3 Replies Latest reply on Jan 9, 2019 6:43 PM by user_306160653

    CY8CKIT-059 Opamp Bandwidth

    user_306160653

      I have tried two samples, neither attached because they are no more than a single  op-amp configured as a voltage follower and a PGA with a gain of 1.

       

      Both are configured as High Power.

       

      When I apply a sine wave signal from a Rigol DG832 waveform generator, configured at 1Vpp amplitude with a 500mVdc offset, the PSOC5LP is unable to maintain the voltage beyond 2kHz. At about this point, the signal drops to about 850mV and continues to with increased frequency.

       

      I was under the impression from the datasheets that it should be able to maintain the voltage in this mode for a signal over 1MHz.

       

      I would greatly appreciate any clarity in the behavior that is being seen. The same signal generator attached to a TI LMC6482 in an identical configuration shows no such issues.

       

      Thanks,

      Karl

        • 1. Re: CY8CKIT-059 Opamp Bandwidth
          user_342122993

          Karl,

          I suspect that load resistance is low and PSoC opamp is overloaded. Providing schematic, project bundle and output traces is always helpful. Try to increase load to >1k. Another thing is that PSoC's Opamps are not rail-to rail, so the output will never goes closer than ~0.15V to zero (which itself caps output to ~85% of 0-1V input). Overall, PSoC's Opamps are not great, but can handle small signals upto ~1MHz, so there is something fishy is going on. Please check it with scope probe switched to 10x; it is possible that cable load capacitance is affecting the frequency cut-off.

          /odissey1

          • 2. Re: CY8CKIT-059 Opamp Bandwidth
            user_306160653

            The signal generator is set to High-Z and the op-amp output is not loaded. I did place a 100k load resistor there however.  Scope probes are fixed at 10x and match the scope's settings.

             

            Have attached the project. Changing the amplitude of the signal does not make any difference.

             

            I spent some additional time (apologies for not responding quicker) to verify the behavior between the PSOC 5LP and the external opamp. In the process I've produced and verified the behavior with a schematic that produces a center offset of ~2.5V.  (embedded below).

             

            I've replicated all including any trace/wire lengths and continue to see expected behavior with the external opamp and the described unexpected behavior with the PSOC 5LP.

             

            Thanks,
            Karl

             

            OpAmp_Test.png

            • 3. Re: CY8CKIT-059 Opamp Bandwidth
              user_306160653

              You were absolutely correct. I looked back to the schematic of the evaluation board and found that the only opamp that wasn't connected to other traces, passives, etc was opamp 1. Once I moved over there, I was able to keep a gain of 6 up to about 100khz.

               

              That explains why one of the opamps was actually outputting a square wave in response to a sine wave.

               

              Karl