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DRV8818 Noise on DIR terminal

HM
Intellectual 460 points

Other Parts Discussed in Thread: DRV8818, DRV8818EVM

Under the functional testing on our engineering board mounted on DRV8818, it occured step-out of the stepping motor.  We cheked waveform on DIR terminal, and found large noise on it.  Also we checked DIR with terminal floating, but noise is still exist on it.

DIR Terminal:  Connected on PCB

DIR Terminal:  Floating

Question

(1) Noise on DIR terminal is caused by lifting device GND.  Is this correct?

(2) Is signal GND separate from power GND? (or is it not separated?)

(3) Is there any effective measre?  An external pull down resistor(zero-ohm?) or an external capacitor will be effectice on this?

Best Regards,

Hide

  • 0
    TI__Guru** 114905 points
    Hi Hide,

    Where is the ground reference for channel 2? It should be as close to pin 7 or 21 as possible.

    This noise may be on the board, but may not have any effect on the device operation. Is there a problem with device operation?

    (1) Noise on DIR terminal is caused by lifting device GND. Is this correct?
    Yes, this could be caused by device GND lifting and falling. It could also be caused by the DIR pin routing.
    Please check that DIR trace is away from the outputs traces.

    (2) Is signal GND separate from power GND? (or is it not separated?)
    All grounds are connected internally at the pins. Then the two GND pins and the thermal pad should to be connected to the GND plane.

    (3) Is there any effective measre? An external pull down resistor(zero-ohm?) or an external capacitor will be effectice on this?
    Prior to adding any circuitry, the source of this bouncing should be determined. If this is due to GND moving, adding an external capacitor should help.
  • 0 in reply to Rick Duncan
    Intellectual 460 points
    Hi Rick,

    Thanks for the explanation.

    The ground reference for channel 2 is close to pin 7. The problem is to stop motor for a moment.

    We checked with floating DIR terminal, but we can see GND moving. So, it has no connection with trace.

    We guess you can see the GND moving with floating DIR terminal on the EVM.


    Best Regards,

    Hide
  • 0 in reply to HM
    TI__Guru** 114905 points
    Hi Hide,

    I looked at the DRV8818EVM and see similar noise on the DIR pin. The noise is about 500mV and matches the AOUT1 output.

    I did not float DIR but see the noise if the DIR pin is driven to GND or 3.3V.

    It appears there is coupling between DIR and AOUT1. This could be due to board layout. For the DRV8818, the DIR trace is long and close to the AOUT1 trace.

    You can try connecting DIR to GND through a zero Ohm resistor if you do not plan to change DIR. If you plan to change DIR, you can try a small capacitor.