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Common Mode Rejection (CMR)
Ideally, the multimeter is completely isolated from the ground - based circuit. However,
there is some resistance between the multimeter input LO terminal and the ground, as
shown in the figure below. This creates an error in measuring the low voltage floating with
respect to the ground.
Noise caused by ground loop
If the multimeter and the device under test are connected to the same common ground
point, measuring the voltage in this circuit will form a "ground loop". As shown below, any
voltage difference (VGround) between two ground reference points will cause current to
flow through the measuring lead. This results in noise and offset voltage (usually
associated with the power supply line), which is added to the measured voltage.
The best way to eliminate the ground loop is to isolate the multimeter from the ground by
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