4 Replies Latest reply on Feb 26, 2016 7:03 AM by user_301021

    Problem with 2 PGA blocks in sequence in PSoC 5LP, CY8CKIT-059 kit.

    user_301021

      Hello,

         

      I think I have found a bug in PSoC Creator 3.3,

         

      I’m trying to make a multi electret microphone system (4), and I need to make a x16 PGA followed by a second x16 PGA and I’m getting only the gain from the first PGA.

         

      I wasn’t getting consistent value with the microphone so I injected a signal with VDAC8. 
      In my experience I have put one VDAC8_2 with the value:
      2304 mV
      90 Hex
      And received in the ADC_SAR_1 values between 2427 and 2496 (in the serial port terminal)

         

      Then made a change in the VDAC8_2 to increase 1 bit (16mV/bit)
      2320mV
      91Hex
      And received in the ADC_SAR_1 values between 2620 and 2693 or an increase of 200.

         

      If we 200 /16mV = 12.5 gain,

         

      This result shows that only the first stage of the opamp is amplifying the signal the second stage isn’t doing nothing, but the signal is passing through it. The 2 PGA's are configured with a x16 and x16 gain.

         

      I’m not certain but I think this is an error in PSoC Creator 3.3, I didn’t experiment with other versions. 

         

      I send the project in attachment.

         

      Thank you,

         

      Best regards,
      João Nuno Carvalho

        • 1. Re: Problem with 2 PGA blocks in sequence in PSoC 5LP, CY8CKIT-059 kit.
          helic_263931

          Wait, you are giving a 2.3V input signal into a PGA chain with a gain of 256? Thats probably not correct. Even just 16mV input signal result in 4.096V at the end of the PGA chain, which is near the analog input range.

          • 2. Re: Problem with 2 PGA blocks in sequence in PSoC 5LP, CY8CKIT-059 kit.
            user_301021

            Hello Hli,

               

            Yes i’m doing that but the voltage reference to the PGA is VDD/2 in a 5V rail and that makes the gain only on the variation without the offset. I need the higher gain because of the microphone small input signal.

               

            Best regards,
            João Nuno Carvalho

            • 3. Re: Problem with 2 PGA blocks in sequence in PSoC 5LP, CY8CKIT-059 kit.
              helic_263931

              Well, even with Vref to the PGA at 5V/2=2.5V you have an input to the PGA of 2500-2304=196mV. Amplify that by 256 and you would have about 50V. I think you are just saturating the PGA output because your input signal swing is not close enough to Vdda/2.

                 

              Maybe you should route Vref to a pin, buffer it there and use this for your circuit. Remember, your input signal swing cannot be larger than 9mV! (5V/256/2)

              • 4. Re: Problem with 2 PGA blocks in sequence in PSoC 5LP, CY8CKIT-059 kit.
                user_301021

                Thank you Hli,

                   

                I have seen the App note AN2214 from Cypress and they use the same method that you are describing, the swing of an electret capsule is really small, even at -44dB of sensitivity. With a 14 bit scope and no amplification I have seen a 4mV amplitude for a quiet speach at 50 cm. The other thing that i noticed is that the  059 kit has the VCC with noise spikes in the order od -8mV below my VDD/2, in the app  note they deal with this noise by spliting the load resistor and using a capacitor do make a low pass filter on the load. 

                   

                I’m making the amplification of 4 electret mic capsules directly in the PSoC 5 and then do some DSP algorithms inside PSoC, but maybe a better solution is to use a dedicated and separate mic amplifier chip for each channel and use only de PSoC to do the sampling of each channel and the processing.

                   

                Best regards,
                João Nuno Carvalho