Rockwell Automation 1769-IF4FXOF2F Compact Combination Fast Analog I/O Module - 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
=