Allen-Bradley 1756-IF8I User Manual page 44

Chapter 3 1756-IF8I Isolated Analog Input Module
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For example, a Notch Filter is typically set to 60 Hz to filter out 60 Hz AC line
noise and its overtones. A 60 Hz Notch Filter setting attenuates frequencies of
60 Hz, 120 Hz, 180 Hz, and so forth.
The following graphic shows 10 Hz Notch Filter selection and how the noise is
dissipated over the entire spectrum but especially at the Notch Filter setting and
its overtones.
Relationship between Noise Rejection Level and RPI Setting
The 1756-IF8I module offers two levels of line noise rejection. Each level has a
filter associated with it. The module automatically determines which filter is used
based on the Notch Filter setting and RPI rate.
A trade-off exists between sampling speed and level of noise rejection:
• The faster sampling speed, ranging from 1 / Notch Filter to 3 / Notch
Filter, the less noise rejection. In this case, the module automatically uses a
SINC^1 filter.
The SINC^1 filter offers 34 dB noise rejection at the Notch Filter
frequency and its overtones.
• The slower sampling rate, > 3 / Notch Filter, the better noise rejection. In
this case, the module automatically uses a SINC^3 filter.
The SINC^3 filter offers 100 dB noise rejection at the Notch Filter
frequency and its overtones.
Rockwell Automation Publication 1756-UM540E-EN-P - December 2017