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TMS320F28379D: Using CMPSS module to decode differential sin/cos signals

Bo Xian Yeh
Intellectual 1425 points
Part Number: TMS320F28379D
Other Parts Discussed in Thread: C2000WARE

Hi Team,

Customer use CMPSS of 28379D to decode differential Sin/Cos signals (Ex: Sin+ to CMPINxP, Sin- to CMPINxN).

 

But customer have a glitch problem on CMPSS output signal when low frequency input signal cross zero.

 

Adding 4x Hysteresis to CMPSS ( 3 volt / 2^12 * 48 = 35 mV) is still not improve the glitch problem.

 

We want to know if there are other solution can avoid glitch, or what reason may cause glitch.

  • 0
    TI__Mastermind 36526 points

    Hi,

    I am having a difficult time understanding which of these above waveforms are connected to CMPINxP and CMPINxN. Is CMPINxP connected to the sine wave output while CMPINxN is connected to ground? 

    What is the difference between SIN+/- and Analog_SIN+/- ? 

    Is the customer using the digital filter? 

    Best Regards,

    Ben Collier

  • 0 in reply to Benjamin Collier
    TI__Intellectual 1425 points

    Hi Ben,

    Here is a simple scheme about our experiment.

     

    We use amplifier to adjust voltage of the encoder differential signals Sin+/Sin-.

     

    Then, Analog_Sin+ connects CMPINxP and Analog_Sin- connects CMPINxN.

    About the digital filter of CMPSS,

     

    We don’t use it because we have no idea how to design the parameter of the digital filter except by trial and error.

     

    If you have some design experience about the digital filter,

     

    please let me know, they may be helpful to us.

  • 0 in reply to Bo Xian Yeh
    TI__Mastermind 36526 points

    Hi,

    For the digital filter, I don't have any suggestions except to say that we have the cmpss_ex2_digital_filter example in C2000WARE. How time sensitive is your use case? That may stop you from using the digital filter. 

    I am also curious about the output of your amplifier, do you have any filter between the output of your amplifier and the comparator? 

    I am also still having trouble understanding the below image. It seems to me like the CMPSS output should be going low sooner.

     

    Could you also share a scope shot of the CMPSS rising edge?

    Best Regards,

    Ben Collier