Noise on Power supply induced by low pass filter?

Discussion in 'General Electronics Chat' started by Andrei, Feb 20, 2011.

  1. Andrei

    Thread Starter New Member

    Apr 18, 2010
    3
    0
    Hi,

    I have a SHARP gp2d120 IR distance sensor. When I apply the required 5 volts to it and watch the output on the scope I noticed some noise at a frequency of 948kHz which I want to get rid of.

    As such I tried putting together a very quick passive lowpass filter to get rid of the noise. However, after putting the sensor's output through the low pass filter, I notice noise on the power supply and even more noise on the sensor output.

    The filter elements are as follows: 13k resistor, 100nF ceramic cap giving me a cutoff frequency of 122.4Hz which should be fine, as anything useful is in frequencies lower than that.

    Any ideas why I am getting even more noise after applying this low pass filter, and noise on the power supply in addition, I must be doing something really stupid. Would an active filter get rid of some of these problems?

    Regards
     
  2. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    Try a CLC filter - preferably a commercial filter - eg from a small switchmode power supply - or a filter module. 1MHz is pretty high to try and get good components and layout to work with if DIY. Mains power supply filters are made to filter that frequency level.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Do you have a 10uF or larger bypass capacitor across the sensors' Vcc and GND pins as recommended in the datasheet?
    http://document.sharpsma.com/files/GP2D120-DATA-SHEET.PDF
    See page 8.
    I'd recommend that you also use a 0.1uF/100nF metal poly film or ceramic cap directly across the Vcc/GND pins.
     
  4. Andrei

    Thread Starter New Member

    Apr 18, 2010
    3
    0
    Thank you for the reply, I completely missed that note in the datasheet. I added the cap and this has canceled some of the noise especially the one on the power supply, however I still get a +500 mV noise at around 1Khz on the output of the sensor, I will try to add another low pass filter on the output and post the results.
     
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