ABB XSeriesG4 User Manual page 115

A.3 Example 2
If the 4-20 mA transmitter cannot drive full range (20 mA through the 250 Ohm resistor), reduce the value of the resistor. For
example drop it to 125 Ohm (or half). A 4-20 mA current would now be converted to a .5 - 2.5 Vdc voltage range. Again, during
field calibration, have the 4 mA current represent 10 bbls per hour and the 20 mA signal represent 100 bbls per hour. The
voltage conversion (using the 125 Ohm resistor) would be as follows:
.5 Vdc = 10 bbls/hour
2.5 Vdc = 1000 bbls/hour
The AD full range is still 0-5 Vdc, but using only about half of its range capability. Just like the resistor value, the ability to resolve
barrels of fluid per hour has been cut in half. Overall accuracy remains unchanged, but only about .006 bbl (6 thousandths of a
barrel, or about one fluid ounce) can be resolved.
A.4 Conclusion
If the 4-20 mA transmitter cannot drive a full 20 mA through the 250 Ohm resistor, one option is to reduce the resistive value.
However, as the resistive value is reduced, resolution (not overall accuracy) will be reduced proportionately. Dropping the
resistive value by half (250 Ohms to 125 Ohms), reduces the resolution also by half.
To reduce the resistive value of an onboard resistor another option is to add an external resistor across the analog input pins.
This places the two resistors in parallel with one another. The parallel combination produces an equivalent resistance that is less
than either of the two resistors alone. The value of this equivalent resistance is calculated as shown in the figure.
Appendix A 4–20 mA transmitters
XSeriesG4 Remote Controller User's manual 2103334 Rev. AB | 103