East Tester ET3260 User Manual page 41

Users manual ET3260 and ET3260A 6 1/2 Digit Multimeter
However, there is certain amount of resistance between multimeter LO input terminal and
grounding terminal, as shown in the following figure, which will cause error to measure the
floated low voltage relative to the grounding terminal.
Vf=Floating ground voltage
Rs=Internal resistance of voltage source under test
Ri=Multimeter isolation resistance (LO terminal against the ground)
Ci=Multimeter input capacitance (LO terminal against the ground)
Noise caused by magnetic loop
If conduct measurement near magnetic field, please pay attention to take measures to avoid
causing sense voltage during measurement wiring, and especially beware when working near
conductor conveying high current. Use twisted-pair in connecting to reduce noise-pickup loop
area, or make all the test leads close to each other as much as possible. Test lead looseness or
vibration may cause induction error voltage. Fasten test leads when operating near magnetic field,
and if possible, try to use magnetic shielding material or keep away from magnetic field source.
Noise caused by grounding loop
If multimeter and equipment under test are connected to the same common ground, measurement
of voltage of the circuit will cause Ground loop. As shown in the following, any voltage
differences (Vground) between two grounding reference points will cause current going through
measurement leads, thus causing noise and offset voltage (generally is associated with
power-line), which are added to voltage under test.
Rl=Lead resistance
Ri=Multimeter ground-isolation resistance'
Vground=Voltage drop on ground bus
The best way to eliminate ground loop is not to conduct grounding to input terminal, and use
multimeter to conduct ground-isolation. If multimeter has to take ground as baseline, connect it
and equipment under test to the same common ground, and if possible, connect them to the same
power outlet.
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