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28 HP E1312A/E1412A Multimeter Application Information Chapter 2
Noise Caused by
Magnetic Loops
If you are making measurements near magnetic fields, you should take the
necessary precautions to avoid inducing voltages in the measurement
conductors. You should be especially careful when working near conductors
carrying large currents. Use twisted-pair connections to the multimeter to
reduce the noise pickup loop area, or dress the input cables as close together
as possible. Also, loose or vibrating input cables will induce error voltages.
Make sure your input cables are tied down securely when operating near
magnetic fields. Whenever possible, use magnetic shielding materials or
physical separation to reduce problem magnetic field sources.
Noise Caused by
Ground Loops
When measuring voltages in circuits where the multimeter and the device-
under-test are both referenced to a common earth ground but at different
points, a “ground loop” is formed. As shown below, any voltage difference
between the two ground reference points (V
ground) causes a current to flow
through the measurement leads. This causes errors such as noise and offset
voltage (usually power-line related), which are added to the measured
voltage.
The best way to eliminate ground loops is to maintain the multimeter's input
isolation from earth; do not connect the input terminals to ground. If the
multimeter must be earth-referenced, be sure to connect it, and the
device-under-test, to the same common ground point. This will reduce or
eliminate any voltage difference between the devices. Also make sure the
multimeter and device-under-test are connected to the same electrical outlet
whenever possible.