Rockwell Automation 1756-OF8H ControlLogix HART Analog I/O Modules - Manual (Page 57)

Rockwell Automation Publication 1756-UM533C-EN-P - February 2011 57
1756-IF8H HART Analog Input Module Chapter 4
Digital Filter
The digital filter smooths input data noise transients. This feature is applied on a
per channel
basis.
The digital filter 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.
Using a step input change to illustrate the filter response, as shown in the figure,
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.
Figure 4 - Filter Response
Yn = Yn-1 + (X
n
– Y
n
-1)
[ Δ t]
Δ t + TA
Yn = present output, filtered peak voltage (PV)
Yn-1 = previous output, filtered PV
Δt = module channel update time (seconds)
TA = digital filter time constant (seconds)
Xn = present input, unfiltered PV