MAX31856MUD+T: A Technical Analysis of High-Precision Thermocouple-to-Digital Converter

In industrial temperature monitoring, thermocouples are widely used due to their wide temperature range and rapid response characteristics. However, their nonlinear output and cold-junction compensation requirements have long posed challenges for system designers. Analog Devices Inc (formerly Maxim Integrated)'s MAX31856MUD+T chip addresses these issues by integrating cold-junction compensation, linearization correction, and digital output functionality, offering a high-precision, easy-to-integrate solution for industrial temperature measurement.

Core Features and Technical Specifications
The MAX31856MUD+T is a 14-pin TSSOP-packaged thermocouple-to-digital converter supporting eight thermocouple types: K, J, N, R, S, T, E, and B. Key specifications are summarized in the table below:

Parameter	Specification
Temperature Resolution	19 bits, 0.0078125°C
Measurement Range	-210°C to +1,800°C (varies by type)
Voltage Accuracy	±0.15% (-20°C to +85°C)
Cold-Junction Accuracy	±0.7°C (-20°C to +85°C)
Input Protection	±45V overvoltage protection
Interface	SPI-compatible
Operating Temperature	-55°C to +125°C
Package Size	4.4mm × 5mm (TSSOP-14)

The chip eliminates thermocouple nonlinearity errors through internal lookup tables (LUTs) for automatic linearization correction. It integrates 50Hz/60Hz filters to suppress power grid noise and supports open-circuit detection and over/under-temperature fault alarms, significantly simplifying system fault management.

Applications and Typical Circuit
The MAX31856MUD+T is widely used in industrial ovens, environmental test chambers, and temperature controllers. For K-type thermocouple measurements, a typical application circuit includes:

Thermocouple Connection: The T+ and T- pins connect to the positive and negative terminals of the K-type thermocouple, with automatic cold-junction compensation performed internally.
Power and Filtering: AVDD and DVDD pins are connected to a 3.3V power supply, with 0.1μF decoupling capacitors added to reduce noise.
SPI Communication: The SDI (data input), SDO (data output), SCK (clock), and CS (chip select) pins interface with a microcontroller to configure the thermocouple type and read temperature data.
Fault Handling: If the chip detects an open circuit or over/under-voltage condition, the FAULT pin outputs a low signal, and the microcontroller can read the fault status register via SPI to identify the specific error.
Common Issues and Solutions
Q1: How to handle OVUV (overvoltage/undervoltage) errors?
OVUV errors may occur when the thermocouple measuring point contacts heating elements, causing electromagnetic interference or voltage fluctuations. Solutions include:

Connecting a 1MΩ resistor between T- (or T+) and GND to provide stable bias.
Checking for loose thermocouple connections to avoid voltage abnormalities.
Ensuring the power supply voltage remains stable within 3.3V ±10%.
Q2: How to improve cold-junction compensation accuracy?
The chip's cold-junction compensation relies on an internal temperature sensor. For environments with significant temperature fluctuations:

Mount the chip close to the thermocouple cold junction to minimize temperature gradients.
Provide independent heat dissipation paths on the PCB to avoid interference from heat-generating components.
Read the cold-junction temperature via SPI and apply secondary correction algorithms in the microcontroller.
Q3: Does the chip support multi-channel expansion?
The MAX31856MUD+T is a single-channel device. For multi-channel measurements:

Use multiple chips in parallel, each with independent SPI addressing (if supported).
Consider Analog Devices' multi-channel thermocouple converters (e.g., MAX31865 series) for simplified system design.
Market Value and Selection Recommendations
The MAX31856MUD+T stands out in industrial automation due to its high precision, integration, and wide temperature range. Its 14-pin TSSOP package suits space-constrained PCB designs, while ±45V input protection and a 125°C maximum operating temperature meet demanding industrial requirements. For cost-sensitive applications, alternatives like the MAX31850 (a simplified version supporting K-type thermocouples) may be considered, but trade-offs in functionality and accuracy must be evaluated.

As a core component for industrial temperature measurement, the MAX31856MUD+T reduces system complexity through integrated design. Its SPI interface and fault detection features enhance data reliability and maintenance efficiency, making it an ideal choice for industrial ovens, power equipment, and environmental monitoring systems.

Fudong Communication (Shenzhen) Group Co., Ltd., established in 2004, is a specialized global first tier semiconductor agent/distributor.

Fudong Mall is an online e-commerce platform belonging to Fudong Communication (Shenzhen) Group Co., Ltd. Fudong collaborates with global electronic component distributors and Chinese spot inventory suppliers.