2 Replies Latest reply on Jul 25, 2016 2:52 PM by user_342122993

    Floating PSoC

    ben.s.

      For one of my projects I am designing a PSoC to measure current from a high side shunt. This shunt will see common mode voltages around 700 volts maximum.

         

      I plan on tying the grounds of the PSoC to one side of the shunt and running the A to D in single ended mode relative to the floating ground. An isolated DC/DC converter will power the PSoC and logical isolators will keep the high voltage off the rest of the circuitry. I have an isolated USB hub if I wind up having to program with high voltage present.

         

      This circuit will need to measure current in both directions. In SE mode, the A to D can only measure between 0 and the reference. How can I set the A to D up to use the lower half of the range to measure the negative voltage and the upper half the positive? This is if the converter were differential, only the input is single ended.

        • 1. Re: Floating PSoC
          user_342122993

          There is no negative input for PSoC, therefore negative voltage should be brought into 0-5V range for ADC measurement. One way to do this was highlighted at this forum by <danaaknight>, which I have liberty to attach below. Basically it is 3-resistor divider; by proper selection of R1-R3 it is possible to squeeze -V...+V into 0..Vdda (PSoC range), where all voltages are "floating" and measured against virtual ground. The price for that will be some lost resolution. Attached are two articles and Excel calculators. I believe that same approach holds for floating PSoC (but it better to check it at low voltages first).

             

          Once input is in PSoC range, ADC can be used in differential mode, where "-inp" is hold by VDAC, exact value can be calculated, or found experimentally.

             

          P.S. Note that ADC_SAR input impedance is low (somewhere in ~20k-"ish" range), it has to be buffered with OPAMP to use with such divider.

             

             

           

             

           

             

          • 2. Re: Floating PSoC
            user_342122993

            Another approach would be to bring voltage to the ground level first, e.g. as shown in this AN:

               

            http://www.linear.com/solutions/5258