8 Replies Latest reply on Feb 19, 2015 5:56 AM by user_14586677

    Delta Sigma Noise

    user_301665782

      Hi Every One !

         

       

         

      I would need some help regarding noise generated for the Delta sigma in single end configuration.

         

      As you can see in the schematic as result of array scanning I am geting pulses which votage depend of the pressure applied to every cell in the array.  My Vdda is 3.3 v . The issue is that with no pulse, ( checked with osciloscope) there is noise that  you can watch in the graphic display picture.

         

      The only way to avoid that is using a PGA before the Delta sigma..

         

      I cannot undestan that. !!!!

         

      And even using a PGA,  I need to filter trough  code , like high pass filter values above 0x80, wich represent almost a half of the ADC range.

         

      I woul appreciate very much any help.

        • 1. Re: Delta Sigma Noise
          user_14586677

          It would be helpful if you posted your project. Note you mentioned something

             

          attached but there is nothing present in your post.

             

           

             

          At high resolution layout, bypassing critical to getting noise floor capability

             

          as shown in datasheet. These may help -

             

           

             

              

             

                    

             

          http://www.cypress.com/?rID=39677     AN57821 - PSoC® 3, PSoC 4, and PSoC 5LP Mixed Signal Circuit Board Layout Considerations

             

          http://www.cypress.com/?rID=40247     AN58827 - PSoC® 3 and PSoC 5LP Internal Analog Routing Considerations

             

          http://www.cypress.com/?rID=39974     AN58304 - PSoC® 3 and PSoC 5LP – Pin Selection for Analog Designs

             

           

             

              

             

                    

             

          http://www.cypress.com/?rID=43337     AN61290 - PSoC® 3 and PSoC 5LP Hardware Design Considerations

             

           

             

           

             

          Consider the following when at hi res -

             

           

             

          1) Shut off all digital I/O on pins adjacent to A/D inputs, also consider

             

          stoping UDB resources to minimize internal ground bounce and coupling

             

          2) Set Vref to external bypass, use a polymer tant to bypass it.

             

          3) Simple averaging helps noise reduction

             

          4) Using specturam analyzer determine noise source by its signature

             

           

             

          Regards, Dana.

             

           

             

              

             

                    

             

          Consider posting your project, makes life easier to troubleshoot.

             

           

             

          “File”                                                             Creator

             

          “Create Workspace Bundle”

             

           

             

          Regards, Dana.

          • 3. Re: Delta Sigma Noise
            user_301665782

            Thank you Dana  for your help!

               

             

               

            It's seem to be that it cannot be uploaded  my pdf where I show you my schematic.

               

            I tried to upload twice.

               

            I will try again.

            • 4. Re: Delta Sigma Noise
              user_301665782
                      I will try with Google Chrome   
              • 5. Re: Delta Sigma Noise
                user_14586677

                Route your Amux output to a pin, do you see 7 Mhz  clock transients

                   

                on it ? I would posit the reason a PGA suppresses transients is because

                   

                of its slew rate limiting effect, eg. it acts as a LPF to edge transients like

                   

                clock feedthru in a CMOS switch.

                   

                 

                   

                Regards, Dana.

                • 6. Re: Delta Sigma Noise
                  user_301665782

                  Thank you Dana !!!

                     

                  May be I cannot understand you complletelly.

                     

                  I undestand that you said a bout PGA acting like a filter .

                     

                  But , what do you mean saying tp pass the Amux output trough a pin ?

                     

                  On the other hand,  If I put  ociloscope's test point at resistance of 2.2 k which is hooked up to pin 3[3] and there is no  array cell touched, there are  information going to the dipaly like random noise even when you cannot see any signal on the osciloscope.

                     

                   

                     

                  Tnak you very much Danna

                  • 7. Re: Delta Sigma Noise
                    user_301665782

                    Hi there !  are there some other input for my question?

                       

                    Thank you all of you.

                    • 8. Re: Delta Sigma Noise
                      user_14586677

                      You accomplished what I stated by connecting your scope to the

                         

                      2K R at the output of the Amux. The idea was to observe the noise

                         

                      at the A/D input from the Amux output at a pin.

                         

                       

                         

                      Do you still get a lot of noise when you short the 2K R to ground, therefore

                         

                      making A/D input also ground ?

                         

                       

                         

                      Best would be to post a project archive as I discussed earlier.

                         

                       

                         

                      Regards, Dana.

                      • 9. Re: Delta Sigma Noise
                        user_14586677

                        Are you setting your reference to allow for external bypassing ?