Example 1; Example 2; Conclusion - ABB XSeries G4 6200 User Manual

6.3.2 Example 1

If the 4-20 mA transmitter can drive full range (20 mA through the 250 OHM
resistor), the user has essentially converted a 4-20 mA current range into a 1-5
VDC voltage range. The analog to digital (A/D) converter used in Totalflow
equipment expects a 1-5 VDC signal. During field calibration, the user might have
the 4 mA of loop current represent 10bbls of fluid per hour and 20 mA represent
100bbls. The conversion to voltage (using the 250 OHM resistor) would be as
follows:
Assuming that the A/D converter is ranged between 0 and 5 VDC and the
converter is resolving 15 bits, the user would expect to resolve about 150μ VDC
(or .0006 mA). These numbers translate to about .003 bbl (3 thousandths of a
barrel or about one half fluid ounce).

6.3.3 Example 2

If the 4-20 mA transmitter cannot drive full range (20 mA through the 250 OHM
resistor), the user may want to reduce the value of the resistor. For this example,
the following will 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, the user
would have the 4 mA current represent 10bbls per hour and the 20 mA signal
represent 100bbls per hour. The voltage conversion (using the 125 OHM resistor)
would be as follows:
The A/Ds full range is still 0-5 VDC; however, the user is only using 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. The overall accuracy remains unchanged, but the
user can only resolve about 0.006bbls (6 thousandths of a barrel or about one
fluid ounce).

6.3.4 Conclusion

If the 4-20 mA transmitter that is being used cannot drive a full 20 mA through the
250 OHM resistor, the user can reduce the resistive value. However, as the
resistive value is reduced, resolution (not overall accuracy) will be reduced
proportionally. If the user drops the resistive value by half (250 OHMs to 125
OHMs), the resolution will be reduced by half.
To reduce the resistive value of an onboard resistor, the user can 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
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Another possible solution would be to reduce the 250 OHM
resistor to a small value, possibly 125 OHMs. An under-
powered transmitter could more easily drive 20 mA into the
lesser resister. Field calibrating the AI would negate any
differences between the 250 OHM and the 125 OHM
resistors. However, as the user reduces the size of the
resistor, they lessen the overall resolution.
1 VDC = 10bbls/hour
5 VDC = 100bbls/hour
.5 VDC = 10bbls/hour
2.5 VDC = 1000bbls/hour
2104349 rev. AD