High input impedence pins catch noise

Discussion in 'General Electronics Chat' started by aamirali, Oct 9, 2012.

  1. aamirali

    Thread Starter Member

    Feb 2, 2012
    415
    1
    Now it is said that High input impedence pins catch noise. So better to pull-up or pull-down them.
    I didn't exactly understand technically how does pin catch noise & low impdence pin not
     
  2. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    The air around us is permeated with magnetic fields from the power line frequency to gigahertz range. When we build circuit boards, we are making a lot of tiny loop antennas. Some of our circuit board loops radiate energy that add to the ambient noise.

    When one of our circuit loops has current induced in it by radiated magnetic fields that current develops a voltage across the resistors. If the resistance of a loop is in the megohm range, the voltage can be significant. By using pull up or pull down resistors in the 10k range, we reduce the induced voltage by a factor of 100 lower than if we used a 1 meg resistor.
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The mess of tangled wires on a breadboard are antennas that pick up mains hum and all kinds of interference. A circuit soldered together on a pcb or on stripboard with its tracks cut short does not pickup interference.

    I have made many high gain audio circuits on stripboard and enclosed in a plastic box. I used shielded audio cables to connect the inputs and outputs and there was no interference. Sometimes a metal box connected to the circuit's ground is needed as a shield.

    Shielded audio cables are used for connecting together audio products especially low level microphones. The shield blocks interference.
     
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