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Noise Rejection
V
test
V
f
U2741A User's Guide
Rejecting power-line noise voltages
An important property of integrating analog- to- digital (A/D)
converters is their ability to reject power- line related noise
that is present on the DC input signals. This is called
normal mode noise rejection, or NMR. The digital multimeter
achieves NMR by measuring the average DC input by
"integrating" it over an integral number of power line cycles.
Common mode rejection (CMR)
Ideally, a digital multimeter is completely isolated from
earth- referenced circuits. However, there is a finite
resistance between the digital multimeter's input LO terminal
and earth ground, as shown below. This can cause errors
when measuring low voltages that are floating in relative to
earth ground.
HI
R
s
LO
Figure 3-1 Common mode source error
V
= Float Voltage
f
R
= DUT Source Resistance Imbalance
s
R
= digital multimeter Isolation Resistance
Ideal
i
Meter
(LO-Earth)
C
= digital multimeter Input Capacitance
i
Error (v) =
C
R
i
i
Measurement Tutorial
V
x R
f
s
R
+ R
s
i
3
49
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