Allen-Bradley CompactBlock LDX 1790D-4T0, Руководство пользователя

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Publication 1790-UM003A-EN-P
3-4 Module Data, Status, and Channel Configuration for DeviceNet
Filter Frequency
The module supports filter selections corresponding to filter frequencies
of 10 Hz, 25Hz, 50 Hz, 60 Hz, 100 Hz, 250 Hz, and 500 Hz. Your filter
frequency selection is determined by the desired range for the input type,
and the required effective resolution, which indicates the number of bits
in the input data that do not vary due to noise. Also consider the required
module update time when choosing a filter frequency. For example, the
10 Hz filter provides the greatest attenuation of 50 and 60 Hz noise and
the greatest resolution, but also provides the slowest response speed.
The choice that you make for filter frequency will affect:
•
noise rejection characteristics for module input
•
channel step response
•
channel cutoff frequency
•
effective resolution
•
module update time
Effects of Filter Frequency on Noise Rejection
The filter frequency that you choose for the module determines the
amount of noise rejection for the inputs. A smaller filter frequency
(e.g. 10Hz) provides the best noise rejection and increases effective
resolution, but also increases channel update time. A larger filter
frequency (e.g. 500 Hz) provides lower noise rejection, but also decreases
the channel update time and effective resolution.
When selecting a filter frequency, be sure to consider channel cutoff
frequency and channel step response to obtain acceptable noise rejection.
Choose a filter frequency so that your fastest-changing signal is below that
of the filter’s cutoff frequency.
Common mode noise rejection for the module is better than 110 dB at 50
Hz (50 Hz filter) and 60 Hz (60 Hz filter). The module performs well in
the presence of common mode noise. Improper earth ground can be a
source of common mode noise.
IMPORTANT
Transducer power supply noise, transducer circuit noise,
and process variable irregularities can also be sources of
common mode noise.