VTI Instruments Ametek EX1200A Series, User Manual

Page: 88 / 165

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EX1200A Triggering 87
Power Line Noise Rejection
An intrinsic characteristic of integrating ADCs is their ability to reject spurious signals. When the
PLCs is set to an integer value, the integrating technique rejects power-line related noise present
with a dc signal on the input. This is called
normal mode rejection, or NMR. (Note that common
mode rejection is with a voltage being common to both input terminals, while NMR is with an ac
voltage between the two input terminals). Normal mode noise rejection is achieved when the
DMM measures the average of the input by “integrating” it over a fixed period..
Noise Caused by Magnetic Loops
When measurements are taken near magnetic fields, precautions should be taken to avoid inducing
voltages in the measurement conductors. This is especially important when working near
conductors carrying large currents. To reduce noise pickup loop area, use twisted-pair connections
to connect to the DMM or dress the input cables as close together as possible. Hanging or
vibrating input cables can also induce error voltages. Be certain that the input cables are tied down
securely when operating near magnetic fields and, whenever possible, use magnetic shielding
materials or physical separation to reduce problems related to magnetic field sources.
DMM Backplane
B
P
L
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H
I
B
P
L
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L
O
B
P
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S
H
I
B
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CH 1
CH 2
CH n
EX1200
Multiplexer Cards
EX1200 Mainframe
Magnetic Source
F
IGURE
4-7: M
AGNETIC
L
OOP
N
OISE