Rockwell Automation 1762-IT4 Thermocouple/mV Input Module - Manual (Page 40)

Publication 1762-UM002A-EN-P - July 2002
3-10 Module Data, Status, and Channel Configuration
Selecting Input Filter Frequency (Bits 2 through 0)
The input filter selection field allows you to select the filter frequency
for each channel and provides system status of the input filter setting
for channels 0 through 3. The filter frequency affects the following, as
explained later in this chapter:
•
noise rejection characteristics for module inputs
•
channel step response
•
channel cut-off frequency
•
effective resolution
•
module update time
Effects of Filter Frequency on Noise Rejection
The filter frequency that you choose for a module channel determines
the amount of noise rejection for the inputs. A lower frequency (50 Hz
versus 500 Hz) provides better noise rejection and increases effective
resolution, but also increases channel update time. A higher filter
frequency provides lower noise rejection, but decreases the channel
update time and effective resolution.
When selecting a filter frequency, be sure to consider cut-off
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 cut-off frequency.
Table 3.2 Open-Circuit Response Definitions
Response
Option Definition
Upscale Sets the input data value to full upper scale value of channel data word. The
full-scale value is determined by the selected input type and data format.
Downscale Sets the input data value to full lower scale value of channel data word. The
low scale value is determined by the selected input type and data format.
Last State Sets the input data value to the last input value prior to the detection of the
open-circuit.
Zero Sets the input data value to 0 to force the channel data word to 0.