Tutorial 7
Keysight 34970A/34972A User’s Guide 273
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 connection 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 especially 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 referenced 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.
Copper-to- Approx. μV/°C
Copper
Gold
Silver
Brass
Beryllium Copper
Aluminum
Kovar or Alloy 42
Silicon
Copper-Oxide
Cadmium-Tin Solder
Tim-Lead Solder
<0.3
0.5
0.5
3
5
5
40
500
1000
0.2
5
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