18
Rockwell Automation Publication 1790-UM001B-EN-P - April 2021
Chapter 2 Installation and Wiring
Figure 7 - Effect of Transducer/Sensor and Cable Length
Where:
Rc = DC resistance of the cable (each conductor) depending on
cable length
Rs = Source impedance of analog transducer/sensor output
Ri = Impedance of the voltage input (500K
for 1790D-TN4VO)
Vs = Voltage source (voltage at the transducer/sensor input device)
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:
Rc = 16
/1000 ft
Rs = 0 (ideal source)
As input source impedance (Rs) and/or resistance (DC) of the cable (Rc) get
larger, system accuracy decreases. If you determine that the inaccuracy error is
significant, implement the following equation in the control program to
compensate for the added inaccuracy error due to the impedance of the source
and cable.
Table 3 - Effect of Cable Length on Input Accuracy
Length of Cable (m)
DC Resistance of Cable, Rc (
)
Accuracy Impact at Input Module
50
2.625
0.00105%
100
5.25
0.00210%
200
10.50
0.00420%
300
15.75
0.00630%
In a current loop system, source and cable impedance do not impact
system accuracy.
V in
Vs
Ri
Rc
Rc
Rs
+
-
Vin =
[Ri x Vs]
[Rs + (2 x Rc) + Ri]
%Ai =
Vin
Vs
(
)
1 -
x 100
Vs = Vin x
[Rs + (2 x Rc) + Ri]
Ri
