Measurement Computing CIO-EXP-RTD16 - Manual (Page 21)

5.6.3 Four-Wire RTD Hookup
EXCITATION CURRENT (+)
SENSE HIGH (+)
SENSE LOW (-)
EXCITATION CURRENT (-)
WIRE
RTD
A four wire RTD has four leads. One to each side
of the temperature sensitive resistor and an
excitation current source and return. This
connection eliminates the inaccuracy associated
with the two-wire RTD hookup. Because no
current flows on the sense lines there is no voltage
drop in the sense lines, thus the error associated
with two-wire RTDs is eliminated. .
Figure 5-6. Four-Wire RTD Hookup
5.7 CALIBRATION AND TEST
After your CIO-EXP-RTD16 is configured and your RTDs are connected, return to InstaCal and
complete the setup information. Then, use calibration to verify the calibration of your RTDs, and finally,
use TEST to make several temperature measurements.
6 CUSTOM GAIN AND EXCITATION CALCULATIONS
There are resistor positions on the CIO-EXP-RTD16 which are unpopulated. These two resistors are
labeled R89 and RX100. R89 is the custom excitation current resistor and RX100 is the custom second
stage gain resistor. The formulas and constraints to needed to calculate a resistor value are given below.
The value of the resistor must be exact to yield the result you desire. There are two ways to insure the
value is exact. The first is to measure the resistor with an Ohmmeter to check its value, rejecting
resistors that do not meet the desired value. The second way is to purchase precision resistors with a 1%
tolerance. Even then, it’s advisable to check the value with an Ohmmeter before soldering the resistor
into the board.
Please solder carefully!
Remember, you are making a modification to the CIO-EXP-RTD16 and if it is not made well the quality
of the signal may be affected. Use only solder with a water soluble flux and be sure to remove all the
flux after you have finished soldering. Flux residue can add capacitance to a circuit which will affect the
signal.
6.1 CUSTOM EXCITATION CURRENT - R89
The formula for IEXC on the CIO-EXP-RTD16 is:
IEXC = (10V
−
Vref) / 5K Where Iexc is Minimum = 0.1 mA and Maximum = 1 mA
Vref is nominally 5V, so Iexc is nominally 1 mA
The procedure for selecting a value for R89 to get a custom excitation current is:
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