Rosemount 1151GP - Digital to Analog Converter Trim

Reference Manual
00809-0100-4360, Rev BA
August 2008
Rosemount 1151
4-10
Analog Electronics
Once again, there are two ways to calibrate an analog transmitter for this
application. One method is to increase the pressure when adjusting the
span. Another method is to apply the span pressure and increase the mA
output.
1. Apply 0 inH
2
O pressure and adjust the output to 4mA. then apply 223.54
inH
2
O pressure and adjust the output to 20 mA.
220 inH
2
O X 1.61% = 3.54 inH
2
O
220 inH
2
O ± 3.54 inH
2
O = 216.46 inH
2
O
2. Apply 0 inH
2
O pressure and adjust the output to 4mA. then apply 220
inH
2
O pressure and adjust the output to 20.257 mA.
16 mA X 1.61% =.257 mA
Smart Electronics
There are also two ways to calibrate a smart transmitter for this
application. One method is to rerange the transmitter. The other method is
to perform a sensor trim.
1. Set the range points to 0–223.54 inH
2
O by using the Field
communicator.
2. Using the Field communicator, perform a Sensor Trim. Select Hi Val, and
apply 216.96 inH
2
O pressure. When the communicator asks what
pressure was applied, enter 216.96 inH
2
O. Set the range points to 0–220
inH2O.
Digital to Analog
Converter Trim
Deciding Whether to Trim the D/A Converter
After the microprocessor conditions the sensor signals, it outputs a digital
word. The digital-to-analog (D/A) output circuitry converts the word to an
analog signal for the 4–20 mA communications line. It may be necessary to
check and trim this circuitry after a period of time. The 4–20 mA output trim
function can also be used to make adjustments to allow for peculiarities of a
particular readout device in the loop.
To determine whether the output must be trimmed, connect the HART-based
communicator and a precision milliammeter capable of reading ± 1 micromole
in the loop as shown in Figure 4-3.
Next, perform a loop test as described on page 3-6. Follow the loop test
procedure and set the transmitter to a 4 mA output. Then check the ammeter.
The reading should be within ±3 A of 4 mA.
Then set the transmitter to a 20 mA output and check the milliammeter. The
reading should be within ± 3 μA of 20 mA. The output should be trimmed if the
values on the meter exceed this tolerance range.
When trimming the output, adjustments are made to the output circuitry. The
appropriate shift will be made for all intermediate points between 4 and 20
mA.
The communicator will allow the D/A converter to be trimmed by using a
current meter or voltage meter. When using a current meter, follow the
sequence outlined in 4–20 mA Analog Output Trim. When using
a voltage meter or a meter whose display does not read out in 4–20 mA,
follow the sequence outlined under 4–20 mA Analog Output Trim Using
Other Scale.