User`s guide

78 34450A User’s Guide
3 Measurement Tutorial
Noise Caused by Magnetic Loops
If you are making measurements near magnetic fields, take
caution to avoid inducing voltages in the measurement
connections. 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 test leads as close together as
possible. Loose or vibrating test leads will also induce error
voltages. Tie down test leads securely when operating near
magnetic fields. Whenever possible, utilize magnetic shielding
materials or increase the distance from magnetic 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, a ground loop is formed. As shown in
Figure 3- 2 on page 79, any voltage difference between the
two ground reference points (V
ground
) causes a current to
flow through the measurement leads. This causes noise and
offset voltage (usually power- line related), which are added
to the measured voltage.