3 Replies Latest reply on Feb 21, 2013 4:08 PM by user_14586677

    Glucose Measurement Using PSoC 3

    rajendra.prasad

      Hi to all,

         

      I am doing glucose measurement , in this i am Interfacing IDAC, VDAC, TIA(Transimpedance Amplifier).Glucose Test Strip gives current so i  am taking IDAC .

         

      The Output Equation at TIA  Vout=Vref-I*R, Normally for I=0 case Vout= Vref , but here I=0 i.e i am not connecting any thing to terminal , but i am measuring Vout, what i am getting is 0.29v, why is this strange output or else i am wrong any where.i am uploading Image of TOPDESIGN.

        • 1. Re: Glucose Measurement Using PSoC 3
          rajendra.prasad

          one Mistake on top post, with I=0, ie not connecting IDAC, not getting any value on multimeter.

          • 2. Re: Glucose Measurement Using PSoC 3
            user_14586677

            What is Vref (VDAC8) set to ? With a scope is TIA output DC or noisy, 60 cycle, etc.....

               

             

               

            Per your diagram Pin_6 = Pin_5 as measure din DC voltage.

               

             

               

            The TIA in this circumstance should act as a follower, although you might

               

            want to disconnect the pin from TIA inverting input to minimize pickup for the

               

            offset test.

               

             

               

            To get an offset of .29 V with 40K TIA G set R one would have to have a leakage

               

            of ~ 7 uA, which would be a pretty high bias current for the OpAmp used in TIA.

               

            I looked for Ibias / Ileakage specs on OpAmp, TIA, saw none. This is just an

               

            observation.

               

             

               

            Regards, Dana.

            • 3. Re: Glucose Measurement Using PSoC 3
              user_14586677

              I set this up, using 1M Ohm TIA G set R to "aggravate" Ibias and Ileak errors.

                 

               

                 

              For VDAC = 0                      I got 29 mV of TIA offset

                 

              For VDAC = .512 V              I got ~ 3 mV of TIA offset

                 

               

                 

              If you look at G error and zero scale error for DAC that pretty much would cover

                 

              the 29 mV I got. To be exact one should calc the spec errors to see what one can

                 

              expect.

                 

               

                 

              Regards, Dana.

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