4 Replies Latest reply on Dec 10, 2015 5:58 AM by user_246598725

    Using PGAs for sensor signal

    user_246598725

      Hi,

         

       

         

      I want to connect a radar sensor to a 5LP. The output signal will be a (more or less) sinusoidal signal, where the frequency determines the speed of the object, the amplitude reflects how much energy is reflected by a given object.

         

      The application note for the sensor states that an amplifier circuit with a gain of ~70-80dB (~3100 to 10k) should be used (. The reference circuit uses two amplifier stages with gains of 100 and 45, so total gain is 4500. The stages are implemented as low pass filters. Input to the circuit and connection between the amplifier stages is AC coupled.

         

       

         

      Now, I wonder how to implement the amplifier effectively with the 5LP's internal programmable gain OpAmps. My goal is to feed the signal into a ADC and run a FFT over it. I want to use the PGAs to avoid saturation if something is really close to the sensor, so I can reduce the gain.

         

      Maximum gain of a PSoC 5LP PGA is 50, so I'd use three of them to get into the 10k range from the reference circuit. So, the first question is about connecting the PGAs: the reference circuit amplifiers are fed by the inverting input, and the non-inverting input is kept on VDD/2. The PGA circuit symbol of PSoC seems to use the non-inverting input as signal source (Vin) and the inverting input for the reference voltage (Vref), reflecting a non-inverting amplifier opposite to the reference circuit. Are there any disadvantages to be expected?

         

      Second question: I'll keep the AC coupling between the PGAs by external capacitors, but I think I can skip the capacitors used for low-pass filtering (capacitor between OpAmp output and input) and make the LPF by software, would this work?

         

       

         

      Regards,

         

       

         

      Ralf

        • 1. Re: Using PGAs for sensor signal
          user_246598725

          Additional question regarding connection: would it work if I use the Vin terminal as VDD/2 reference and the Vref pin set to external for signal input?

             

           

             

          Regards,

             

           

             

          Ralf

          • 2. Re: Using PGAs for sensor signal
            user_342122993
                    RA1981, There is a shortcut: the gain recommended by radar manufacture (~5000) gives an idea that the output signal has typical amplitude in ballpark of about 200uV or so (sorry, you didn't provide radar part number, so it's a guess). Using 16-bit DelSig ADC with buffer amplifier set to gain 8, and selecting range +/- .125V will give you resolution ~500nV/bit without any PGA attached, which is more than sufficient for recording radar data with 9-bit resolution. Try this first, without going into ordeal with PGAs, as it may be sufficient.   
            • 3. Re: Using PGAs for sensor signal
              user_342122993

              And if you really need double PGAs on the front end, here is a schematic (attached) from working project (lock-in). The trick is that both PGAs set to operate in the inverted mode,  other combinations (inv/non-inv, non-inv/non-inv, etc.) do not work, likely due to the offset produced by PGAs. You can fight the offset voltage by introducing a capacitor between the PGAs (and all associated resistors, bypass cap, references), but design shown alleviates all that (in expense of low input impedance).

                 

              • 4. Re: Using PGAs for sensor signal
                user_246598725

                Hi Odissey1,

                   

                 

                   

                thank you for your hints, I'll try them and report back.

                   

                 

                   

                Regards,

                   

                 

                   

                Ralf