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Effect of Device and Cable Output Impedance on Output Module
Accuracy
The maximum value of the output impedance is shown in
creates the largest deviation from an ideal voltage source.
Figure 8 - Effect of Device and Cable Output Impedance
+
+
Vs
-
–
Where:
Rc = DC resistance of the cable (each conductor) depending on cable
length
Rs = Source impedance of 1790D-TNOV2 (0.5 )
R
= Impedance of the load device
load
Vs = Voltage at the output of 1790D-TNOV2
V
= Measured potential at the load device
load
%Ai = Percent added inaccuracy in a voltage-based system due to source
and cable impedance.
[R
x V
]
load
s
V
=
load
[R
+ (2 x R
) + R
s
c
For example, for Belden 8761 two conductor, shielded cable and a
1790D-TNOV2 module:
Rc = 16 /1000 ft
Rs = 0.5
V
load
%AV
= (1 -
) x 100
load
V
s
Table 4 - Effect of Output Impedance and Cable Length on Accuracy
DC resistance of the
Length of Cable (m)
cable, Rc ()
1
0.0525
10
0.525
50
2.625
100
5.25
Rockwell Automation Publication 1790-UM001B-EN-P - April 2021
Rs
Rc
Vload
Rc
Rc
]
load
Accuracy impact at the load
1000
0.0605%
0.155%
0.575%
1.1%
Chapter 2
Installation and Wiring
Figure
8, because it
Rload
Ri
10,000
100,000
0.00605%
0.000605%
0.0155%
0.00155%
0.0575%
0.00575%
0.11%
0.011%
19
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