Separating Thermopile Package Temperature From Object Temperature

A thermopile’s infrared response depends on the radiation reaching it and on its own thermal condition. A board temperature change can therefore affect the calculated object temperature even when the target is unchanged. Review package temperature and optical geometry together before assigning every drift to the target.

Identify the measured quantities

Record the sensor’s raw thermopile signal, local temperature reading, and calculated object temperature where available. The TI TMP007 calibration guide describes calibration using sensor voltage and die temperature to relate received radiation to object temperature. Its system specific calibration approach is a useful reminder that one coefficient set does not automatically fit every mechanical arrangement.

Distinguish the temperature of the target surface from the temperature of the surrounding scene. The sensor receives radiation through a finite field of view. Changing distance, aperture, or target size can change what contributes to the reading.

Control local thermal disturbances

Identify nearby processors, regulators, LEDs, and airflow paths that change the sensor’s thermal environment. Test the operating modes that turn those heat sources on and off. A calibration performed only with the board idle may not represent the active product.

Log local temperature and raw sensor response during the transition. Look for transient error as well as the final steady value. A local temperature channel may track one part of the package while the optical or mechanical assembly responds on a different time scale.

Calibrate the intended geometry

Use a stable reference target and an independently established target temperature. Keep the target surface, orientation, distance, aperture, and enclosure configuration documented. Avoid changing the target’s optical properties while assuming only its temperature has changed.

Collect data across relevant object and ambient conditions, following the selected sensor’s calibration procedure. Reserve additional points for verification rather than using every measurement to fit the coefficients. This helps reveal whether the model interpolates adequately outside its fitted examples.

Verify with the finished assembly

Repeat the test after enclosure installation and during representative power state changes. Include enough settling time to characterize both transient and steady performance. Record any conditions in which the reading must be marked invalid or delayed.

Save raw signal, local temperature, reference target temperature, geometry, and calibration coefficients together. The resulting accuracy statement should describe the tested assembly and operating range. It should not imply that the PCB measures arbitrary surfaces or targets with the same accuracy simply because the thermopile provides a digital temperature output.