0001 # SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
0002 %YAML 1.2
0003 ---
0004 $id: http://devicetree.org/schemas/iio/afe/temperature-sense-rtd.yaml#
0005 $schema: http://devicetree.org/meta-schemas/core.yaml#
0006
0007 title: Temperature Sense RTD
0008
0009 maintainers:
0010 - Liam Beguin <liambeguin@gmail.com>
0011
0012 description: |
0013 RTDs (Resistance Temperature Detectors) are a kind of temperature sensors
0014 used to get a linear voltage to temperature reading within a give range
0015 (usually 0 to 100 degrees Celsius).
0016
0017 When an io-channel measures the output voltage across an RTD such as a
0018 PT1000, the interesting measurement is almost always the corresponding
0019 temperature, not the voltage output. This binding describes such a circuit.
0020
0021 The general transfer function here is (using SI units)
0022
0023 V = R(T) * iexc
0024 R(T) = r0 * (1 + alpha * T)
0025 T = 1 / (alpha * r0 * iexc) * (V - r0 * iexc)
0026
0027 The following circuit matches what's in the examples section.
0028
0029 5V0
0030 -----
0031 |
0032 +---+----+
0033 | R 5k |
0034 +---+----+
0035 |
0036 V 1mA
0037 |
0038 +---- Vout
0039 |
0040 +---+----+
0041 | PT1000 |
0042 +---+----+
0043 |
0044 -----
0045 GND
0046
0047 properties:
0048 compatible:
0049 const: temperature-sense-rtd
0050
0051 io-channels:
0052 maxItems: 1
0053 description: |
0054 Channel node of a voltage io-channel.
0055
0056 '#io-channel-cells':
0057 const: 0
0058
0059 excitation-current-microamp:
0060 description: The current fed through the RTD sensor.
0061
0062 alpha-ppm-per-celsius:
0063 description: |
0064 alpha can also be expressed in micro-ohms per ohm Celsius. It's a linear
0065 approximation of the resistance versus temperature relationship
0066 between 0 and 100 degrees Celsius.
0067
0068 alpha = (R_100 - R_0) / (100 * R_0)
0069
0070 Where, R_100 is the resistance of the sensor at 100 degrees Celsius, and
0071 R_0 (or r-naught-ohms) is the resistance of the sensor at 0 degrees
0072 Celsius.
0073
0074 Pure platinum has an alpha of 3925. Industry standards such as IEC60751
0075 and ASTM E-1137 specify an alpha of 3850.
0076
0077 r-naught-ohms:
0078 description: |
0079 Resistance of the sensor at 0 degrees Celsius.
0080 Common values are 100 for PT100, 500 for PT500, and 1000 for PT1000
0081
0082 additionalProperties: false
0083 required:
0084 - compatible
0085 - io-channels
0086 - excitation-current-microamp
0087 - alpha-ppm-per-celsius
0088 - r-naught-ohms
0089
0090 examples:
0091 - |
0092 pt1000_1: temperature-sensor0 {
0093 compatible = "temperature-sense-rtd";
0094 #io-channel-cells = <0>;
0095 io-channels = <&temp_adc1 0>;
0096
0097 excitation-current-microamp = <1000>; /* i = U/R = 5 / 5000 */
0098 alpha-ppm-per-celsius = <3908>;
0099 r-naught-ohms = <1000>;
0100 };
0101 ...
