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Filtering AC line noise

  1. May 23, 2007 #1
    Hello,

    I'm designing a circuit with a linear regulator to supply power to an RFID reader. The input to the regulator is 12VDC supplied by an ordinary external AC/DC desktop switching power supply. When I look at the output of the linear regulator with a scope it looks pretty clean except for 50-60hz (approx. 35mV pk-pk) noise. The same noise is present at the output of the desktop power supply so I'm fairly sure it's from the AC line. Should a well designed desktop supply filter this out or is it up to me to use something like an inline AC line filter on the AC input to the desktop supply?

    Thanks
     
  2. jcsd
  3. May 23, 2007 #2
    Why use a switched supply when linear regulator power supplies are generally less expensive and will not generate switching noise? Since you're going to use a linear regulator anyway, why not just use a rectifier bridge followed by an integrator... and feed the output of that circuit into your regulator?

    Are you trapped on a desert island with MacGyver and your only way out is to use the half tube of toothpaste, flare gun, and switched power supply you just happen to have?
     
  4. May 24, 2007 #3

    NoTime

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    Noise?
    Or do you mean sine wave?

    If it is a switching power supply, then what you are seeing is more likely more likely to be measurement error.

    At 35mv it is unlikely to cause problems.
     
  5. May 24, 2007 #4
    Thanks for your response NoTime. I'm sorry for my incorrect terminology; I'm still learning in this field. What I'm seeing is a sine wave that is approx. 50-60hz. Do you mean measurement error produced by the scope? Why would this measurement error exist if it's a switching power supply (I checked a linear supply and the sine wave does not exist)?
     
  6. May 24, 2007 #5

    berkeman

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    The output of a switching supply will not have any 60Hz component. It will have ripple noise at the switching frequency of the supply, not at the input AC mains frequency.

    If your 12Vdc supply is an unregulated, filtered full-wave rectified transformer output, then yes, it will have some ripple at 60Hz.

    I agree with NoTime that you probably have a measurement error. Like, you have a floating ground or some common-mode return current that is showing up on your 'scope. When you measure this ripple with a floating DMM on ACV setting, what does it read?
     
  7. May 24, 2007 #6

    berkeman

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    Also, if you are post-regulating this input anyway, that will eliminate any ripple input.
     
  8. May 24, 2007 #7
    Unfortunately I don't have a DMM that is good enough to make that measurement. I did do a little research into the measurement errors that you're talking about though and found the following excerpt from an Agilent tutorial:

    "When measuring voltages in circuits where the multimeter and the device-under-test are both referenced to a common earth ground, a "ground loop" is formed. As shown below, any voltage difference between the two ground reference points (Vground) causes a current to flow through the measurement leads. This causes errors, such as noise and offset
    voltage (usually power-line related), which are added to the measured voltage. The best way to eliminate ground loops is to maintain the multimeter’s isolation from earth; do not connect the input terminals to ground. If the multimeter must be earthreferenced, be sure to connect it, and the device-under-test, to the same common ground point. This will reduce or eliminate any voltage difference between the devices. Also make sure the multimeter and device-under-test are connected to the same electrical
    outlet whenever possible."

    The complete document is at http://www.home.agilent.com/upload/cmc_upload/All/EPSG073122.pdf

    I think this is what you're talking about and it makes a lot of sense to me now that I've read through it. Another interesting thing that I discovered before I read your post is that if I plug the switching desktop supply into my Belkin UPS (just a surge protection only outlet not battery backup) and the scope into the wall outlet that the UPS is plugged into the sine wave goes away completely. Do these results agree with the theory that the sine wave is a measurement error?
     
  9. May 24, 2007 #8

    berkeman

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    Probably. Another way to check for measurement errors like this is to do a "ground minus ground" measurement with your 'scope. Hook up your probe for your measurement, and you see the extra ripple that you suspect is coming from a gound loop or other common-mode problem. Move your probe tip to the same place as your probe's ground clip, and check to see that you have a flat line. If the ripple/noise is still there, it is a measurement problem. If the ripple/noise goes away, it's probably a real signal.

    BTW, another possibility is that you are seeing swithching output ripple, but have the time base of the 'scope turned down to a lower sweep speed and are aliasing the higher frequency noise into a ~60Hz waveform. Turn up the timebase to a faster sweep speed where a several 100kHz switching supply output ripple can be seen -- does the 60Hz ripple turn into higher frequency noise at some point? Aliasing with a digital 'scope is always something you have to keep in mind, and keep your timebase in the range of the signals that you are expecting to see and measure.
     
  10. May 24, 2007 #9
    Well I learned a valuable lesson today....plug the measurement equipment into the same wall outlet as the device your measuring. It was a ground loop created by having the scope plugged into a different wall outlet than the desktop power supply that was creating the sine wave. Thanks so much for all the help; I really appreciate it.

    BTW, I'm going to do a "ground minus ground" measurement just for the heck of it. That type of measurement will probably come in handy in the future to diagnos these kinds of things.
     
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