Rockwell Automation 1771-IFE , D17716.5.90 USER MNL. ANALOG INPUT Module - Manual (Page 33)

Module Configuration
Chapter 4
4-4
The module has hardware-based high frequency filters on all channels to
reduce the effect of electrical noise on the input signal. Software digital
filtering is meant to reduce the effect of process noise on the input signal.
Digital filtering is selected using BTW word 3, bits 00-07.
The digital filter equation is a classic first order lag equation (Figure 4.1).
Using a step input change to illustrate the filter response (Figure 4.2), you
can see that when the digital filter constant time elapses, 63.2% of the total
response is reached. Each additional time constant achieves 63.2% of the
remaining response.
Figure 4.1
Digital Filter Equation
Y
n
= Y
n-1
+
t
t + TA
(X
n
- Y
n-1
)
Where: Yn = present output, filtered peak voltage (PV)
Yn -1 = previous output, filtered PV
t = module channel update time (seconds)
X n = present input, unfiltered PV
TA = digital filter time constant (seconds)
Figure 4.2
Digital Filter Lag Equation Illustration
100%
63%
0 0.01 0.5 0.99 Time in Seconds
16723
TA = 0.99 sec
TA = 0.5 sec
TA = 0.01 sec
Unfiltered Input
0
Amplitude
Digital filter time constant values of 0.00 BCD to 0.99 BCD (0.00 BCD =
no filter; 0.99 BCD = maximum filter) are set in bits 00 through 07 of
word 3 of the block transfer write instruction. If an invalid digital filter
value is entered (i.e., 0.1F), bit 02, word 1 of the block transfer read
instruction will be set. If an invalid digital filter value is entered, the
module will not perform digital filtering. If you use the digital filtering
feature, the filter time constant value chosen will apply to all input signals.
Digital Filtering