6 Replies Latest reply on Jan 20, 2014 5:45 PM by user_14586677

    input impedance of comparator

    travis.fagerness

       What is the input imedance of the v2.0 comparator module? I've been seeing some errorenous readings when I am using a voltage divider into the comparator. The divider is on the order of 50kohms.

        • 1. Re: input impedance of comparator
          user_14586677

          I do not see in module datasheet or device datadsheet or TRM

             

          any discussion. If you are using Hysterisis I suspect you may be

             

          seeing +fdbk R's used to develop fdbk.

             

           

             

          File a case at -

             

           

             

              

             

                    

             

          www.cypress.com

             

          “Support”

             

          “Technical Support”

             

          “Create a Case”

             

           

             

           

             

          Or use DelSig, and a sampling R to measure the current going into Comparator,

             

          that might be instructive. Are you using Hysteresis ?

             

           

             

          If you file a case let forum know what answer was.

             

           

             

          Regards. Dana.

          • 2. Re: input impedance of comparator
            user_78878863

            In the CY8C38 family data sheet, the (typical) input current is specified as 2 nA, or 14nA for SIO pins. It might be higher, but not _that_ much.

               

            Are you sure you have the pins really configured as analog high-Z?

            • 3. Re: input impedance of comparator
              user_14586677

              I am not sure of what the applicability of the SIO pin input leakage is to this

                 

              issue ?

                 

               

                 

              Pins being used are GPIO configed as analog, connected to a comparator,

                 

              unless I misunderstood something...which wouldn't be the first time.

                 

               

                 

              Regards, Dana.

              • 4. Re: input impedance of comparator
                user_14586677

                My apologies, I see you might have been think SIO comparator functionality

                   

                was being used -

                   

                 

                   

                 

                   

                Regards, Dana.

                • 5. Re: input impedance of comparator
                  user_78878863

                  No, I meant to calculate the input impedance from the current flowing in the pin (2 nA would, at 5 V, result in an input resistance of 2.5 GigaOhm). This is the only specification I can find for that value. The Comparator compoinent data sheet states that the comparator is implemented as trimmer differential amplifier, and hysteresis is not handled by a feedback resistor it seems.

                     

                  So I guess the problem doesn't stem from the input impedance.

                  • 6. Re: input impedance of comparator
                    user_14586677

                    Pretty much all comparators implemented by using high gain diff amp,

                       

                    and trimmed as it states. It does say offsetting currents are added to

                       

                    input stage to generate Hysteresis, that could be done with I source or

                       

                    simple R's, either way that would affect Zin, or so I think. Thats why a

                       

                    case needs to be filed, we are fishing with guesses at this point.

                       

                     

                       

                    Regards, Dana.