MAX31856 | GUILCOR SENSORS

MAX31856
Temperature sensors

Advanced thermocouple interface with high accuracy, fault detection, and wide thermocouple compatibility for industrial applications.

Maximum precision
+/- 0,5°K

Minimum temperature
-270°C

Maximum temperature
+1800°C

Minimum dimensions
32 x 15 x 5

Response time

Fast

Drift

Low

Self-warming
Low

Price
High

What is a MAX31856 sensor ?

The MAX31856 is the most comprehensive thermocouple converter in the MAX series.

It allows for reading all standardized types (K, J, N, R, S, T, E, B) via a 4-wire SPI interface, while automatically managing:

cold junction compensation,
19-bit ADC conversion,
and connection or short-circuit error detection.

This module is now a reference in industrial, scientific, and precision thermal control systems.

Operating principle

The MAX31856 measures the voltage generated by the thermocouple (a few µV/°C), converts it via a very low-noise 19-bit ADC, and applies a digital conversion according to the ITS-90 coefficients of the selected thermocouple type.

It integrates:

precise cold junction compensation (internal sensor ±0.5 °C),
digital filtering of 50/60 Hz noise,
and a programmable alarm system for threshold exceedances.

Technical Specifications

Parameter	Typical value
Compatible thermocouple type	K, J, N, R, S, T, E, B
Maximum measurement range.	−270 → +1800 °C
Resolution	19 bits (0,0078 °C)
Precision	±0,5 K (type K, 0–700 °C)
Interface	SPI (read/write)
SPI Frequency	Up to 5 MHz
Power supply voltage	3,0 → 3,6 V
Typical current	1.5 mA typical
Cold welding compensation	Integrated, 14-bit internal sensor
Cold welding compensation	Integrated, 14-bit internal sensor
Case	TQFN-20 / module breakout
Annual drift	Low (±0.1 K/year)

Wiring Configuration

Brooch	Name	Function
1	T-	Negative thermocouple input
2	T+	Positive thermocouple input
3	VCC	Power Supply (3.3 V)
4	GND	Mass
5	SCK	SPI Clock
6	CS	Chip Select
7	SDO	SPI Output (MISO)
8	SDI	SPI Input (MOSI)
9	DRDY	(optional) Data ready signal
10	FAULT	Configurable alarm signal

Self-warming

Almost zero (< 0.05 °C) thanks to ultra-low current ADC conversion.

Suitable for sensitive applications or precision measurement chains

Application areas

⚙️ High-temperature industrial processes (metallurgy, glassmaking, extrusion)

🔬 Laboratory instrumentation and thermal test benches

🧠 Precise control in 3D printing, ovens, and drying chambers

💡 Multi-type measurement systems (K, J, T, R...)

🧱 Embedded systems requiring high precision and integrated diagnostics

Should I choose a MAX31856 sensor ?

Strengths points

🌡️ Universal multi-thermocouple compatibility
→ The MAX31856 supports all standardized types (K, J, T, N, E, R, S, B). A single module for all temperature ranges, from cryogenic to +1800 °C, a true thermal chameleon.
🎯 Unmatched precision and resolution
→ With its 19-bit ADC and cold solder joint compensation of ±0.5 °C, it achieves a typical accuracy of ±0.5 K.
It is the ideal choice for test benches, high-demand industrial processes, or thermal R&D.
⚙️ Integrated diagnostics and filtering
→ Automatic error detection (short circuit, disconnected thermocouple), configurable 50/60 Hz digital filtering, and FAULT alarm output for intelligent real-time monitoring.

Weaknesses points

💰 Higher cost
→ Its versatility and precision come at a price: the MAX31856 costs 2 to 3 times more than a MAX6675 or 31855, reserved for professional or critical systems.
🔋 Power supply 3.3 V only
→ Like the MAX31855, it requires a level shifter with 5 V microcontrollers (Arduino Uno, Mega, etc.).
🧩 More complex SPI configuration
→ Multiple registers (configuration, filters, alarms, offset) require more advanced software initialization, even though libraries exist (Adafruit, STM32 HAL, etc.).

Useful information

Here is some useful information regarding the MAX31856 sensors.

Resolution and precision

The MAX31856 features a 19-bit analog-to-digital converter, a 14-bit cold junction sensor, and digital correction based on ITS-90 coefficients.

It thus offers exceptional accuracy, even in electrically noisy environments.

