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long wires on digital signal

Discussion in 'General Electronics Discussion' started by Rajinder, Dec 19, 2016.

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  1. Rajinder

    Rajinder

    426
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    Jan 30, 2016
    Hi,
    I am driving a PWM circuit and low pass filter from a microcontroller. The output from this circuit will have a voltage in the range of 5-15mV.
    However there is a cable (twisted pair with overall screen) between the microcontroller and this PWM circuitry. The cable is around 12-18 inches length.
    I'm concerned about noise pick up as I only have a few mV that I am using.
    Any help would be appreciated.
    Thanks
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,883
    2,096
    Nov 17, 2011
    There are a few possibilities:
    1. Put the low pass filter at the end of the cable. That way the filter will suppress noise that's been picked up, too.
    2. Make the PWM signal generate not 5 mV ... 150 mV but e.g. 150 mV ... 4500 mV (a factor of 30). put an attenuator 1/30 at the and of the cable to get 5 mV ... 150 mV from the higher voltage input. This will attenuate any noise, too.
    3. Combine both methods: generate a PWM suitable for 150 mV ... 4500 mV as input to the cable. place the low pass filter + attenuator 1/30 at the end of the cable to filter and attenuate any noise.
    4. In addition: use shielded cable, shield connected to ground, to minimize pick-up of noise as your first line of defense.
    Harald
     
    BobK likes this.
  3. Rajinder

    Rajinder

    426
    7
    Jan 30, 2016
    Thanks Harald.
    What would be the best way to attenuate, a potential divider?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,883
    2,096
    Nov 17, 2011
    That's what I was thinking of.
     
  5. Rajinder

    Rajinder

    426
    7
    Jan 30, 2016
    Thanks
     
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