Rockwell Automation 1769-IF4FXOF2F Compact Combination Fast Analog I/O Module User Manual
Page 22
22
Publication 1769-UM019A-EN-P - October 2008
Chapter 2 Installation and Wiring
Effect of Transducer/Sensor and Cable Length Impedance on
Voltage Input and Output Accuracy
For voltage inputs and outputs, the length of the cable used between
the transducer/sensor/load and the module can affect the accuracy of
the data provided by the module.
Voltage Input Accuracy
Where:
Rc = DC resistance of the cable (each conductor) depending on
cable length
Rs = Source impedance of analog transducer/sensor input
Ri = Impedance of the voltage input (220 k
Ω
)
Vs = Voltage source (voltage at the transducer/sensor input device)
Vin = Measured potential at the module input
%Ai = Percent added inaccuracy in a voltage-based system due
to source and cable impedance
For example, for Belden 8761 two conductor, shielded cable:
Rc = 5.25
Ω
/1000 m
Rs = 0 (ideal source)
V in
Vs
Ri
Rc
Rc
Rs
+
-
Effect of Cable Length on Input Accuracy
Length of Cable,
m (ft)
DC Resistance of the Cable,
Rc (
Ω
)
Accuracy Impact at the
Input Module
50 (164)
2.625
0.002385%
100 (328)
5.25
0.00477%
200 (656)
10.50
0.00954%
300 (984)
15.75
0.01431%
Vin
Ri
Vs
×
[
]
Rs
2
Rc
×
(
)
Ri
+
+
[
]
--------------------------------------------------------
=
%
Ai
1
Vin
Vs
---------
–
⎝
⎠
⎛
⎞
100
Ч
=