Rockwell Automation 1756-OF8I ControlLogix Eight-channel Isolated Analog I/O Modules - Manual (Page 52)

52 Rockwell Automation Publication 1756-UM540A-EN-P - May 2014
Chapter 3 1756-IF8I Isolated Analog Input Module
Digital Filter
The digital filter smooths input data noise transients on each input channel.
This value specifies the time constant for a digital, first-order lag filter on the
input. It is specified in units of milliseconds. A value of 0 (zero) disables
the filter.
The digital filter equation is a classic, first order lag equation.
As shown in the following graphic, by using a step input change to illustrate the
filter response, you see that 63.2% of the total response is reached when the
digital filter time constant elapses. Each additional time constant achieves 63.2%
of the remaining response.
To see where to set the Digital Filter, see page 127 .
Y
n
= Y
n -1
+ X
n
- Y
n -1
[
Δ t
]
Δ t
+ TA
Y
n
= Present output, filtered peak voltage (PV)‘
Y n -1
= Previous output, filtered PV
Δ t
= Module channel update time (seconds)
TA = Digital filter time constant (seconds)
X
n = Present input, unfiltered PV
0 0.01 0.5 0.99 Time in Seconds
100%
63%
0
Amplitude
Unfiltered Input
TA = 0.01 second
TA = 0.5 second
TA = 0.99 second