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Viewing PWMA and OUT_A using an Oscilloscope

Michael Mina1
Intellectual 480 points

why is it when I view PWMA and OUT_A on an oscilloscope (DRV8312-69M-KIT), they look alike (PWM signal, or square waves).  I know what PWMA should like, it's a PWM signal, but, I'm at al loss as to why OUT_A looks exactly like it.  I'm assuming OUT_A is the voltage signal to the motor, and should look like a sine wave, but it doesn't.

  • 0
    TI__Guru** 119680 points

    Michael,

    The square wave pulses to the mosfet switches are at high frequency (default of 45 KHz for this kit in user.h).  These are used to construct the low frequency (0 to motor max frequency, for the kit motor it is rated at under 300 Hz) sinewave.  You can think of this as a bunch of small steps to construct a smooth sinewave.  For example, at 200 Hz operation there will be 5mS (200 Hz) / 22.2 uS (45 KHz) = 225 "steps" in the full sinewave cycle.  The inductance of the motor combined with the Bemf will determine how "smooth" this sinewave appears to the motor windings.  If you have a current probe this is quite easy to view.

  • 0 in reply to ChrisClearman
    Intellectual 480 points

    Hello again Chris.  In the past you metioned that I need a current probe to measure the test points on the EVMs.  Please tell me where I can find a current probe small enough to use on the test small test points that are on the boards.  I am at a loss.  Your help is greatly appreciated.  I have all the necessary oscilloscopes to use and plot the waveforms, but i don't to plot the PWM for my reports. Thank you.

  • 0 in reply to Michael Mina1
    TI__Guru** 119680 points

    I've mentioned previously to you that current probes don't need to use the test points. The probes clip around a phase wire. 

  • 0 in reply to ChrisClearman
    Intellectual 480 points

    Thank you for answering my question promptly.  I already built a box with 3 current probes around the 3 phases, as you suggested earlier.  I was just wondering why the EVMs have the test points, if they're only PWM signals.  That's all.

  • 0 in reply to Michael Mina1
    TI__Guru** 119680 points

    it can be useful to use an O-scope to see PWM or PWM-DAC signals during initial bring-up of the HW / driver level SW.