Rockwell Automation 1769-OF4VI Compact I/O Isolated Analog Modules - Manual (Page 28)

Publication 1769-UM014B-EN-P - May 2010
2-12 Installation and Wiring
Where:
Rc = DC resistance of the cable (each conductor) depending on
cable length
Rs = Source impedance of analog transducer/sensor input
Ri = Impedance of the voltage input (1 M for 1769-IF4I)
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, implementing the following equation in the control program can
compensate for the added inaccuracy error due to the impedance of the source
and cable.
Table 2.2 Effect of Cable Length on Input Accuracy
Length of Cable,
m (ft)
DC Resistance of the Cable,
Rc ()
Accuracy Impact at the
Input Module
50 (164) 2.625 0.000525%
100 (328) 5.25 0.00105%
200 (656) 10.50 0.0021%
300 (984) 15.75 0.00315%
TIP
In a current loop system, source and cable impedance do
not impact system accuracy.
Vin
Ri Vs
  
Rs
2 Rc    Ri + +  
------------------------------------------------------- =
% Ai
1
Vin
Vs
--------- –
 
  100
 =
Vs Vin
Rs
2 Rc    Ri + +  
Ri
-------------------------------------------------------  =