circuit in our board if the required accuracy is met.

         

        The CN0287 specification says that accuracy of 0.5°C. Is this accuracy guaranteed over a specific temperature range?

         

        For our accuracy analysis , we have identified the error introducing components as below :

         

        RTD measurement :

         

        • Resistor Rref  - Tolerance & Temperature coefficient
        • ADC AD7193 - ENOB , INL , Offset error, Gain error , Offset drift , Gain drift

        Due to ratio-metric cancellation,  change in excitation current & ADC reference voltage cancel out each other.

         

        Thermo-couple measurement :

         

        • ADC AD7193 - ENOB , INL , Offset error, Gain error , Offset drift , Gain drift
        • ADC Reference ADR3425 - Initial Accuracy, Line regulation , Load regulation, Output Noise
        • Cool Junction Compensation ADT7310 - Accuracy ( 0.5°C )

         

        Please let us know if we have missed out any parameter in our analysis.

         

        Also, it will be helpful if you can share any document related to accuracy analysis done on CN0287.

         

        Looking forward to your response.

         

        Regards,

        Suresha N S

        • 2. Re: CN0287: Isolated 4-Channel, Temperature Measurement Circuit
          jtillack

          Hello,

          Is there any reference on how the CN0287 demonstration software is using / configuring the AD7193? Specifically, it is not clear how the AD7193 should be configured for the different sensor types. Could someone write a brief description such as:

          For 2 wire RTD measurements, the AD7193 will be configured as follows...

          For 2 wire thermocouple measurements the AD7193 will be configured as follows....

          etc

          This information would make the reference design much more useful.

          Any help will be greatly appreciated.

          -Jonathan

          • 3. Re: CN0287: Isolated 4-Channel, Temperature Measurement Circuit
            Wyue

            @Hello Sir

             

            We did the CN0287 evaluation software by Labview and I attached the snapshot of default configuration used by Evaluation software for different type of sensor.

             

            All the configuration for different types of thermocouples and RTDs are almost the same except the gain setting which is depend on the output signal scale. The gain were decided by the type of sensor and the temperature range of the application. We prefer to occupy at much of dynamic range as possible by setting the biggest reasonable gain to process the signal.

             

            With best regards,

            Willie.

            • 4. Re: CN0287: Isolated 4-Channel, Temperature Measurement Circuit
              nmykris

              This circuit has been implemented in a design that is interfaced with a proprietary CPU. Having been given the task of writing the driver, I'm trying to understand how to perform temp measurement with a type K thermocouple. The "Default Configuration.png" is helpful in configuring the devices but I'm not sure how to apply the data read from the ADT7310 and the AD7193 to compute the temperature at the thermocouple end. Can someone walk me through the steps/equations or point me to a tutorial on how to accomplish this?

               

              Any help would be greatly appreciated,

              Nick

              • 6. Re: CN0287: Isolated 4-Channel, Temperature Measurement Circuit
                rmiddleton

                Hello,

                 

                I'm trying to change the value of the AD5201 digitial pot on the board, however because the chip select is tied to P3 on the AD7193 I can't properly change the value on the AD5201.  What happens is that I write a SPI message to the AD7193 to set the P3 pin, write out to the SPI bus to change the value of the AD5201, and then write back out on the SPI bus to change the P3 pin value, so the old value in the AD5201 is overwritten.  Is there some way around this, or do I need to modify the board to pull the chip select off?

                • 7. Re: CN0287: Isolated 4-Channel, Temperature Measurement Circuit
                  Brandon

                  Hi Robert,

                   

                  In the default implementation of the design, what you are doing is proper and only way to change the value of the AD5201.  Because the CS of the AD5201 is tied the AD7193, you need to active the P3 output and then set it low, after which you can send SPI data out to the AD5201.  Once completed you'll need to reset the P3 output to tristate mode through the AD7193.

                   

                  I think the design was done this way because of the isolation channels on the board, and trying to minimize those GPIO connections to the processor. 

                   

                  I wouldn't recommend modifying the board as something might go wrong, and you would lose that digital isolation for sure.

                   

                  Cheers,

                  Brandon

                  • 8. Re: CN0287: Isolated 4-Channel, Temperature Measurement Circuit
                    rmiddleton

                    Brandon,

                     

                    I'm confused because I can't change the P3 output without writing out to the SPI bus, which will overwrite the value that the AD5201 just clocked in.  I haven't seen anything in the datasheet which would imply that the AD5201 only clocks in one byte, it looks to me like it will clock in data on every clock change.

                     

                    I've attached a scope capture, 1 = P3 pin, 2 = AD5201 output, 3 = MOSI.  I set the P3 pin, write out a byte to the AD5201, and then write out two bytes to the AD7193 to change the value of P3.

                    • 9. Re: CN0287: Isolated 4-Channel, Temperature Measurement Circuit
                      Brandon

                      Hi Robert,

                       

                      You bring up an interesting point, because how can you bring the AD5201 CS line low, without writing out on the SPI bus to the AD7193 and writing in a new value on the AD5201?? 

                       

                      Let me check with our guy who did the software, and we'll get back to you on what he did for this.

                       

                      Cheers,

                      Brandon

                      • 10. Re: CN0287: Isolated 4-Channel, Temperature Measurement Circuit
                        Wyue

                        Hello Robert

                         

                        It did confuse me when I was working on the circuit but it finally solved with 30% effort and 70% of good luck .

                        Please see the attached email from my colleague with the same confusion as you do.

                         

                        With best regards,

                        Willie.

                        • 11. Re: CN0287: Isolated 4-Channel, Temperature Measurement Circuit
                          rmiddleton

                          Thanks, that makes more sense now at least.