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8834 filtering for EMI mitigation

Joe Scott
Prodigy 30 points

I’m part of a team working on a motor control application in an imaging probe.  We’ve been working to integrate the 8834 as a stepper motor driver and need some help to mitigate what we believe to be EMI noise coupling into the image signal lines.

 We’ve added a 330uF cap to the power supply, which helped reduce the magnitude of the noise on the motor power supply line, but it still looks peaky.   We don’t have any filtering on the AOUT/BOUT pins and think that may help address the issue.

 I found a post in the forums that seems to address our question (http://e2e.ti.com/support/applications/motor_drivers/f/38/p/64662/241363.aspx#241363).  Can someone please help to determine if the snubber suggestion would translate well to our application.  I'm a newbie trying to help with some legwork and would appreciate help with the values and topology. 

  • 0
    TI__Mastermind 21320 points

    Hi Joe,

    The suggestions in that thread are all good pieces of advice. I would recommend them if you believe this is the issue. Are the image signal lines running close to your motor outputs? It may be a good idea to try and isolate these as much as possible as there will always be some noise coming from the motor.

    What kind of operating conditions are you running at?

  • 0 in reply to Nicholas Oborny
    TI__Guru 55615 points

    Joe,

    In that same thread that you linked to, you can also see a recommendation to add a series ferrite bead on each output.  This is the most effective means to limit radiation and image noise.

    A good example of a bead to use is linked below.  Place one of these beads on each motor output as close to our IC as possible and then terminate the bead (on the motor side) with a 1nF capacitor to GND.  Keep the loop to GND as short as possible.  I would also recommend the same bead in series with the power supply. 

    http://www.digikey.com/product-detail/en/MPZ1608S101A/445-1564-2-ND/571871

    A snubber will certainly help "round the edges" of the PWM, but a bead is the best approach if you really want to ensure success.  You could put footprints for both on your board and experiment. 

     

  • 0 in reply to Nicholas Oborny
    Prodigy 30 points

    Hi Nicholas -

    We're unfortunately pretty set in the proximity of the signal and motor lines.  We've shielded everywhere we can and that has helped, too, but has not eliminated the issue. 

    We're running with a bipolar motor that has parallel windings; 1.2A/phase, .95 ohms and .4 mH.  The step rate is on the order of 50-100 us steps.  We noticed the noise went down significantly when running at a lower voltage (like 5 V), but we have other constraints that require we stay at 10V input.

    Thanks,

    Joe

  • 0 in reply to Ryan Kehr
    Prodigy 30 points

    Thank you Ryan!

    We'll try the ferrites and see if that helps.