Allen-Bradley MicroLogix 1200 User Manual page 51

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
(e.g. 10Hz) provides the best noise rejection and increases effective
resolution, but also increases channel update time. A larger filter
frequency (e.g. 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 (±10V) of the module. Improper earth ground can be a
source of common mode noise.
Transducer power supply noise, transducer circuit
TIP
noise, and process variable irregularities can also be
sources of common mode noise.
Channel Step Response
Another module characteristic determined by filter frequency is
channel step response, as shown in the following table. The step
response is the time required for the analog input signal to reach 100
percent of its expected final value, given a full-scale step change in
the input signal. Thus, if an input signal changes faster than the
channel step response, a portion of that signal will be attenuated by
Module Data, Status, and Channel Configuration
Publication 1762-UM003A-EN-P - February 2003
3-17