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SOP-8 / TSSOP-14 · MAX31855
ActiveThermocouple-to-Digital Converter (SPI)
MAX31855
Maxim Integrated · SOP-8 / TSSOP-14 · Thermocouple-to-Digital Converter (SPI)
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Overview
The MAX31855 is Maxim's cold-junction-compensated thermocouple-to-digital converter, and it removed almost all of the analogue pain from thermocouple measurement. A single part performs the amplification of the microvolt-level thermoelectric signal, measures the temperature of the junction where the thermocouple meets the PCB, applies the linearisation table for the selected thermocouple type, and outputs the result as a signed 14-bit word over a read-only SPI-compatible interface. No external amplifier, no cold-junction thermistor, no lookup table in firmware and no analogue layout headaches — just the thermocouple, a 0.1 µF supply bypass and a recommended 10 nF differential capacitor across T+ and T-.
The device is offered in versions for K, J, N, T, S, R and E thermocouples, selected by the suffix on the part number. Temperature resolution is 0.25 °C (14 bits), the reading range extends from -270 °C to +1800 °C, and K-type accuracy is ±2 °C from -200 °C to +700 °C, widening to ±4 °C from +700 °C to +1350 °C and ±6 °C across the full range. The internal cold-junction sensor resolves 0.0625 °C (12 bits) and holds ±2 °C from -20 °C to +85 °C, or ±3 °C across the -40 °C to +125 °C ambient range; those figures exclude thermocouple nonlinearity. Conversion takes 70 ms typical and 100 ms maximum, the SPI clock runs to 5 MHz, and fault-detection bits flag an open thermocouple, a short to VCC or a short to GND — which turns a broken probe into a diagnostic message instead of a silent wrong answer. Power and packaging are straightforward: VCC is 3.0 V to 3.6 V with 900 µA typical and 1.5 mA maximum supply current, operating ambient is -40 °C to +125 °C, thermocouple input bias current is under ±100 nA, and the device comes in an 8-pin SO package with 170 °C/W junction-to-ambient thermal resistance. Because the interface is read-only, the host simply toggles CS and clocks out 32 bits: 14 bits of thermocouple temperature, 12 bits of internal temperature and a fault bit plus three fault-reason bits.
Two practical design points matter more than the headline numbers. First, cold-junction accuracy is board-level accuracy: the MAX31855 measures the temperature of its own package, so a warm voltage regulator, a sunny enclosure or self-heating from the SPI bus will bias every reading. Keep the converter thermally coupled to the connector block and away from heat sources, and add a second temperature reading if your application is metrology-grade. Second, noise: thermocouple leads are high-impedance microvolt sources acting as antennas. Twist the leads, keep them short, route them away from switching supplies and motor cables, and fit the 10 nF differential capacitor the datasheet recommends.
Sourcing note: the MAX31855 is a Maxim design now owned by Analog Devices, and the low part count plus the popularity of the breakout boards make it a clone magnet. Counterfeit and relabelled parts typically show up as a fixed or all-zero SPI word, missing fault flags, wrong linearisation for the claimed thermocouple type, or a device that reports a plausible temperature but no cold-junction field. Verify the SPI word structure bit by bit against the datasheet, check that the fault bit asserts when you disconnect the probe, and confirm the internal temperature tracks a reference within ±2 °C. MOQ is 1 reel (2,500 pcs SO-8) or 50 pcs loose, shipping in 1-3 days from Shenzhen.
Key Features
- Cold-junction compensation and linearisation on-chip — no analogue front end needed
- 14-bit, 0.25 °C resolution from -270 °C to +1800 °C
- ±2 °C accuracy for K-type from -200 °C to +700 °C
- Detects open thermocouple and shorts to GND or VCC
- Read-only SPI at up to 5 MHz from a 3.0 V to 3.6 V supply
Typical Applications
- Industrial furnace, kiln, reflow oven and 3D printer hotend control
- HVAC, appliance and process temperature monitoring above 300 °C
- Engine, exhaust and automotive test-cell thermocouple acquisition
- Laboratory and calibration equipment with K, J, T or E probes
- Food processing, brewing and brewing-kettle temperature logging
Specifications
- Thermocouple Types
- K, J, N, T, S, R, E (suffix selected)
- Temperature Range (K-Type)
- -270 to 1800 °C
- Thermocouple Resolution
- 14-bit (0.25 °C)
- Thermocouple Accuracy (K-Type)
- ±2 (-200 to 700 °C) °C
- Cold-Junction Accuracy
- ±2 (-20 to 85 °C) °C
- Digital Interface
- SPI, Read-Only (5 MHz Max Clock)
- Supply Voltage
- 3.0 to 3.6 V
- Operating Temperature
- -40 to 125 °C
- Pins
- 8
- RoHS
- Compliant
Procurement Tips
Always specify the thermocouple-type suffix explicitly on the BOM and purchase order — MAX31855KASA+ and MAX31855JASA+ look identical on a reel label but produce completely different readings from the same probe.
