4 Replies Latest reply on Sep 2, 2016 11:20 AM by dingram9640_1725166

    SigData using VREF as input.

      I am new to PSOC and am trying to become familiar with the CY8CKIT-059 PSoC 5LP Prototyping Kit.  I would like to use it as a low cost, fairy accurate multi channel data acquisition system.  I would like to use  the internal 1.024 VREF as one of the input signals and use the resulting value to judge the accuracy of the remaining channels.  Unfortunately,  I don't get the expected results.    I would like to use the 20 bit Sigma Delta A2D with a single input range of 0.0 to 6 *Vref.  When I connect the Vref through an analog mux I get values of approximately 1.78 volts.  When I buffer the input through an op amp follower I get results of 2.33 volts.  I am obviously missing something basic. 


      Can someone give me a hint??

        • 1. Re: SigData using VREF as input.

          Welcome in the forum!


          Can you please post your complete project, so that we all can have a look at all of your settings. To do so, use
          Creator->File->Create Workspace Bundle (minimal)
          and attach the resulting file.





          • 2. Re: SigData using VREF as input.

            Attached is a cut down version of using a 3 channel digital mux.  The DVDAC is just a test signal.   The timer is just  is my attempt to let the mux stabilize after switching signals. The TestTimer_1 just provides a basic 10 ms clock count.  There is a warning that there is no direct path for the 1.024 ref to the mux. I am not sure how that will affect the results.  

            • 3. Re: SigData using VREF as input.

              There is absolutely no need to measure the unbuffered 1.024V reference. Delete that from your topdesign.


              Set the ADC's reference to "Internal Bypassed on P0_3" and provide a blocking cap (1µF) to GND on P0_3. Otherwise you will never get 20bit resolution.





              • 4. Re: SigData using VREF as input.

                I couldn't get a good value for either the buffered or unbuffered VREF.  The message that there was no direct signal path from VREF to the MUX probably means that the input was floating.  That explains why that value is bad.  


                The reason the buffered value was floating and yielding bad results is I DID NOT START THE OP AMP in main.c   MY BAD!!!!!! 


                Documentation indicates that the bypass of PO.3 is needed for high frequency.  I did put the bypass in (after I started the op amp) and there is a difference n values obtained from the SIGDEL.


                Now I have to address the accuracy


                I set the input to mux and corresponding pins to different sources and used my DMM to measure the voltage on the pins as the program was sending data.


                Source                                   DMM            Sig Del      Ratio
                DVDAC set for 1000mv      0.9975            1.0234       1.025
                Voltage Regulator                 3.3553            3.4535       1.029
                Buffered Vref                       1.0233            1.0496       1.026


                My tendency is to believe the DMM and conclude that the Mux/SigDel yields results that are 2.6% high and  I should scale the data from other channels accordingly.


                Thanks for your response.