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xxxiv Keysight E1412A Digital Multimeter User Guide
Keysight E1412A Multimeter Application Information DC Voltage Measurements
Common Mode Rejection (
CMR
)
Ideally, a multimeter is completely isolated from earth-referenced circuits.
However, there is finite resistance between the multimeter's input LO terminal
and earth ground as shown below. This can cause errors when measuring small
voltages which are floating relative to earth ground.
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