User`s guide

266
Chapter 7 Tutorial
Measurement Fundamentals
Rejecting Power-Line Noise Voltages A desirable characteristic of
an integrating analog-to-digital (A/D) converter is its ability to reject
spurious signals. Integrating techniques reject power-line related noise
present with DC signals on the input. This is called normal mode
rejection or NMR. Normal mode noise rejection is achieved when the
internal DMM 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 (PLCs) of the spurious input, these errors (and their
harmonics) will average out to approximately zero.
When you apply power to the internal DMM, it measures the power-line
frequency (50 Hz or 60 Hz), and uses this measurement to determine the
integration time. The table below shows the noise rejection achieved with
various configurations. For better resolution and increased noise
rejection, select a longer integration time.
The following graph shows the attenuation of AC signals measured in
the DC voltage function for various A/D integration time settings. Note
that signal frequencies at multiples of 1/T exhibit high attenuation.
PLCs Digits Bits
Integration Time
60 Hz (50 (Hz)
NMR
0.02
0.2
1
2
10
20
100
200
4½
5½
5½
6½
6½
6½
6½
6½
15
18
20
21
24
25
26
26
400 s (400 s)
3 ms (3 ms)
16.7 ms (20 ms)
33.3 ms (40 ms)
167 ms (200 ms)
333 ms (400 ms)
1.67 s (2 s)
3.33 s (4 s)
0 dB
0 dB
60 dB
90 dB
95 dB
100 dB
105 dB
110 dB
Signal Frequency x T
Signal Gain
0 dB
-10 dB
-20 dB
-30 dB
-40 dB
0.1
1
10
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