Single voltage input (24v or 5v), Single voltage input (24v or 5v) -6 – Rockwell Automation System Design for the Control of Electrical Noise User Manual
Page 82
Publication GMC-RM001A-EN-P — July 2001
9-6
High Speed Registration Inputs
Single Voltage Input (24V or 5V)
The figure below illustrates a typical registration filter circuit.
Figure 9.4
Registration filter circuit
•
R1 lowers the circuit impedance which improves noise immunity.
It also ensures that the signal voltage falls rapidly when the
detector turns off. A lower R value is better, but is limited by the
drive capability of the detector and the dissipation in the resistor.
A 470 ohm resistor will dissipate 1.2W at 24V dc if on
continuously, hence it should be rated at 2W.
•
The maximum value of R2 depends on the impedance of the
registration input (a volt drop to 10% of nominal is ideal). If input
impedance is less than 4.7k ohms, then R2 will require a lower
value (i.e., 10% of input impedance). If R2 is changed, use this
formula for the on-delay: Delay (uS) = R2 (ohm) x C (
µ
F). A value
of 470 ohms should be acceptable for most cases.
•
Capacitor C, together with R2, determines the on-delay. Capacitor
C, together with R1 + R2, determines the off-delay (as shown in
the table below).
R1
R2
C
R1
k ohm
R2
k ohm
C
nF
Delay
On
µ
s
Delay
Off
µ
s
Noise
Attenuation
factor
@ 1MHz
Noise
Attenuation
@1MHz
dB
0.47
0.47
4.7
2
4
14
23
0.47
0.47
10
5
9
30
29
0.47
0.47
22
10
21
65
36
0.47
0.47
47
22
44
139
43
0.47
0.47
100
47
94
295
49
0.47
0.47
220
100
207
649
56