Rockwell Automation 1769-IF4XOF2 Compact 8-Bit Low Resolution Analog I/O Combination Module User Manual
Page 35
Publication 1769-UM008A-EN-P - November 2001
Installation and Wiring 3-13
Effect of Device and Cable Output Impedance on Output 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.
Where:
Rc = DC resistance of the cable (each conductor)
depending on cable length
Rs = Source impedance of the 1769-IF4XOF2 output (10
Ω
)
Ri = Impedance of the voltage input (150 K
Ω
for the voltage input
channel of the 1769-IF4XOF2)
Vs = Voltage at the output
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 an
input channel:
Rc = 16
Ω
/1000 ft
Rs = 10
Ω
Ri = 150 K
Ω
Table 3.2 Effect of Output Impedance and Cable Length on Accuracy
Length of Cable (m)
dc resistance of the cable
Rc (
Ω
Ω
Ω
Ω
)
Accuracy impact at the
input module
50
2.625
0.01017%
100
5.25
0.01366%
200
10.50
0.02066%
300
15.75
0.02766%
V in
Vs
Ri
Rc
Rc
Rs
+
-
Vin
Ri
Vs
×
[
]
Rs
2
Rc
×
(
)
Ri
+
+
[
]
-------------------------------------------------------
=
%
Ai
1
Vin
Vs
---------
–
100
Ч
=