34405A User’s and Service Guide 55
Measurement Tutorial 3
Noise Rejection
Rejecting Power–Line Noise Voltages
A desirable characteristic of integrating analog–to–digital (A/D) converters
is their ability to reject power–line related noise present with DC input
signals. This is called normal mode noise rejection, or NMR. The
multimeter achieves NMR by measuring the average DC input by
"integrating" it over a fixed period.
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 low voltages which are floating relative to earth ground.
Refer to “Measurement Noise Rejection” on page 137 for the multimeter’s
NMR and CMR characteristics.
Noise Caused by Magnetic Loops
If you are making measurements near magnetic fields, take caution to
avoid inducing voltages in the measurement connections. 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 test leads as close together as possible. Loose or
vibrating test leads will also induce error voltages. Tie down test leads
securely when operating near magnetic fields. Whenever possible, utilize
magnetic shielding materials or increased distance from magnetic sources.
V
f
R
s
R
i
C
i
Ideal
Meter
V
f
= Float Voltage
R
s
= DUT Source Resistance
Imbalance
R
i
= Multimeter Isolation Resistance
(LO-Earth)
C
i
= Multimeter Input Capacitance:
V
f
x R
s
Error (v) =
R
s
+ R
i
HI
LO
V
test
34405A users guide.book Page 55 Saturday, September 2, 2006 3:38 PM
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