Voltages on probe grounding

Discussion in 'General Electronics Chat' started by petewh, Sep 22, 2016.

  1. petewh

    Thread Starter New Member

    Mar 23, 2016
    I'm refining my measurements with the scope by grounding the probe nearer the point of interest. But I've measured a small voltage on the ground strip of solderless breadboards with the volt meter. The voltage is on the order of 50mV up to 150mV or more. It seems to me that it is OK to ground the scope probe there since my power supply and function generator are floating. But I'm not really sure. I know the probe's ground is connected to earth ground. Since it must be common practice can I get some feedback?
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
    If there is a voltage difference between where you connect the probe ground and the ground at the point you are trying to measure then your readings from the scope will be incorrect by that difference. Note that the difference voltage may include both DC and AC components depending on your circuit.

    If you are connecting two or more pieces of grounded equipment to your circuit and you connect the grounds to different places then you have created a ground loop which may also affect your measurements.
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    The issue stems from the scope ground being connected to chassis ground.

    A power supply typically has full isolation from the AC line. I am less certain if your function generator is also isolated, though probably it is.

    Unplug the units and measure from output return to the AC plug ground. If it is en you should be safe.