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TIDA-00774: RC selection for speed reference

John Pan
Intellectual 990 points
Part Number: TIDA-00774

Hello,

      I am wondering how R45 and C34 were selected for the speed reference circuit in TIDA-00774.

1) Is the selection designed to filter noise caused by the MOSFET switching? The cutoff frequency calculated based on R45 = 10K and C34=0.1 uF is about 150 Hz. The switching frequency in this application is 20 KHz.

2) or is the selection designed to filter noise to MCU MSP430F5132IDA? The time constant calculated based on R45 = 10K and C34=0.1 uF is about 1 ms.  Based on the MSP430F5132IDA datasheet, page 40, the sample time, ts is 1 ~ 3 us. The RC time constant should be less than the sample time. But calculated RC time constant is about 1000 times higher.

3) or other considerations?

      I will appreciate you could provide the reasoning for this selection?  I see other design applications using much lower R value (say 100 ohms instead of 10k) and much lower C value (say 3300 pF), both are several orders smaller.

      Thanks,

      John

  • 0
    TI__Intellectual 2585 points

    Hi, 

    The RC design is to design a LPF to avoid the noise from POT. The pot input is an analog input. We need avoid the high-freq noise. And also, to protect the ESD, we need add a large value resistance. 

    Also, for the speed input pin, we add a large filter to avoid the sudden voltage change because of noise. It can get a much stable speed command. 

    As you mentioned, the small resistance with small cap usually design for PWM input circuit. It is different.

    B.R

    Hawken

  • 0 in reply to Hawken Li
    Intellectual 990 points

    Hi Kawken,

    Thank you so much for your quick reply. Your answer is valuable. So the RC design is to provide a LPF for the noise from POT and to protect the ESD.

    However, there is a cutoff frequency of a LPF in a RC design. I am wondering how the cutoff frequency was determined? Is it related to the MOSFET switching?  or is it related to MCU MSP430F5132IDA? Any explanation would be greatly appreciated.

    Thanks and best regards,

    John

  • 0 in reply to John Pan
    TI__Intellectual 2585 points

    Hi, 

    It should be many considerations:

    - Wire length from pot to input circuit

    - PCB layout: noise of crystal, motor driver and other signals.

    You need change the cutoff frequency with your system. It is not related to the ADC sampling freq. It is just related to the noise level of your input circuit and board. 

    Usually, for POT input, 50-200Hz cutoff freq is preferred. You need tune it with actual system.