Rockwell Automation 1790D-NOV2/TNOV2 CompactBlock LDX Analog Modules - Manual (Page 27)

Publication 1790-UM001A-EN-P - March 2002
Installation and Wiring 2-11
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.
Where:
Rc = DC resistance of the cable (each conductor)
depending on cable length
Rs = Source impedance of 1790D-NOV2/TNOV2 (0.5
Ω
)
R
load
= Impedance of the load device
Vs = Voltage at the output of 1790D-NOV2/TNOV2
V
load
= Measured potential at the load device
%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
1790D-NOV2/TNOV2 module:
Rc = 16
Ω
/1000 ft
Rs = 0.5
Ω
Ri
Rc
+
-
Rs Rc
Rc
Vs
+
–
Vload Rload
43266
V
load
=
[R
load
x V
s
]
[R
s
+ (2 x R
c
) + R
load
]
%AV
load
= (1 - ) x 100
V
s
V
load