9 Replies Latest reply on Nov 6, 2013 6:09 AM by user_14586677

    PSoC5 amplifier

    junhui.yuan

      The result of PGA amplifier (internal) was much more than gain setting when input voltage was small.

         

      Is there any problem with my kit or something wrong?

        • 1. Re: PSoC5 amplifier
          junichi.hiraoka
                  How much greater?   
          How much original gain setting?   
          You would be better upload the simplified design.   
          • 2. Re: PSoC5 amplifier
            ki.leung

            How is your set up? ie what is your input and how do you measure your output?

            • 3. Re: PSoC5 amplifier
              user_14586677

              Sounds like you are observing noise. If you short the

                 

              input then you will see just noise on the output. Several

                 

              contributions -

                 

               

                 

              1) Thermal due to fdbk R's used

                 

              2) Noise due to PSRR

                 

              3) Noise due to layout and bypassing

                 

              4) Internal/external noise due to C coupling effects

                 

               

                 

              To investigate -

                 

               

                 

              1) If you have a spectrum analyzer look at output noise to determine

                 

              largest fundamental components, then examine if thats correlated to

                 

              an internal clock or divider or pin bit banging. Turn off all internal UDB

                 

              digital and other system resources, one at a time, to see what internal

                 

              resource may be largest contributor.

                 

              2) Caps, best bulk caps bypassing are polymer tantalums paralled my ceramics.

                 

              3) Look at any hi z nodes in signal path, they will be susceptable to coupling via

                 

              stray C.

                 

              4) If driving high C loads, like gate of external MOSFET, look at return current

                 

              paths, make sure they are "stiff".

                 

              5) Using scope on infinite persistance look at supply rails, to get an idea of

                 

              what pk-pk noise levels you are dealing with.

                 

               

                 

              Regards, Dana.

              • 4. Re: PSoC5 amplifier
                junhui.yuan

                 the input voltage was 1.6mv, the gain set is 22, then the result was more than 100mv

                • 5. Re: PSoC5 amplifier
                  user_14586677

                  Don't forget the PGA is not RRIO -

                     

                   

                     

                   

                  • 6. Re: PSoC5 amplifier
                    user_1377889

                    Additionally: input offset max = 10mV times 22 gives 220mV which is well within your measured 100mV

                       

                     

                       

                    Bob

                    • 7. Re: PSoC5 amplifier
                      user_14586677

                      Thtas true if sign of input offset is +, otherwise the results quite different.

                         

                       

                         

                      You can always measure the offset with A/D, and subtract it out. Thats because

                         

                      DelSig CM in is 100 mV ouytside the rails.

                         

                       

                         

                      The beauty of PSOC, ability to do a lot of self test.

                         

                       

                         

                      Regards, Dana.

                      • 8. Re: PSoC5 amplifier
                        user_78878863

                        Out of curiosity - how can I measure the (internal) offset voltage of an OpAmp? (Apart from shorting the inputs and then measuring the output)

                        • 9. Re: PSoC5 amplifier
                          user_14586677

                          This basic approach might be usable (you can always use muxing, S/H in general designs) -