Rejecting Power Line Noise Voltages; Common Mode Rejection (Cmr) - Agilent Technologies Agilent E1412A User Manual

Rejecting Power
Line Noise
Voltages
Common Mode
Rejection (
Chapter 2
A desirable characteristic of integrating analog-to-digital (A/D) converters
is their ability to reject spurious signals. The integrating techniques reject
power-line related noise present with a dc signal on the input. This is called
normal mode rejection or
when the multimeter measures the average of the input by "integrating" it
over a fixed period. If you set the integration time to a whole number of
power line cycles (
to approximately zero.
The E1412A provides three A/D integration times (1, 10 and 100PLCs) to
reject power line frequency noise (and power-line frequency harmonics).
Power line frequency defaults to 60Hz unless you specifically set it to 50Hz
with the
CAL:LFR
integration time based on which power line frequency is set. Table 2-2
shows the noise rejection achieved with various configurations. Select a
longer integration time for better resolution and increased noise rejection.
Power Line
Cycles (PLCs)
Ideally, a multimeter is completely isolated from earth-referenced circuits.
However, there is finite resistance between the multimeter's input LO
)
CMR
terminal and earth ground as shown below. This can cause errors when
measuring small voltages which are floating relative to earth ground.
. Normal mode noise rejection is achieved
NMR
s) these errors (and their harmonics) will average out
PLC
command. The multimeter determines the proper
Table 2-2. Noise Rejection
Integration Time
60Hz
0.02 400s
0.2 3ms
1 16.7ms
10 167ms
100 1.67sec
Agilent E1412A Multimeter Application Information
(50Hz)
NMR
(400s) NONE
(3ms) NONE
(20ms) 60dB
(200ms) 60dB
(2sec) 60dB
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