Rockwell Automation 1756-XXXX ControlLogix Analog I/O Modules - Manual (Page 62)

62 Publication 1756-UM0009C-EN-P - December 2010
Chapter 4 Non-isolated Analog Voltage/Current Input Modules (1756-IF16, 1756-IF8)
Digital Filter
The digital filter smooths input data noise transients for all channels on the
module. 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
illustration, 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 how to set the Digital Filter, see page 210 .
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
Δ
t = Module channel update time (seconds)
TA = Digital filter time constant (seconds)
Xn = Present input, unfiltered PV
0 0.01 0.5 0.99 Time in Seconds
16723
100%
63%
0
Amplitude
Unfiltered Input
TA = 0.01 sec
TA = 0.5 sec
TA = 0.99 sec