Parameter	Valeur typique
Thermocouple resolution	19 bits (~0,0078 °C / LSB)
Cold welding resolution	14 bits (~0,0156 °C / LSB)
Precision	±0,5 K (type K, 0 → +700 °C)
Measurement range	−270 → +1800 °F (depending on type)
Annual drift	±0,1 K
Conversion time	100 → 200 ms
Power supply voltage	3,0 → 3,6 V
Max. SPI Frequency	5 MHz
Digital filters	Selectable 50 / 60 Hz
Cold solder joint	±0.5 °C typical

🔹 Remarks:

Internal automatic calibration, no external correction required.
Compatible with all ITS-90 thermocouples.
Exceptional stability due to multi-channel digital compensation.

Example of application – Digital reading SPI

The MAX31856 operates in SPI (4-wire) read/write mode.

Each transaction reads a 4-byte (32-bit) word, containing the thermocouple temperature, internal temperature, and status codes.

🔹 SPI Read Structure (32 bits)

Bits	Name	Description
[31 → 13]	Thermocouple temperature (19 signed bits)	Value × 0.0078125 °C
[12 → 4]	Cold soldering temperature (9 bits)	Value × 0.015625 °C
[3]	FAULT	1 = error detected
[2]	OC	Open thermocouple
[1]	SCG	Short circuit to GND
[0]	SCV	Short circuit to VCC

🔹 Conversion Equations

Thermocouple Temperature:

T_TC(°C) = Value_19b x 0,0078125

Cold weld temperature:

T_CJ(°C) = Value_14b x 0,015625

Final temperature:

TTOTAL = TTC + TCJ

🔹 Practical example

SPI reading (4 bytes) → 0x0010A4C0

Decoding:

Thermocouple temperature: 0x0010A = 426₁₀
Cold weld temperature: 0x4C = 76₁₀

✅ Measured temperature = 4.51 °C

T_TC = 426 × 0,0078125 = 3,32 °C

T_CJ = 76 × 0,015625 = 1,19 °C

T_TOTAL = 4,51 °C

🔹 Automatic error codes

OC = 1 → open thermocouple
SCV = 1 → short circuit to VCC
SCG = 1 → short circuit to GND
FAULT = 1 → error detected (check wiring or type)

Integration into an electronic system

The MAX31856 connects to any SPI-compatible microcontroller (Arduino, ESP32, STM32, Raspberry Pi, etc.).

It requires 4 SPI wires + power and supports software configuration (type of thermocouple, filter, offset, alarms).

🔹 Typical components

Composant	Function
MAX31856	Universal thermocouple converter SPI
Thermocouple (K, J, T, N, R, S, E, B)	Temperature sensor
Microcontroller	SPI management and data processing
Logic level converter (5V → 3.3V)	Optional
3.3 V Power Supply	Source stable
Capacitor (100 nF)	Power filtering
Pull-up resistor (10 kΩ)	Alarm Lines FAULT / DRDY (optional)

🔹 Functional diagram (ASCII)

+3.3V │ [Thermocouple] T+ ───┐ │ [MAX31856] │ T− ───┘ │ GND ───┴───────────────┐ │ ┌────────────────────┴────────────────────┐ │ Microcontroller │ │────────────────────────────────────────│ │ CS → GPIO D5 (chip select) │ │ SCK → GPIO D6 (SPI clock) │ │ SDI → GPIO D7 (MOSI) │ │ SDO → GPIO D8 (MISO) │ │ DRDY → GPIO D9 (data ready, optional) │ │ FAULT→ GPIO D10 (alarm, optional) │ │ VCC → 3.3V │ │ GND → GND │ └────────────────────────────────────────┘

🔹 Operating Principle

1️⃣ The microcontroller initializes the type of thermocouple (CONFIG0 register).

2️⃣ The MAX31856 measures the thermocouple voltage + internal temperature.

3️⃣ An SPI read returns the 32 bits of data + any error codes.

4️⃣ The system can trigger an alarm via the FAULT output if the thresholds are exceeded.

Practical advantage

Advantage	Description
🌡️ Universality	Compatible with 8 different types of thermocouples
⚙️ Extreme precision and stability	19-bit resolution, drift ±0.1 K/year
🔧 Configurable digital filtering	Eliminate 50/60 Hz noise
🔥 Automatic cold-junction compensation	High-precision internal correction
🧠 Integrated diagnostics	Automatically detected OC, SCV, SCG errors
🧩 Programmable alarms	High/low thresholds, configurable FAULT signal
💻 Bidirectional SPI Interface	Lecture + complete configuration from MCU