Rockwell-automation 1769-OF4VI Compact I/O Isolated Analog Modules User Manual download pdf (Page 29)

Publication 1769-UM014B-EN-P - May 2010

Installation and Wiring 2-13

Effect of Device and Cable Output Impedance on Output Module Accuracy

The maximum value of the output impedance is shown in the example below,

because it creates the largest deviation from an ideal voltage source.

Figure 2.5 Output Module Accuracy

Where:

Rc = DC resistance of the cable (each conductor)

depending on cable length

Rs = Source impedance (1  for 1769-OF4VI)

Ri = Impedance of the voltage input (220 k for 1769-IF4I)

Vs = Voltage at the output of 1769-OF4VI

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 and a

1769-OF4CI output module:

Rc = 16 /1000 ft

Rs = 15 

Ri = 220 k

Table 2.3 Effect of Output Impedance and Cable Length on Accuracy

Length of Cable (m) dc Resistance of the Cable

Rc ()

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%

V in

RcRs

Vin

Ri Vs

Rs 2 RcRi++

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Rockwell-automation 1769-OF4VI Compact I/O Isolated Analog Modules User Manual Page 29