3 Replies Latest reply on Nov 28, 2019 9:09 PM by SampathS_11

    LPF2 minimum sampling frequency

    EmDr_1490911

      Estimated Cypress community and technical staff. I would like to ask you what's the minimum sampling frequency (analog clock frequency) admited by LPF2 low pass filters. I'm currently working on low frequency applications with these filters in 29466 PSoC1 devices.

       

      Thank you in advance.

                                             Emanuel Dri

        • 1. Re: LPF2 minimum sampling frequency
          SampathS_11

          Kindly refer to LPF2 Datasheet. You can go to a minimum of 12kHz sampling frequency. You will need a column clock of 48kHz.

           

          Best regards,

          Sampath Selvaraj

          • 2. Re: LPF2 minimum sampling frequency
            EmDr_1490911

            Thank you for your answer Sampath @SampathS_11.

             

            I must say that this feature (the lowest clock frequency admited) is presented in a way that seems to be inconsistent in the datasheet (LPF2 V4.10) .

            Let me explain:

            1. On tables 3 and 4 it indicates that the minimum corner frequency (assuming a Butterworth filter as a case study) is 1KHz, with an oversamplign ratio (OSR) of 100 times. Therefore, the sampling frequency should be 1KHz x 100= 100KHz (and the corresponding column clock of 400KHz).
            2. In the section Features and Overview, it says that the corner frequency ranges from 20Hz to 180KHz. This contradicts what tables 3 and 4 state.
            3. The table (whithout nubmering) between tables 4 and 5 depicts a filter configuration having a sample frequency of 12KHz, which again conflicts with the content of tables 3 and 4.
            4. The section Operational Limits says that the corner frequency has been tested down to 10Hz with an OSR of 100 times. This implies a sampling frequency of 10Hz x 100=1000Hz. This contradicts what is stated in Features and Overview, and tables 3 and 4.

             

            I would like to ask you if you can kindly clarify these issues or indicate a correct way to interpret the documentation of this module.

             

            Thank you in advance

                                                  Emanuel Dri

            • 3. Re: LPF2 minimum sampling frequency
              SampathS_11
              1. On tables 3 and 4 it indicates that the minimum corner frequency (assuming a Butterworth filter as a case study) is 1KHz, with an oversamplign ratio (OSR) of 100 times. Therefore, the sampling frequency should be 1KHz x 100= 100KHz (and the corresponding column clock of 400KHz). Tables 3 and 4 are performance characteristics, over a specific span of sampling frequencies.
              2. In the section Features and Overview, it says that the corner frequency ranges from 20Hz to 180KHz. This contradicts what tables 3 and 4 state. This is the recommended range of corner frequencies.
              3. The table (whithout nubmering) between tables 4 and 5 depicts a filter configuration having a sample frequency of 12KHz, which again conflicts with the content of tables 3 and 4. This table depicts the values of the switched capacitor block for the corner/sampling/coumn frequency.
              4. The section Operational Limits says that the corner frequency has been tested down to 10Hz with an OSR of 100 times. This implies a sampling frequency of 10Hz x 100=1000Hz. This contradicts what is stated in Features and Overview, and tables 3 and 4. This is the lowest frequency which has been tested. Designers should limit the corner frequencies to the range specific in the Features and Overview.

               

              My answers are given in bold font. If designers are operating in the boundary of the range specified in Features and Overview, it is best to test if their particular design meets the necessary accuracy.

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