5 Replies Latest reply on Sep 4, 2018 1:33 PM by user_436055692

    Alternate Vref bypass capacitor connection

    user_436055692

      On PSOC5LP P0[2] and P0[4] are dedicated to the connection of an bypass capacitor for ExtVrefL_1 and EXTVrefL_2 (internal Vref of each SAR ADC).

      Alas when using the internal OPA of the PSOC5 these two pin are assigned to the non inverted input.

      Using internal OPA and Vref bypass capacitor at the same time is the goal.


      Is it possible to bypass the internal Vref by connecting it to another analog output and place a discrete capacitor to GND?
      Any suggestion of an alternate connection point is welcome.

        • 1. Re: Alternate Vref bypass capacitor connection
          vsrs

          Hello Jacques,

           

          The pins 0[2] and 0[4] can act as dedicated pin for SAR bypass capacitor connection and as the Opamp negative input. These connections are not programmable. Unfortunately it is if you want to use the same piar of opamp and bypass option in SAR.The solution out of this(if you are using only 2 opamps) is to use the next pair of opamps Opamp2 and Opamp3 which make use of port3.

           

          Best Regards,
          VRS

          • 2. Re: Alternate Vref bypass capacitor connection
            user_436055692

            Thank you for the answer.

            If I use a Psoc with only two OPA the SAR bypass capacitor pin are not more available when the OPA are activated.

            Then I am looking for a backup plan.
            If Vref is output to a analog pin  where an external bypass capacitor is connected, will it improve the SAR behavior and Vref noise level?

            • 3. Re: Alternate Vref bypass capacitor connection
              vsrs

              Hi,

               

              You need to make sure that the opamp pair that do not use the 0[2],0[4] pins are used in your project. To make sure that you are using the right opamps, you can go to CYDWR -> Analog tab and check where the opamps are placed. You can right click on the placed component and relocate to the position you want.

               

              Your second statement is right. The bypass capacitor enable SAR to work in higher datarate with better noise immunity. Without bypass capacitor the conversion rate will be limited.

               

              Best Regards,
              VRS

              • 4. Re: Alternate Vref bypass capacitor connection
                user_78878863

                Even though you are forced to place the Vref bypass caps on the dedicated pins, you are not also forced to do the same with the OpAmp pins. The latter ones are routable in the PSoC routing matrix.

                I just places two OpAmps and a SAR ADC, and let Creator route anything by itself:

                screenshot_pins.png

                Routing for Vref:

                screenshot_analog_vref.png

                Routing for OpAmp2 positive input (which is on P4_0):

                screenshot_analog_opamp.png

                The only drawback is that these routed connections now have an higher internal resistance (the Ohmmeter in the Analog tab says 1350 ohm from pin to Opamp input, vs. 450 ohm for the other OpAmps connection).

                • 5. Re: Alternate Vref bypass capacitor connection
                  user_436055692

                  Thank you for the advice.
                  On Friday I tried to move the +input pin of the opamp and it seems suitable.
                  It brings around 800 Ohms additional serial resistor when the opamp input current is 15pA typical.

                  12 nV unbalace is neglectible in our design where the opamp gain is 1.

                   

                  This is a good way to have the SAR Vref connected to a bypass capacitor and still use the opamp.