Rockwell Automation 1756-OF8I ControlLogix Eight-channel Isolated Analog I/O Modules - Manual (Page 71)

Rockwell Automation Publication 1756-UM540A-EN-P - May 2014 71
1756-IRT8I Combined Temperature-sensing Isolated Analog Module Chapter 4
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 disables the filter.
The digital filter equation is a classic first order lag equation.
By using a step input change to illustrate the filter response, you can see that when
the digital filter time constant elapses, 63.2% of the total response is reached.
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
1672
100%
63%
0
Amplitude
Unfiltered Input
TA = 0.01 s
TA = 0.5 s
TA = 0.99 s