Publication 1769-UM002B-EN-P - July 2005
2-14 Installation and Wiring
For example, for Belden 8761 two conductor, shielded cable and a 1769-IF4
input module:
Rc = 16 Ω/1000 ft
Rs = 15 Ω
Ri = 220 KΩ
As output 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 output
module and cable.
Table 2.3 Effect of Output Impedance and Cable Length on Accuracy
Length of Cable (m) dc resistance of the cable
Rc (Ω)
Accuracy impact at the
input module
50 2.625 0.00919%
100 5.25 0.01157%
200 10.50 0.01634%
300 15.75 0.02111%
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
-------------------------------------------------------
×=
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