Rockwell Automation 1769-IF4XOF2 Compact 8-Bit Low Resolution Analog I/O Combination Module - Manual (Page 34)

Publication 1769-UM008A-EN-P - November 2001
3-12 Installation and Wiring
For example, for Belden 8761 two conductor, shielded cable:
Rc = 16
Ω
/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 3.1 Effect of Cable Length on Input Accuracy
Length of Cable (m) dc resistance of the
cable, Rc (
Ω Ω Ω Ω
)
Accuracy impact at the
input module
50 2.625 0.00350%
100 5.25 0.00700%
200 10.50 0.01400%
300 15.75 0.02100%
TIP
In a current loop system, source and cable
impedance do not impact system accuracy.
% Ai 1
Vin
Vs
--------- –  
 
100
×
=
Vs Vin
Rs 2 Rc
× ( )
Ri + +
[ ]
Ri
-------------------------------------------------------
Ч
=