Chapter 8 Tutorial

Measurement Fundamentals

y wsu

I tuqyys v

qww qq”w3tywyI5M/ s y y us

 

y q”4 Sqsxy uzus u”yt u x ts ”xu y dxy y sq””ut

XWb4 Xu u usyuut xu q”

MWW 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 (ZVLs) of the spurious input, these errors (and their harmonics) will average out to approximately zero.

When you apply power to the internal MWW, 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.

 

 

 

Integration Time

 

PLCs

Digits

Bits

60 Hz (50 Hz)

NMR

 

 

 

 

 

0.02

412

15

400 μs (400 μs)

0 dB

0.2

512

18

3 ms (3 ms)

0 dB

1

512

20

16.7 ms (20 ms)

60 dB

2

612

21

33.3 ms (40 ms)

90 dB

10

612

24

167 ms (200 ms)

95 dB

20

612

25

333 ms (400 ms)

100 dB

100

612

26

1.67 s (2 s)

105 dB

200

612

26

3.33 s (4 s)

110 dB

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.

Signal Gain

0dB

-10 dB

-20 dB

-30 dB

-40 dB

0.1

1

10

Signal Frequency x T

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Image 340
HP 34970A manual Wsu, Integration Time PLCs