3-30
IM DR231-01E
Balance each other
integration time
DC voltage (average value)
Input voltage (instantaneous value)
Normally, an integration time of 20ms (50Hz) or 16.7ms (60Hz) is selected depending on the
commercial power supply frequencies. A 100-ms integration mode is added to the instrument to
achieve superior noise rejection. However, when using the 100ms setting, the smallest
measurement interval is longer than in case of the 20ms or 16.7ms setting. The integration effect
enables the PWM A/D converter to perform the following two functions.
• Rejection of frequency determined by the reciprocal of the integration time and frequencies
which are whole multiples of that frequency;
• First-order lag filter provided with cut-off frequency proportional to the reciprocal of the
integration time.
The following table compares the integration times of 16.7ms, 20ms and 100ms.
Integration time
Rejection frequency
Cut-off frequency
Remarks
16.7ms
n
×
60Hz
approx. 19Hz
for 60Hz
20.0ms
n
×
50Hz
approx. 16Hz
for 50Hz
100.0ms
n
×
10Hz
approx. 3.2Hz
for both 50Hz/60Hz
n=1,2,3...
As shown in the table, the merit of 100.0-ms integration is not only that it applies to both 50 and
60Hz, but also that it provides a low cut-off frequency as the first-order lag filter and improves
the noise rejection ability.
The following figure shows the calculation values of the NMRR for three integration times and an
example of actual measurement of the NMRR for a 100-ms integration signal.
• Calculated values of NMRR
• Example of actual measurement of NMRR
(100ms)
0
-20
-40
-60
-10
-30
-50
-70
5
10
20
50
100
200
Freq ency (Hz)
dB
16.7ms
20.0ms
100ms
F
(H )
Noise Filter
This instrument is equipped with a low-pass filter (cutoff-frequency of 10Hz (for both 50/60 Hz),
50Hz,60Hz) which functions as a way of noise rejection. Also exponential averaging functions as
a noise filter.
Anti-Noise Measures: Applications
Practical Measures
• Reducing noise itself
The basics of this practical measures dictates using the instrument in conditions where noise is
suppressed as much as possible.
• for power lines: an increase of impedance;
Separate the power lines for noise source equipment (inverter, thyristor, etc.) from those for
the measuring instrument.
3.8 Countering Noise
Summary of Contents for DR231
Page 12: ...10 IM DR231 01E...
