Overview

The MAX31865 is a high-precision 15-bit resistance-to-digital converter manufactured by Maxim Integrated (Analog Devices). Designed specifically to interface with Platinum Resistance Temperature Detectors (PT100 and PT1000 RTDs), it replaces complex analog instrumentation op-amp circuits with a single digital SPI IC.

Supporting 2-wire, 3-wire, and 4-wire RTD configurations, the MAX31865 achieves an ultra-fine resolution of $0.03125^\circ\text{C}$ ($0.0009765625\ \Omega$ per LSB) across extreme temperature ranges ($-200^\circ\text{C}$ to $+850^\circ\text{C}$). It incorporates built-in fault detection (detecting open RTD elements, short circuits to ground/$V_{CC}$, and over-range temperatures).

Quick reference

Pinout

Breakout module header & Terminal Block:

Specifications

RTD Resistance & Temperature Calculation Math

1. Calculate RTD Resistance ($R_{RTD}$):

$R_{RTD} = \left( \frac{\text{15-bit Raw ADC Register Value}}{32768} \right) \times R_{REF}$

2. Callendar-Van Dusen Equation ($T \ge 0^\circ\text{C}$):

$R_{RTD}(T) = R_0 \cdot \left( 1 + A \cdot T + B \cdot T^2 \right)$

Where $A = 3.9083 \times 10^{-3}\ ^\circ\text{C}^{-1}$ and $B = -5.775 \times 10^{-7}\ ^\circ\text{C}^{-2}$.

$T (^\circ\text{C}) = \frac{-A + \sqrt{A^2 - 4B \left( 1 - \frac{R_{RTD}}{R_0} \right)}}{2B}$

Wiring (3-Wire PT100 Probe)

Example (Adafruit_MAX31865 Library)

#include <Adafruit_MAX31865.h>

// Use hardware SPI with CS pin 10
Adafruit_MAX31865 thermo = Adafruit_MAX31865(10);

// Set Reference Resistor (430 ohms for PT100)
#define RREF 430.0
// Set Nominal 0°C Resistance (100 ohms for PT100)
#define RNOMINAL 100.0

void setup() {
  Serial.begin(115200);
  Serial.println("Adafruit MAX31865 PT100 RTD Test");

  thermo.begin(MAX31865_3WIRE); // Configured for 3-wire PT100
}

void loop() {
  uint16_t rtd = thermo.readRTD();
  float ratio = rtd / 32768.0;
  float resistance = RREF * ratio;
  float temp = thermo.temperature(RNOMINAL, RREF);

  Serial.print("RTD Raw ADC: "); Serial.print(rtd);
  Serial.print(" | Resistance: "); Serial.print(resistance); Serial.print(" Ω");
  Serial.print(" | Temperature: "); Serial.print(temp); Serial.println(" °C");

  // Check for faults
  uint8_t fault = thermo.readFault();
  if (fault) {
    Serial.print("Fault 0x"); Serial.println(fault, HEX);
    thermo.clearFault();
  }

  delay(1000);
}

Common mistakes

• Mismatched $R_{REF}$ value in software: Standard Adafruit/generic breakout boards use a $430\ \Omega$ reference resistor ($R_{REF}=430.0$) for PT100. Setting $R_{REF}=400.0$ in code introduces a $+7^\circ\text{C}$ measurement error.
• Selecting wrong wire jumpers: Failing to configure 2-wire / 3-wire PCB solder jumpers to match the connected physical PT100 probe triggers constant RTD INLOW or RTD INHIGH fault flags.

Notes

• MAX31865 vs MAX31855: MAX31865 interfaces with RTDs (PT100/PT1000); MAX31855 interfaces with Thermocouples (K-Type).