12 Replies Latest reply on Apr 2, 2015 1:17 PM by user_242978793

    Noisy Differential Preamp into SAR ADC

    content.librarian

       Once I realized that I hadn't blown my opamps, I resumed work on my load cell project - schematic attached.  I am reading voltage values that make some sense in the context (e.g. values get proportionaly larger when more weight is placed on the cell), but the signal coming off the differential preamp is swinging wildy in value. The SAR readings are therefore swinging wildly too, sometimes even more so than their analog counterparts.

         

       

         

      I'm too delirious to ask a more detailed question or do other research at this exact point, but since this is for a project on a tight deadline, I thought I'd at least get the conversation started.

         

       

         

      Thanks,

         

      Don

        • 1. Re: Noisy Differential Preamp into SAR ADC
          user_342122993

          As test measure I suggest to remove OPAMPs and measure voltage from bridge directly by ADC. Preferably 20-bit DelSigADC.

             

           

             

          Useful Cypress notes:

             

          Instrumentation Amplifier http://www.cypress.com/?docID=41827

             

          Maintaining Accuracy With Small Magnitude Signals http://www.cypress.com/?docID=26838

             

           

             

          odissey1

          • 2. Re: Noisy Differential Preamp into SAR ADC
            user_78878863

            Even though you have not completely fried your OpAmps, you still might have damaged the pins. Then this might be the reason that it doesn't work as expected. Can you try with a board thats surely not damaged? Or try the project to use OpAmps / pins that are surely not affected?

            • 3. Re: Noisy Differential Preamp into SAR ADC
              user_14586677

              Both these ap notes odissey1 mentioned have severe errors in them due to the fact

                 

              the PGA G matching is very poor, hence CMR in the IA severly crippled. Don't implement

                 

              an IA that way, either 2 or 3 OpAmp versions. An analysis follows.

                 

               

                 

                  

                 

                       

                 

              http://www.cypress.com/?app=forum&id=0&rID=82151     IA Analysis Matlab

                 

               

                 

              www.dropbox.com/s/plck7e95v7pw33c/CMR%20Analysis%20IA.pdf

                 

               

                 

              Some questions -

                 

               

                 

              1) What is matching accuracy of your 120K Rfdbk resistors ? Even if they have

                 

              great matching do the analysis of Aol G matching for the OpAmps, might not

                 

              be good enough for your design.

                 

              2) What is pk-pk A/D result noise you are getting ?

                 

              3) Using a DSO, infinite persistence, look at A/D inputs to see what

                 

              total noise looks like into A/D

                 

              4) Ground loops present ?

                 

               

                 

              Regards, Dana.

              • 4. Re: Noisy Differential Preamp into SAR ADC
                content.librarian

                Thank you to everyone for the assistance.

                   

                 

                   

                To answer Dana's questions, I have attached a scope shot.

                   

                 

                   

                1. Resistors are not matched. Once I recalculate the gain necessary, I will order some resistor networks or precision-matched resistors ASAP.

                   

                 

                   

                2. Noise is as high as 60 mV peak-to-peak based on the attached scope shot, a 6% error in a 1.024 V measuring range.

                   

                 

                   

                3. Also from the scope shot, noise appears to be largely 100 MHz sinusoid with other random noise.

                   

                 

                   

                4. I will check for ground loops. Since the source of the noise seems to be external RF (nothing on the board is running faster than 24 MHz), ground loops might be a likely culprit.

                • 5. Re: Noisy Differential Preamp into SAR ADC
                  user_14586677

                  Polymer caps for bulk bypass best, ~ 1 order magnitude better f vs Z

                     

                  curves, to replace traditional tants. Look at actual datasheets, same

                     

                  for ceramics, not all caps are as 'good" as we assume.

                     

                   

                     

                  Some useful ref material on PCB layout and noise....

                     

                   

                     

                  www.dropbox.com/s/ruaf9booe17jk8n/PCB%20Layout.zip

                     

                   

                     

                  Regards, Dana.

                  • 6. Re: Noisy Differential Preamp into SAR ADC
                    user_14586677

                    From tech support -

                       


                       

                          

                       

                                  

                       

                    The front-end of the SAR ADC can be modeled as a first-order RC (low-pass) network- with R = 2.2

                       

                    kohm and C = 10pF. So, R = 2.2kohm is not really the net input resistance.

