Rockwell Automation 1769-OF4 Compact Analog Output Module - Manual (Page 22)

22 Publication 1769-UM020A-EN-P - December 2009
Chapter 2 Installation and Wiring
Effect of Transducer/Sensor and Cable Length Impedance on
Voltage Output Accuracy
For voltage outputs, the length of the cable used between the load
and the module can affect the accuracy of the data provided by the
module.
Voltage Output Accuracy
Where:
Rc = DC resistance of the cable (each conductor)
depending on cable length
Rs = Source impedance (1
Ω
)
Ri = Impedance of the voltage input
Vs = Voltage at the output of 1769-OF4 module
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-IF4 input module as the load:
Rc = 52.5
Ω
/1000 m
Rs = 1
Ω
Ri = 220 K
Ω
Effect of Output Impedance and Cable Length on Accuracy
Length of Cable DC Resistance of the Cable Rc Accuracy Impact at the
Load
50 m (164 ft) 2.625
Ω
0.00284%
100 m (328 ft) 5.25
Ω
0.00523%
200 m (656 ft) 10.50
Ω
0.01%
300 m (984 ft) 15.75
Ω
0.01477%
V in Vs
Ri
Rc
Rc Rs
+
-
Vin
Ri Vs
×
[ ]
Rs 2 Rc
× ( )
Ri + + [ ]
-------------------------------------------------------- =
% Ai 1
Vin
Vs
--------- –
⎝ ⎠
⎛ ⎞
100
Ч
=