8 Replies Latest reply on Apr 7, 2015 3:41 PM by user_14586677

    Please Help Me Understand SAR ADC

    content.librarian

      This is actually related to my thread Noisy Differential Preamp into SAR ADC, but it is a somewhat different subject, so I'm starting a new topic.

         

       

         

      I have made some progress on the noise but am still working on it. While I wait for precision resistors and a few other components to arrive tomorrow, hopefully, I've started looking deeper into the SAR ADC.

         

       

         

      Using ADC_StartConvert(); and the EOC ISR, I have the conversions runnig in sync with the switching polarity of the excitation pins of the load sensor, as verfied by the debugger. I realize, however, that I don't understand what the results mean. I guess I'm still thinking in old-fashioned ADC terms in that I expect values between 0 and 4095 from a 12 bit ADC, but even on individual calls of GetResult16, I'm seeing values well beyond that. I've read the parts of the data sheet related to signed values, left vs right justification, but it hasn't become clear yet.

         

       

         

      I'm confusing things by trying to gain a few bits more resolution by averaging over 256 samples called within the firmware. I wonder if that somehow clashing with or amplifying what is happening in the ADCs own "Samples averaged" settings. Again, reading that section of the datasheet has not yet clarified things for me.

         

       

         

      This turning into another of my "too-uninformed-to-even-state-my-question-clearly" posts, but you all have provided useful information in response to them in the past. Hopefully I will someday be able to return the favor.

         

       

         

      Thanks,

         

      Don

        • 1. Re: Please Help Me Understand SAR ADC
          user_14586677

          I noticed you have your Vref set to Vdd/2. That is a pretty noisy ref selection.

             

          You would be better off using internal 1.024, bypassed as the selection. But

             

          then you have a CM problem in that you would need to offset the bridge, maybe

             

          an R in the hot leg......an analysis would be needed as that would alter G.

             

           

             

              

             

                    http://www.maximintegrated.com/en/app-notes/index.mvp/id/3426

             

           

             

           

             

          Regards, Dana.

          • 2. Re: Please Help Me Understand SAR ADC
            user_342122993

            Instead of wasting time fighting analog circuits why not to switch to one of the low-cost PSoC5LP boards (-059 kit, FreeSoC, SchmartBoard, ...)? Then you can use DelSigADC directly (no OPAMPs necessary). Moreover, I notice that you want to modulate voltage to the bridge, so it will be essentially lock-in detector. For that DelSigADC has special modulation input to change polarity. If you switch to PSoC5LP, I will provide you a lock-in project example.

               

             

               

            Other thing is that if you provide voltage to the bridge from digital pins, do not expect it to be nice and steady. There surely be digital noise and instabilities.

               

            odissey1

            • 3. Re: Please Help Me Understand SAR ADC
              user_242978793
              SAR
              Ultra-High Speed when power consumption not primary concern?
              N bits - 2^N -1 Comparators Caps increase by a factor of 2 for each bit.
              Thermometer Code Encoding
              Sparkle codes / metastability, high power consumption, large size, expensive.
              Conversion Time does not change with increased resolution.
              Component matching typically limits resolution to 8 bits.
              2^N-1 comparators, Die size and power increases exponentially with resolution.
                 

               

                 

               

                 

               

                 

              Here is a chart of the SAR ADC  .  I looked at your scope trace and I think you have a great oscillator.  Start adding some bypass caps and inductors to shunt that oscillaiton to ground.  I still say to try the star ground process..

              • 4. Re: Please Help Me Understand SAR ADC
                content.librarian

                The development started on the PSoC 4 BLE because it was shiny and new and I didn't know all its limitations yet. Now the client wants a demo in a few days so there is no time to get new parts and change direction at this point. I'll just get it to look as good as it can by any means necessary.

                   

                 

                   

                The irony is that we were already talking about going to a PSoC 5 plus a PRoC for the next version.

                   

                 

                   

                Don

                • 5. Re: Please Help Me Understand SAR ADC
                  content.librarian

                  After carefully hand-building a prototype "shield" with perf board (I know, I know),  I was still seeing noise equal to as much of 20% of full scale. I was pretty discouraged at that moment, but I went ahead and tried some heavy smoothing (time-weighted moving average) of the samples in firmware (256 to 512 samples averaged, perhaps overkill).

                     

                   

                     

                  Well, unlike the near-oscillator-like noise I was getting earlier, this noise must have been pretty Gaussian because the averaging generally yielded results within a gram or two across a range of close to 500 gm, or less than 1% error.

                     

                   

                     

                  If we do move on to a second alpha or beta design, however, I am DEFINITELY looking at the PSoC 5 and its 20-bit delta-sigma converters.

                  • 6. Re: Please Help Me Understand SAR ADC
                    user_14586677

                    Yes, diminishing returns, as the square root -

                       

                     

                       

                     

                       

                     

                       

                     

                    • 7. Re: Please Help Me Understand SAR ADC
                      content.librarian

                      Dana:

                         

                       

                         

                      You have a very impressive library of documents and it seems you can recall where and what they all are at a moment's notice.

                         

                       

                         

                      This one should be useful in improving the perceived overall speed/timing performance of the device, which is important for a proof-of-concept demo. Thank you.

                      • 8. Re: Please Help Me Understand SAR ADC
                        user_14586677

                        Who am I again ?........

                           

                         

                           

                        When I was a student in 1970 I discovered Motorola published ap notes.

                           

                        So I wrote them a letter, explaining who I was etc.., and asked if I could

                           

                        get some of them. In a week several boxes showed up, ~ 250 ap notes

                           

                        at the time as I recall, and I was estatic. Over the next several years I

                           

                        read I think ~ 80 % of them.

                           

                         

                           

                        That was the start, now the net is my presonal library so to speak. My

                           

                        only complaint is cannot get access to IEEE library w/o some steep

                           

                        dues. But I think that will change, hopefully.

                           

                         

                           

                        Regards, Dana.