Rockwell Automation 1769-IR6 Compact I/O 1769-IR6 RTD/Resistance Input Module - Manual (Page 61)

Rockwell Automation Publication 1769-UM005B-EN-P - March 2012 61
Module Data, Status, and Channel Configuration Chapter 4
Setting Filter Frequency (Bits 0…2)
The module supports filter selections corresponding to filter frequencies of
10 Hz, 50 Hz, 60 Hz, 250 Hz, 500 Hz, and 1 kHz. 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 channel
configuration word 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
· module autocalibration
· effective resolution
· module update time
Effects of Filter Frequency on Noise Rejection
The filter frequency that you choose for a channel determines the amount of
noise rejection for the inputs. A smaller filter frequency (such as 10Hz)
provides the best noise rejection and increases effective resolution, but also
increases channel update time. A larger filter frequency (such as 1 kHz)
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 as long as the signals applied to the input
terminals do not exceed the common mode voltage rating (±2.5V) of the
module. Improper earth ground can be a source of common mode noise.
TIP Transducer power supply noise, transducer circuit noise,
and process variable irregularities can also be sources of
common mode noise.