Thermocouple & RTD Sensor Interface IC Guide: MAX6675 vs MAX31855 vs ADS1220

Temperature acquisition looks simple until the sensor signal has to become reliable digital data.

Industrial ovens, injection molding machines, HVAC equipment and process-control systems commonly use K-type thermocouples or PT100/PT1000 RTDs. The problem is that these sensors do not behave like ordinary voltage-output sensors. A thermocouple produces a very small voltage and requirescold-junction compensation (CJC). An RTD is a resistance-based sensor and normally needs excitation, accurate measurement and compensation for lead resistance.

This is why choosing athermocouple converter ICor anSPI RTD AFE ICshould start with the sensor type and measurement architecture, not simply with ADC resolution.

Key Takeaways

TheMAX6675is a dedicated K-type thermocouple-to-digital converter with integrated cold-junction compensation, a 12-bit output, 0.25°C resolution and an 8-pin SO package. ADI currently lists the MAX6675 asPRODUCTION, so its lifecycle should not be described as NRND without checking the exact ordering code.

TheMAX31855adds 14-bit signed output, 0.25°C resolution, cold-junction compensation and thermocouple fault detection. The K-typeMAX31855KASA+is specified for -200°C to +1350°C and uses an 8-pin SO package.

TheADS1220is fundamentally different. It is a 24-bit delta-sigma ADC with PGA, programmable excitation current sources, internal reference and SPI, designed to measure small signals from RTDs, thermocouples, thermistors and bridges. It is offered in 16-pin TSSOP and VQFN packages.

Basic Comparison

Parameter	MAX6675ISA+	MAX31855KASA+	ADS1220IPW / ADS1220IRVA
Main function	K-type thermocouple converter	Thermocouple-to-digital converter	Precision multi-purpose ADC / AFE
Sensor	K-type thermocouple	Multiple thermocouple versions	RTD, thermocouple, thermistor, bridge
Resolution	12-bit	14-bit	24-bit ADC, up to 20-bit effective
Resolution step	0.25°C	0.25°C	Depends on configuration
Cold-junction compensation	Integrated	Integrated	Requires measurement architecture
Interface	SPI-compatible, read-only	SPI-compatible, read-only	SPI
PGA	No	No	1 to 128
Excitation current	No	No	10 µA to 1.5 mA programmable
Package	8-pin SO	8-pin SO	TSSOP-16 / VQFN-16
Main application	Simple K-type measurement	Wider-range thermocouple systems	Flexible RTD/TC precision AFE

Specifications are based on current manufacturer information and exact ordering codes should be verified before substitution.

1. MAX6675 vs MAX31855: Is MAX31855 a Direct Upgrade?

This is probably the first question engineers ask.

At the functional level, the answer isyes in some designs, but not as a universal drop-in replacement.

The MAX6675 is designed specifically for K-type thermocouples and measures from 0°C to +1024°C. It provides cold-junction compensation and a simple SPI-compatible read-only interface.

The MAX31855 supports K-, J-, N-, T-, S-, R- and E-type versions and provides a wider measurement capability. The datasheet specifies 14-bit output with 0.25°C resolution and includes open-thermocouple and short-to-ground/VCC detection.

For a K-type design, theMAX31855KASA+is an attractive architectural alternative when the wider range or fault diagnostics are useful.

But engineers should not stop at "both are 8-pin SO."

The firmware must also be checked. Both devices use SPI-compatible interfaces, but the data format and status information are not identical. A MAX6675 driver should therefore not simply be copied to a MAX31855 without reviewing the register/frame interpretation.

That is the difference between afunctional replacementand a truedrop-in replacement.

2. What Does Cold-Junction Compensation Actually Do?

Thermocouples do not directly measure absolute temperature.

They measure a voltage related to the temperature difference between the measurement junction and the reference junction. In a PCB-based temperature acquisition circuit, the IC therefore needs to know the temperature around the thermocouple connection point.

This is the reason CJC matters.

The MAX6675 and MAX31855 integrate this function. The engineer does not need to build a separate precision temperature-sensing chain simply to compensate the connector junction.

But there is a practical question:

Can an IC with higher ADC resolution automatically provide better temperature accuracy?

No.

Thermocouple accuracy also depends on sensor tolerance, CJC accuracy, PCB thermal gradients, connector construction, noise, calibration and the signal path.

A 24-bit ADC does not magically turn an ordinary thermocouple into a laboratory-grade thermometer.

3. Where ADS1220 Changes the Design Approach

TheADS1220is not simply another MAX6675-style converter.

TI describes it as a 24-bit, 2-kSPS, four-channel delta-sigma ADC with an integrated PGA, reference, SPI interface and two programmable excitation current sources. The PGA can be programmed from 1 to 128, and the IDACs can be programmed from 10 µA to 1.5 mA.

This makes the ADS1220 particularly interesting forPT100 RTD sensor sourcing.

A PT100 does not produce a thermocouple-style voltage. Its resistance changes with temperature. The measurement circuit therefore needs a known current or voltage reference and a precision ADC to determine the resistance.

The ADS1220 integrates many of the building blocks needed for that architecture.

