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USB 5V noise filtering

Discussion in 'Electronic Design' started by Maquis, Apr 14, 2006.

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  1. Maquis

    Maquis Guest

    Recently the internal audio device in my laptop went dead. I replaced it
    with a USB sound adapter tapping its power from the USB +5V. The adapter
    works (almost) fine but...
    the audio, especially the microphone input, is noisy. Some kind of a digital
    noise of switching noise. After inquiry, the noise is coming from the laptop
    5V supply coming down the USB cable, about 50mVp-p on a scope, may be more
    as my scope is low bandwidth. The adapter is surely lacking effective
    filtering.

    What would be the most simple and effective way to filter this out without
    to much voltage drop?
    The adapter can work down to 3.5Vdc and draws about 20mA. I look for
    suggestions or ideas form people who got the same kind of noise problem from
    USB supply.

    Thanks!
    John
     
  2. Ian Stirling

    Ian Stirling Guest

    Maybe - the noise may be coming from somewhere else.
    First, I'd try a clean 5V supply.
    Maybe even 4.5V from batteries.
     
  3. artie

    artie Guest

    USB uses half-duplex differential signaling. One of the tricks you can
    try is to get a short USB cable, open up the +5 line (RED), put a 10
    ohm resistor in series, and bypass caps from the load side of the
    resistor to the ground (BLK) line. I usually dig through the junk box
    and put in a surface-mount slug tantalum (Hobson's choice, usually 4 to
    10 uF) paralleled with a .01 uF. That will tell you if you've got
    noise on the +5 line messing things up.

    But how about ground loops? Open up BOTH the +5 and ground lines, and
    put 1 ohm resistors in series (1 to 10 ohms, again, hobson's choice
    depending on what you have and how much current the thing on the end of
    the USB cable wants), and bypass that.

    A pair of these cables makes it easier to find/diagnose noisy (cheap)
    designs.
     
  4. Maquis

    Maquis Guest

    I build a small filter with 22 ohms with bypass caps 15uF/0.01uF on the
    supply line. I still have 4.3V at the output. This got rid of most noise by
    about 80% (subjectively...).
    But I still have some kind of steady high frequency wistle noise when
    recording with the mic. I know for sure the mic is good.
    Just to be sure, I tried 4a clean 4.5V supply from three "AA" batteries as
    suggested by Ian and the result is the same. At least the filter is good for
    filtering noise from the computer.
    I guess the remaining noise is "internal" to the adapter just before the
    microphone signal is sampled and converted into digital form.
     
  5. Maquis

    Maquis Guest

    Finally I reduced (drastically) the mic remaining noise by adding a bigger
    cap right at the supply input of the adapter. At least 220uF does the trick.
    I kept the resistors in the circuit.

    Thanks for your suggestions guys!
    John
     
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