Rockwell-automation 1769-OF4VI Compact I/O Isolated Analog Modules User Manual download pdf (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

---------

–





100=

Vs Vin

Rs 2 RcRi++

-------------------------------------------------------

=

1 2 ... 23 24 25 26 27 28 29 30 31 32 33 ... 127 128

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Rockwell-automation 1769-OF4VI Compact I/O Isolated Analog Modules User Manual Page 28

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