1 2 Previous Next 16 Replies Latest reply on May 30, 2013 1:22 PM by user_14586677

    Sample and hold module VREF

    ki.leung

      HI,

         

      I am checking on example project and found the sample and hold example. The sample and hold component can use internal and exteranl as Vref, but I could not find any reference on why and when you should use the internal or external vref. Any idea where can I find the information?

         

      Tks

        • 1. Re: Sample and hold module VREF
          user_78878863

          I know two reasons for using an external reference:

             
                
          • the internal one is not precise enough, ot to unstable (temp coefficent)
          •     
          • the internal one has the wrong voltage (e.g. you need 2.048V but the internal one only goes to 1.2V)
          •    
             

          On the -050 kits there is even a spare footprint for an external reference with higher precision (and I think on the -030 kits too). The internal reference is accurate for about 0.9%, with a temp coefficent of 50ppm. The proposed LM4140 gives you 0.1% and 3ppm.

          • 2. Re: Sample and hold module VREF
            user_14586677

            1) You have to do an end to end error analysis to see what reference tolerance you need.

               

            2) The internal reference can be scaled with PGA, its gain and drift would contribute to ref error analysis.

               

            3) The S/H probably does not warrant need for high precision external reference, as its own errors

               

            probably dominate end to end error analysis. But my conjecture can only be established by doing

               

            the error analysis.

               

            4) LM4140 A grade an expensive part, and only a 0 - 70 C part. Digikey shows ~ $4 - $5

               

            part. The 10 ppm part more like $ 1.50 - $ 3.00.

               

             

               

            The onchip reference is speced over I temp range, and its specs look like -

               

             

               

             

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            • 3. Re: Sample and hold module VREF
              ki.leung

              But how is the reference affecting the S&H?

                 

              May be I missed something here. Where is the reference come into play with the S&H process?

              • 4. Re: Sample and hold module VREF
                user_14586677

                This should help -

                   

                 

                   

                 

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                • 5. Re: Sample and hold module VREF
                  ki.leung
                          Thanks Bob. May be I should make it clearer at the beginning. I read the datasheet already. My question is. If I use internal reference . What is the effect on the S&H output, how is it being affected. Would it be +-x% of the input, where x is the error of VREF?. Would there be noise on the output which is the noise of the VREF? If I use external VREF of 2.5V, would it mean that the input must be 2.5V or above? Would it mean voltage below the VREF would be different to those ABOVE 2.5V? I got the small text box again....!!!!   
                  • 6. Re: Sample and hold module VREF
                    ki.leung

                    Also, is the effect of Vref post-samplig or pre-sampling?

                    • 7. Re: Sample and hold module VREF
                      user_78878863
                              This took a while to find, it is hidden well in the TRM. When you look at the PSoC5 TRM, part 30.9 ("Sampled Mixer"), there is the statement: The use of the internal ground can cause different step sizes up versus down because the amplifier does not respond identically when the negative terminal jumps below ground. To avoid this distortion, use the external reference option and set it to 500 mV or greater. There is also a schematic showing how the S&H internally works (and I must to admit that I still not fully comprehend it). And sorry for the formatting, there is none right now for me. One should think hat after 4 weeks or so it should work. Can I redeem some of my reward points for a proper editor, please?   
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                      • 8. Re: Sample and hold module VREF
                        user_78878863
                                I should have used some quotation marks around the quote :( So what it comes down to (as far as I understand it): The internal reference for the S&H is not Vref, but ground (bad wording, I think, then). Using it seems to affect the linearity of the S&H, and can cause maybe jumping signals (the S&H samples at both the falling and rising edge, and the internal amplifier behaves not linear when the negative input goes below ground (which it will in the S&H configuration). One should guess this should be part of the component data sheet, and not be hidden in the TRM...   
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                        • 9. Re: Sample and hold module VREF
                          user_78878863
                                  @summon dana: I'm pretty sure you know what this means exactly, you are way better in these things than I am...   
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                          • 10. Re: Sample and hold module VREF
                            ki.leung
                                    Thanks hli, Shall check the TRM for details. As you said. it should be included in the data sheet or at least give a link to the TRM.   
                            • 11. Re: Sample and hold module VREF
                              user_296761584

                              Went thru the TRM, it seems that using the internal one would be a bad option. I think it should state this in the datasheet and give more details of what would happen as well as the requirement of having external reference to more than 500mV.

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                              • 12. Re: Sample and hold module VREF
                                ki.leung

                                I think Cypress should update the data sheet.

                                • 13. Re: Sample and hold module VREF
                                  user_14586677

                                  I filed a case with Cypress as there are two different schematics showing two different

                                     

                                  T/H, S/H capacitor sizes.

                                     

                                   

                                     

                                  I also address this step size issue. I think this reference is to mixer operation, CMR

                                     

                                  allowable. But I have seen ap notes discussing an allowable CMR operation that

                                     

                                  does not comply to datasheet.

                                     

                                   

                                     

                                  Will keep forum informed when I get answer.

                                     

                                   

                                     

                                  Regards, Dana.

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                                  • 14. Re: Sample and hold module VREF
                                    user_14586677

                                    I filed a CASE on this to clarify -

                                       

                                     

                                       

                                    1) Two different schematics in TRM show two different cap sizes for S/H, T/H

                                       

                                     

                                       

                                    2) I think the Vref comment about step size in mixer is a CMR issue of OpAmp,

                                       

                                    not sure that it applies to S/H, T/H, just mixer. But I have been wrong before on guesses, 

                                       

                                    so have asked Cypress to clarify. However the Vref issue in module datasheet

                                       

                                    totally incomplete as to what it is doing when applied to S/H, T/H. I think it is a CMR

                                       

                                    issue again with internal OpAmp.

                                       

                                     

                                       

                                    Will keep forum informed.

                                       

                                     

                                       

                                    Regards, Dana.

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