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--- resources:eval:user-guides:eval-ad7441x:tools:3wrtd [31 Aug 2021 14:13] – [Implementation] Bríde Ní Riagáin
+++ resources:eval:user-guides:eval-ad7441x:tools:3wrtd [30 Jun 2023 17:08] (current) – RMEAS2 formula typo update (RTD + RL) (not 2RL) Arnost Pavlik
@@ Line 3: / Line 3: @@
 ====Introduction====
 The AD74413R, software configure I/O IC is designed to interface with industrial, process & building control sensors and actuators. The IC comes with a range of programmable I/O capabilities, including analog output, analog input, digital input and temperature measurement. Temperature measurement is crucial in many industrial environments, with thermocouples and RTDs (Resistance Temperature Detectors) being the most commonly used temperature sensor types. The AD74413R has in-built thermocouple and 2-wire RTD measurement capability. 3-wire RTD measurements are not directly supported on chip but this Wiki page will show how to use the AD4413R to make 3-wire RTD measurements, while maintaining all of the existing functionality of the IC.
+\\
 ====RTDs====
+An RTD (Resistance Temperature Detector) is a sensor used to measure temperature. RTDs have a repeatable resistance vs temperature relationship which can be used to accurately determine the temperature. RTDs are used in a wide variety of applications and are particularly suitable for industrial applications due to their with wide operating temperature ranges and reliability.
+\\ To measure temperature with an RTD, a small excitation current is passed through it and the corresponding voltage drop is measured. The corresponding resistance is calculated and used to determine the ambient temperature of the sensor.
+\\
+\\ ===RTD Types===
+-wire RTDs are typically used for low accuracy applications and when the distance to the sensor is short (lead lengths are short). Longer leads result in larger lead resistances, which add error to the overall resistance measurement. If long lead lengths are in use or higher accuracy is required, 3-wire RTDs are used. In this case, it is assumed that each of the leads are of similar length and resistance. A 3-wire RTD measurement allows for an estimate of this lead resistance to be made and this in turn is factored out of the overall resistance measurement. Finally, 4-wire RTDs provide the best accuracy - 2 wires are used to source the excitation current through the sensor while the remaining 2 wires are used to directly sense the voltage drop across the RTD sensor.
 \\
 ====External Component changes to AD74413R circuit to support 3-wire RTD====
@@ Line 55: / Line 61: @@
 \\
 \\ V <sub>(I/OP to I/ON)</sub> and I<sub>EXCITE</sub> are used to determine the combined resistance of the RTD and the two connected lead wires according to the following equation:
+\\
 \\ R<sub>MEAS1</sub> = R<sub>RTD</sub> + 2R<sub>L</sub> = V <sub>(I/OP to I/ON)</sub> / I<sub>EXCITE</sub>
 \\
@@ Line 68: / Line 75: @@
 \\ V<sub>MIN</sub> is the minimum voltage of the selected ADC range
 \\
-\\ V <sub>(I/OP to I/O AUX)</sub> and I<sub>EXCITE</sub> are used to determine the combined resistance of the RTD and one R<sub>L</sub> according to the following equation:
+\\ V <sub>(I/OP to I/O AUX)</sub> and I<sub>EXCITE</sub> are used to determine the combined resistance of the RTD and one R<sub>L</sub> according to the equation:
-\\ R<sub>MEAS2</sub> = R<sub>RTD</sub> + 2R<sub>L</sub> = V <sub>(I/OP to I/O AUX)</sub> / I<sub>EXCITE</sub>
+\\
+\\ R<sub>MEAS2</sub> = R<sub>RTD</sub> + R<sub>L</sub> = V <sub>(I/OP to I/O AUX)</sub> / I<sub>EXCITE</sub>
 \\
 \\
@@ Line 75: / Line 83: @@
 ===RTD Calculations===
 \\ These 3 measurements are then used to determine the lead resistance and as a result, the RTD resistance
-R<sub>L</sub> = R<sub>MEAS1</sub> - R<sub>MEAS2</sub>
+\\ R<sub>L</sub> = R<sub>MEAS1</sub> - R<sub>MEAS2</sub>
+\\
+\\
 R<sub>RTD</sub> = R<sub>MEAS2</sub> - R<sub>L</sub>
 \\
 \\
 ===Conversion Rates===
-A choice of conversion rates are available for these measurements. See the AD74413R datasheet for the full range of conversion rates. To minimize the time taken for these measurements, it would be possible to only make one measurement continuously, as the RL resistance is not expected to change over time. The second measurement can be made less often to spot check that no significant changes have occurred to the RL value.
+A choice of conversion rates is available for these measurements. See the AD74413R datasheet for the full range of conversion rates. To minimize the time taken for these measurements, it would be possible to only make one measurement continuously, as the RL resistance is not expected to change over time. The second measurement can be made less often to spot check that no significant changes have occurred to the RL value.
 \\
@@ Line 112: / Line 122: @@
 ====Open Wire Detect====
 The IOUT open circuit fault is asserted when an open wire is present on either I/OP or I/ON screw terminals. This open circuit fault is indicated on the ALERT pin, which can be used as an interrupt to the host microcontroller.
-For the third lead, on I/O AUX,
-[[:resources:eval:user-guides:ad7441x|Back to other AD7441xR Wiki Topics]]
+[[:resources:eval:user-guides:ad7441x|Back to AD74412R/AD74413R Table of Contents]]

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