TI also provides a reference design demonstrating thermocouple measurement using the ADS1220 together with an RTD for cold-junction compensation. In that design, a Pt100 is placed near the thermocouple connector and measured using the ADS1220's excitation current sources.

This is an important distinction:

MAX6675/MAX31855 simplify thermocouple measurement. ADS1220 gives the engineer more freedom to design the measurement chain.

4. Why ADS1220 Is Not a Pin-to-Pin MAX6675 Alternative

This is where procurement teams need to be careful.

A buyer searching for aMAX6675 alternativemay find ADS1220 because both can be used in temperature measurement.

Technically, however, these are very different replacement categories.

The MAX6675 is an 8-pin thermocouple converter.

The ADS1220 is a 16-pin precision ADC available in TSSOP-16 or VQFN-16.

So ADS1220 is afunctional architecture alternative, not a PCB drop-in replacement.

Moving from MAX6675 to ADS1220 normally means reviewing:

· Sensor interface circuitry

· CJC method

· Reference design

· Excitation-current configuration

· SPI firmware

· PCB footprint

· Analog filtering

· Calibration procedure

That may be completely reasonable for a new design, but it is not the same job as replacing one 8-pin device with another.

5. What About a PT100 and PT1000 Design?

For a simple K-type thermocouple product, a dedicated converter can reduce development effort.

For an industrial controller that needs several sensor types, the situation changes.

A system may need:

PT100 + PT1000 + thermocouple + thermistor

In that case, a flexible ADC/AFE architecture can potentially reduce the number of different front-end designs.

ADS1220 supports RTDs, thermocouples, thermistors and bridge sensors, while its integrated PGA and current sources reduce the number of external building blocks required.

For an industrial temperature transmitter, this flexibility can be valuable.

But there is a trade-off: software, calibration and analog design become more complicated.

So I would ask one question before choosing the IC:

Does the product need one sensor type, or does it need a reusable multi-sensor measurement platform?

For the first situation, a dedicated converter may be simpler. For the second, a precision ADC/AFE can make more engineering sense.

6. Package Compatibility Is a Procurement Issue, Not Just a PCB Issue

A common search phrase is:

SOIC-8 K-type thermocouple IC in stock

This usually indicates that the buyer is trying to preserve an existing PCB footprint.

The MAX6675 and MAX31855 K-type versions both use 8-pin SO packages, which makes package-level comparison easier.

ADS1220 is different because it uses 16-pin TSSOP or VQFN packages.

Therefore, an RFQ should specify the complete part number and package rather than just saying:

"Need a 24-bit temperature ADC."

For production replacement, I would include:

RFQ Requirement	Example
Original MPN	MAX6675ISA+
Sensor	K-type thermocouple
Interface	SPI
Package	8-pin SO
Temperature range	Required application range
CJC	Required
Quantity	Sample / production
Alternative	Pin-compatible or functional only
Traceability	Lot/date code requirement

This prevents a supplier from proposing a technically capable part that cannot actually be assembled onto the customer's PCB.

7. What Should Be Checked Before Approving a Replacement?

ForMAX6675 vs MAX31855 cold junction compensation comparison, I would use five checks:

Sensor compatibility:K-type, J-type, RTD, PT100 or PT1000.

Temperature range:The application's real operating range must remain inside the converter's specified range.

Accuracy and resolution:Do not compare resolution alone. Check actual accuracy under the required temperature range.

Digital interface:Verify SPI mode, data frame, fault bits and firmware requirements.

Mechanical compatibility:Check package, pinout and PCB land pattern.

For ADS1220, add another layer: verify the RTD connection method, IDAC value, reference resistor and PGA configuration.

Conclusion

MAX6675, MAX31855 and ADS1220 can all appear in the same industrial temperature acquisition project, but they solve different levels of the problem.

MAX6675remains a straightforward K-type thermocouple converter with integrated cold-junction compensation and an 8-pin SO package. Importantly, ADI currently lists the exact MAX6675ISA+ and MAX6675ISA+T models asPRODUCTION, so its lifecycle should be checked by exact MPN rather than described broadly as obsolete.

MAX31855provides a wider thermocouple family, 14-bit output, 0.25°C resolution and additional fault detection, making it a useful alternative where those functions are required.

ADS1220is a different class of solution: a flexible 24-bit ADC/AFE with PGA, excitation current sources and multiple inputs, making it suitable for PT100/PT1000 RTDs as well as thermocouple and other low-level sensor measurements.

Forthermocouple converter ICsourcing, the safest workflow is therefore:

Sensor type → temperature range → CJC → accuracy → interface → package → pinout → firmware → exact MPN availability.

For an urgent requirement such asindustrial temperature transmitter IC fast dispatchorSOIC-8 K-type thermocouple IC in stock, the exact manufacturer part number should be included in the RFQ. For a new multi-sensor design, ADS1220 can be evaluated as a functional AFE platform rather than treated as a simple MAX6675 replacement.

This distinction is what separates a realtemperature-interface cross referencefrom a list of parts that merely contain the word "temperature" in their specifications.