                       

                     

                       

                    And the input impedance of the SAR is a function of the sampling rate and is given by, Zin = 1/ (fs*C),

                       

                    where C = 10pF.

                       

                     

                       

                    We should not take the 2.2Kohm as the input resistance of the SAR as it is.

                       

                     

                       

                    Regards, Dana.

                    • 7. Re: Noisy Differential Preamp into SAR ADC
                      user_14586677

                      Rough approximation (at the pin, does not include your interface R, C) -

                         

                       

                         

                       

                      • 8. Re: Noisy Differential Preamp into SAR ADC
                        content.librarian

                        Dana:

                           

                         

                           

                        Thank you very much for doing all this analysis. Since I utimately want to minimize off-PSoC components, it will be put to good use.

                           

                         

                           

                        In the interest of meeting a tight deadline for an in-process demonstration, however, I thought of using an INA826 I have handy to pre-condition the sensor signal, and then going straight in to the PSoC SAR ADC as shown in the attached image. This circuit is not built yet nor are the resistor and cap values calculated yet.

                           

                         

                           

                        A few questions I have if this is even worth pursuing:

                           

                         

                           
                              
                        • Would it help to use one of the PSoC op amps just as a follower before the ADC?
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                        • Should the filter caps, C_1 and C_2, go to Vref instead of Vss?
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                        • The reason I want to go to a single-ended inst amp: I'm having a hard time in firmware getting the polarity switching of the sensor excite pins to sync up with the ADC sampling. The single-ended ADC with Vref as the negative input seems to give the greatest dynamic range for positive-only sensor readings. Thoughts?
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                        Thank you again for all the help.

                        • 9. Re: Noisy Differential Preamp into SAR ADC
                          user_14586677
                                
                          • Would it help to use one of the PSoC op amps just as a follower before the ADC? 
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                          No, I think the INA826 takes care of that problem, eg. presenting a low z Vx source to the SAR

                             

                           

                             
                                
                          • Should the filter caps, C_1 and C_2, go to Vref instead of Vss?
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                          I am sure the Vref of INA should be bypassed and at Vdda / 2 to maximize dynamic range.

                             

                          There is another problem with this part, its input does not common mode to positive rail.

                             

                          Is that OK with your expected output of the bridge ? The Caps C1 and C2 should be OK

                             

                          to help noise suppression as long as the latency of a reading due to bridge to C1/C2

                             

                          settling time OK.

                             

                           

                             

                           

                             
                                
                          • The reason I want to go to a single-ended inst amp: I'm having a hard time in firmware getting the polarity switching of the sensor excite pins to sync up with the ADC sampling. The single-ended ADC with Vref as the negative input seems to give the greatest dynamic range for positive-only sensor readings. Thoughts?
                          •    
                             

                           

                             

                          You could synch the SAR be selecting trigger mode and using PSOC comparator from excite

                             

                          signal to generate the Trigger.

                             

                           

                             

                          Regards, Dana.

                          • 10. Re: Noisy Differential Preamp into SAR ADC
                            content.librarian

                            Again - thank you!

                               

                             

                               

                            I will try a few different schemes over the next few days to see which gives the best practical results, but I think I will initially focus on the alternating excite voltage because that provides some noise cancelation while also increasing dynamic range. I'd also gotten that working once before with the INA826 and a different microcontroller, except that micro would lock up after a while, so I didn't pursue develpment on it further once I moved to the PSoC.

                               

                             

                               

                            I will report which scheme I end up using once I make that decision.

                            • 11. Re: Noisy Differential Preamp into SAR ADC
                              user_14586677

                              Great, glad to have been of assitsance.

                                 

                               

                                 

                              Regards, Dana.

                              • 12. Re: Noisy Differential Preamp into SAR ADC
                                user_242978793

                                Use a Star ground for all the analog devices and also you might try a shielded area ground on the inputs.  A star ground is one where all the analog device are tied to one point on the board.  Also a shielded area ground has The input trace surrounded by a ground shield which is tried to star ground.  Also it is hard to get good fidelity  with a bread boad or other temporary wiring setups.  I assume that you have followed the Cypress layout guide for the PSOC device and have the ground planes and by pass caps.  You could also add a Ferrite bead on the power leads.