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Digital filter – Rockwell Automation 1756-XXXX ControlLogix Analog I/O Modules User Manual

Page 62

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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