Rockwell Automation System Design for the Control of Electrical Noise - 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