Designing for Thermocouples and IC Temperature Sensors
Bruce Schreiner posted on June 27, 2014 |
Simplify designs with thermocouple circuit filtering and analog IC temperature sensors.

Texas Instruments has sponsored the following story


A common issue with various temperature sensors is that many require filtering, circuitry, calibration, and conversion tables to determine the temperature results. Texas Instruments (TI) has come up with analog sensors and thermocouple circuit filtering IC solutions to address these issues.

 

Analog IC Temperature Sensors
Essentially, TI is expanding their current analog temperature sensing LMT series with 7 new devices. I predict that these devices will maintain TI's innovation and dominance in the area of temperature sensing. 

The LMT devices are designed to become essential parts of protection, control, and calibration circuits in automotive and industrial applications. The output from these sensors will have accuracy and repeatability without the need for compensating circuitry.  This will make the LMT series temperature ICs excellent choices to replace NTC thermistors as explained in this comparison video from TI engineers.

The LMT devices provide a linear output over a wide measuring range (-50°C to +150°C) that will simplify your microcontroller embedded support software.  This translates to less board space for the implementation of monitoring and control circuits. Furthermore, dropping the look-up tables in the software design translates to cost savings for high volume applications. Additionally, the LMT series is valued for performance, high accuracy, and lower power draw.

The LMT8x-90 devices include evaluation modules (LMT84-7EVM/LMT88-9EVM/LMT90 EVM) and documentation to make an easy integration, testing and implementation into your new designs.  Finally, the selection between devices is made simple through the online analog temperature sensor application. However, if you are offline, a product selection chart can also be used.

 

Integration of Thermocouples
Thermocouples are the most expensive temperature measurement device. They are used in extreme environments such as high temperature ranges that would otherwise melt your analog temperature ICs.  However, unlike analog temperature ICs, thermocouples require support circuitry. If your application requires the use of thermocouple-based measuring sensors, then TI has a range of circuit filtering solution ICs.

Cold junction compensation is a requirement for temperature determination given the differential sensing nature of the thermocouple circuit. In other words, thermocouples cannot measure absolute temperatures, only temperature difference using voltage. Therefore, a temperature monitor is needed at the cold end of the thermocouple to determine an absolute temperature.

The LM94022 reference junction devices provides the function of cold end temperature determination.  Additionally, the LMP7715 operational amplifier eliminates common mode pick-up from long thermocouple cables, another common design challenge to thermocouples.

Due to the differential measurement setup of thermocouples, they often experience issues working at near ambient temperatures. Therefore, the LM7705 negative bias generator allows for improved measurement of voltages when a thermocouple is operating near system ambient temperature and the resulting thermocouple voltage is close to 0V.

Similarly, the LMP7716 provides a negative buffer and circuit amplification to measure below system temperature voltages.

Conclusion
If your design requires temperature protection, control or calibration then I suggest analog temperature IC's from TI to help to reduce board space and simplify your design. These ICs are even finding their way as NTC thermistor replacements.

However, if your design must operate in extreme temperatures that requires thermocouples, then I suggest you search the TI WEBENCH® sensor design tool online. This will allow you to take advantage of the series of TI thermocouple signal conditioning components.

 Texas Instruments has sponsored promotion of their industrial communications solutions on ENGINEERING.com. They have no editorial input to this post - all opinions are mine.  Bruce Schreiner

     

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