VTI Instruments Corp.
82 Basic EX1200 DMM Operation
When using switch multiplexer modules such as the EX1200-3048, each input channel pair (HI
and LO) is routed through relays which are close to one another on the PCB to maintain the same
average temperature. Because of this proximity, the relays generate approximately equal
thermoelectric voltages on both the HI and LO channels, making the differential reading across
these two switches approximately cancel the thermoelectric voltages.
DMM
VT1
VT2
HI
LO
Switch Pair
VM
V
S
= Source Voltage
V
T1/
V
T2
= Thermoelectric Voltage for Switch 1 and 2
V
READING
= Voltage Displayed by DMM
V
READING
= V
S
+ (V
T1
- V
T2
)
FIGURE 4-4: NEGATION OF SWITCH THERMOELECTRIC VOLTAGES
Measuring Voltages with High Output Resistance
Source loading errors occur when the output resistance of the DUT is a significant percentage of
the DMM’s own input resistance.
R
S
V
S
HI
DMM
LO
R
i
V
S
= Ideal DUT Voltage
R
S
= DUT Source Resistance
R
i
= Multimeter Input Resistance
(10 M or 10 G)
Error % = 100 x R
S
R
S
+ R
i
FIGURE 4-5: ERROR CALCULATION FOR HIGH OUTPUT RESISTANCE LOADS
To reduce the effects of source loading error and to minimize noise pickup, the input resistance of
the DMM can be set to HIZ mode (> 10 GΩ) for the 100 mV dc, 1 V dc, and 10 V dc ranges. The
input resistance is maintained at 10 MΩ for the 100 V dc and 300 V dc ranges.
Common Mode Rejection
An ideal DMM would be floating and isolated from earth-referenced circuits. In practice, though,
there is a finite resistance between the DMM’s input LO terminal and earth/chassis ground (see
Figure 4-6). This can cause errors when small voltages which are actually floating relative to earth
ground are measured (as for a photovoltaic cell, for example).
V
F
= Float Voltage
R
s
= DUT Source Resistance Imbalance
R
i
= Multimeter Isolation Resistance
Error (v) = V
f
x R
s
R
s
+ R
i
V
TEST
V
f
R
S
HI
LO
DMM
R
i
> 10 G
FIGURE 4-6: COMMON MODE REJECTION ERROR
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