Noise in system when laptop is connected to external supply

Discussion in 'General Electronics Chat' started by Vindhyachal Takniki, Feb 22, 2016.

  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    I have a system in which pin is toggled. Pin toggle is viewed on CRO with 10x setting & peak detect. There are two cases:

    Case1: MCU board is powered by laptop usb power. CRO earth is connected to MCU board ground. laptop is powered from internal battery & no external supply is connected to laptop. SO CRO earth is connected to MCU ground which in turn connected to laptop ground via usb connector shield.
    In attaced pdf, page 1 shows the pin toggle waveform. It is nice & clean, no noise or whatever.

    Case1: Now external supply is connected to laptop also.
    Now suddenly there is noise in system. Page 2, shows the noise in pin toggle. Pin toggle time is still the same. (However pic shows it larger, I think its due to camera speed). But noise is present in system as shown.
    Then I connected CRO probe to ground of MCU where CRO earth probe is connected. Page 3 shows the noise in MCU ground also.

    1. What is this noise? One reason could be that now laptop is connected to earth, which is now connected to MCU ground. But this happened in first case also where CRO earth is connected to CMU ground?

    2. Is this actually a noise? Because noise is present both in pwer & ground of MCU, so I think if it is measured by differential probe then there will ne no noise in case2.

    3. is this noise dangerous for circuit, i.e it affects analog circuitry. If yes, how to get rid of this noise.

    PS: I have long wire coming thorugh CRO i.e separte earth probe & separte CRO probe. Another erason could be this is due to large CRO probe wire length. But if that would be case, it should happen in case 1 also.
  2. StayatHomeElectronics

    AAC Fanatic!

    Sep 25, 2008
    Where are you connecting the measurement probe ground of your oscilloscope (DSO)? Or is that what you are calling CRO earth ground?

    If you are connecting the separate oscilloscope earth ground to the MCU ground, do you really need to? Try disconnecting it. Having it connected in several locations can cause problems with a circuit.