Rockwell Automation 1769-OF4VI Compact I/O Isolated Analog Modules User Manual
Page 29
Publication 1769-UM014B-EN-P - May 2010
Installation and Wiring 2-13
Effect of Device and Cable Output Impedance on Output Module Accuracy
The maximum value of the output impedance is shown in the example below,
because it creates the largest deviation from an ideal voltage source.
Figure 2.5 Output Module Accuracy
Where:
Rc = DC resistance of the cable (each conductor)
depending on cable length
Rs = Source impedance (1 for 1769-OF4VI)
Ri = Impedance of the voltage input (220 k for 1769-IF4I)
Vs = Voltage at the output of 1769-OF4VI
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 and a
1769-OF4CI output module:
Rc = 16 /1000 ft
Rs = 15
Ri = 220 k
Table 2.3 Effect of Output Impedance and Cable Length on Accuracy
Length of Cable (m)
dc Resistance of the Cable
Rc ()
Accuracy Impact at the
Input Module
50
2.625
0.00919%
100
5.25
0.01157%
200
10.50
0.01634%
300
15.75
0.02111%
V in
Vs
Ri
Rc
Rc
Rs
+
-
Vin
Ri
Vs
Rs
2 Rc
Ri
+
+
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=