VTI Instruments Corp.
84 Basic EX1200 DMM Operation
Noise Caused by Ground Loops
When measuring voltages in circuits where the DMM and the DUT are referenced to a common
earth ground, but at different points, a “ground loop” is formed. As shown in Figure 4-8, 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.
R
L
= DUT Source Resistance Imbalance
R
i
= Multimeter Isolation Resistance
V
ground
= Voltage Drop on Ground Bus
V
TEST
R
L
HI
LO
DMM
R
i
> 10 G
V
ground
R
L
FIGURE 4-8: A GROUND LOOP
The best way to eliminate ground loops is to maintain the DMM’s input isolation from earth
ground by not connecting the input terminals to ground. If the DMM must be earth-referenced, be
sure that the DMM and the DUT are connected to the same common ground point. This will
reduce or eliminate any voltage difference between the devices. It is also recommended that the
DMM and DUT be connected to the same electrical outlet whenever possible.
AC VOLTAGE MEASUREMENTS
The EX1200 DMM can measure the true RMS ac voltages within the limits specified. AC voltage
(ACV) measurements differ from the DCV measurements because simple integration and
averaging of the input signal will not yield the RMS value.
Taking an ACV measurement uses the same set up as a DCV measurement, with the HI and LO
leads being connected to the source. Be certain to observe the maximum specification ratings
before connecting to the DMM. As with DCV measurements, the source can be connected through
the front panel or through the DMM Bus backplane bus via a switch multiplexer module or the
DMM bus rear panel connector.
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