Frequently Asked Questions
What is the difference between the MAX31855 and the MAX6675?
The MAX6675 is the older, simpler part: K-type only, 12-bit resolution (0.25 °C), 0 °C to +1024 °C range and no cold-junction fault reporting beyond an open-thermocouple flag. The MAX31855 keeps the same 0.25 °C thermocouple resolution but extends to 14 bits signed, covers -270 °C to +1800 °C, supports seven thermocouple types via suffix, adds a 12-bit internal cold-junction readout at 0.0625 °C resolution and reports shorts to VCC or GND. Use the MAX6675 only for legacy designs; new builds should use the MAX31855 or MAX31856.
Should I use the MAX31855 or the MAX31856?
The MAX31856 is the more capable successor: 19-bit thermocouple resolution (0.03 °C), programmable mains-frequency rejection at 50 Hz or 60 Hz, selectable thermocouple types via register rather than part suffix, support for a voltage-input mode so you can drive it with other sensors, and on-chip linearisation tables. It costs more and needs a full SPI read/write driver instead of the MAX31855's simple read-only word. Choose the MAX31855 for cost-sensitive K-type measurements in the -200 °C to +700 °C window, and the MAX31856 for precision, multi-type or mains-noisy environments.
Which suffix do I order for my thermocouple type?
The letter after MAX31855 selects the linearisation: K, J, N, T, S, R or E. The common industrial order code for a K-type in the 8-pin SO package is MAX31855KASA+ (lead-free), with JASA+, NASA+, TASA+, SASA+, RASA+ and EASA+ covering the other types. The suffix is fixed at the factory — you cannot change thermocouple type in firmware, unlike the MAX31856. Confirm the type printed on your probe before ordering, since a K-type converter fed a J-type probe will read tens of degrees wrong at temperature.
How do I decode the 32-bit SPI word?
Clock out 32 bits with CS low, SCLK idle low (SPI mode 0) at up to 5 MHz. Bits D31 to D18 are the signed 14-bit thermocouple temperature in 0.25 °C steps, D16 is the fault flag, D15 to D4 are the signed 12-bit internal cold-junction temperature in 0.0625 °C steps, and D2, D1, D0 are the short-to-VCC, short-to-GND and open-thermocouple indicators. Shift right by 18 and sign-extend from 14 bits for the hot junction, shift right by 4 and mask 12 bits for the cold junction. D17 is reserved and should be ignored.
Why does my reading look noisy or drift with board temperature?
Two causes. Noise: the thermocouple is a high-impedance microvolt source, so twist the probe leads, keep them away from switching supplies and motor wiring, fit the 10 nF differential capacitor across T+ and T-, and keep input leakage paths clean — the part only draws ±100 nA of bias current, so contamination on the PCB is enough to shift readings. Drift: cold-junction compensation is referenced to the MAX31855's own die temperature, so anything that heats the package — a nearby regulator, a sealed enclosure in the sun, continuous SPI traffic — biases the result. Thermally couple the converter to the connector block and away from heat sources.
Can I use the fault detection to protect a process?
Yes, and you should. The open-thermocouple bit detects a broken or disconnected probe, and the short-to-VCC and short-to-GND bits detect a probe shorted to the chassis or supply. Poll these bits every conversion and drive the process to a safe state when they assert, rather than trusting the temperature value alone — a broken thermocouple can otherwise read as a plausible low temperature and let a heater run away. Remember that conversion takes up to 100 ms, so your watchdog timing should allow at least two conversions before declaring a fault persistent.
How do I detect counterfeit or relabelled MAX31855 parts?
Test the SPI word structure, not just the temperature output. A genuine device returns a 12-bit cold-junction field that tracks ambient within ±2 °C, asserts the fault bit when you disconnect the thermocouple, and returns the correct short-to-VCC or short-to-GND flags when you jumper T+ to the relevant rail. Clones often return a fixed value, all zeros, or a plausible temperature with a dead cold-junction field. Physically check laser marking, SO-8 lead finish and date-code traceability to Maxim/ADI. We screen every reel against the datasheet fault behaviour before shipment.
What is the MOQ and lead time?
MOQ is 1 reel (2,500 pcs, 8-pin SO) or 50 pcs loose for prototyping, and breakout modules are available in packs of 10. Shenzhen stock ships in 1-3 business days; the ADI authorized channel is typically 10-16 weeks. Specify the exact suffix (for example MAX31855KASA+) on the purchase order so a J-type or T-type part is not substituted.
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