Noise caused by magnetic fields
If you are making measurements near magnetic fields, you should take precautions to avoid inducing
voltages in the measurement connections. Voltage can be induced by either movement of the input con-
nection wiring in a fixed magnetic field or by a varying magnetic field. An unshielded, poorly dressed input
wire moving in the earth’s magnetic field can generate several millivolts. The varying magnetic field
around the AC power line can also induce voltages up to several hundred millivolts. You should be espe-
cially careful when working near conductors carrying large currents.
Where possible, you should route cabling away from magnetic fields. Magnetic fields are commonly
present around electric motors, generators, televisions, and computer monitors. Also make sure that your
input wiring has proper strain relief and is tied down securely when operating near magnetic fields. Use
twisted-pair connections to the instrument to reduce the noise pickup loop area, or dress the wires as
close together as possible.
Noise caused by ground loops
When measuring voltages in circuits where the internal DMM and the device-under-test are both ref-
erenced to a common earth ground, 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 LO measurement
lead. This causes an error voltage (V
L
) which is added to the measured voltage.
Keysight DAQ970A User's Guide 181
4Measurement Tutorials
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