Compensating for thermal rise of temperature sensor. - Page 1
Forum section: electronics / projects

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 13, 2014, 08:50:27 pm
content: Hello there,I have an ADT7420 (http://www.analog.com/static/imported-files/data_sheets/ADT7420.pdf) mounted on a stand-off board using 100 mil header to a pcb at ambient temperature, no enclosure. I intend to measure the surrounding air. In the range -30*C to 100*C. The problem is that when the ic powers up, it gives readings which agree with my fluke 87V, however over the course of 10 minutes, the temp output climbs from ambient temp to ambient + 2*C. This is a disaster as the ic is 0.25*C accurate.If I add forced airflow over the sensor, I can lower the temperature to the same as its startup temperature, and it once again agrees with the fluke.Heat sinks don't seem to make much of a difference either way.(1) what do you reckon is causing the temperature reading to rise?(2) is it logical to measure the temperature rise at ambient as a function of time, and subtract this difference to correct for the error, regardless of the measured temperature? In other words, can I always expect a 2*C rise regardless of whether the temperature is -10*C or +80*C? That way I could avoid using forced airflow, which is very intrusive in the environments I may wish to probe.(3) any other ideas to correct the readings?Thank you for your time.Kind regards,Marius

author: AndyC_772
author_rank: Super Contributor
timestamp: January 13, 2014, 10:08:45 pm
content: Is there anything else on the PCB which could be warming the sensor?It's rated at 700uW dissipation when fully active, and has a junction-to-ambient thermal resistance of 37 deg C per W. So, the self heating of the sensor should be 0.026 deg C, which is about 100x smaller than you're measuring.It sounds as though there could be heat rising from the PCB up the connector pins into your sensor board. You need to thermally isolate the sensor board a bit better, and/or improve its thermal contact with the air. If its thermal contact with the PCB is too good, you're effectively measuring the PCB temperature rather than the air temperature. Longer, thinner wires may be called for.

author: Psi
author_rank: Super Contributor
timestamp: January 14, 2014, 12:11:19 am
content: The copper tracks to/from the pins of the temp sensor to the rest of the circuit will conduct heat really well. All electronics produce heat, even when you don't notice it.There are a few techniques you can use...- Make the PCB tracks to the sensor as thin as possible to reduce the thermal conduction.Obviously if you make them too thin the Vdrop may start to cause problems but if the temp sensor is digital (i2c/spi) this matters less since your not measuring analog with respect to a non zero gnd point.- Snake the tracks back and forward so they are effectively very long which also reduces thermal conduction. (Same vdrop issue if you go too far)- Put the sensor on wires instead of on the PCB and located it away from the heat source.- Use a 2nd temp sensor much closer to the heat source and use that value to correct the error from real temp sensor in realtime.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 14, 2014, 01:09:47 am
content: Thank you guys,I don't have much of an heat source, only a 16MHz MCU, linear reg and small 8x2 LCD with led backlight.Is it possible to use some kind of insulation, to isolate the stand-off board pins thermally, but not electrically? I am already using long, thin tracks.Thanks.M

author: jeremy
author_rank: Super Contributor
timestamp: January 14, 2014, 02:08:25 am
content: Perhaps try connecting the thermocouple from your fluke thermally to the sensor (thermal transfer compound) and see if the fluke tracks the rise in the same way as the sensor? At least then you will know that it is functioning correctly (aside from the heating). Also try swapping it out for a second one.PS linear regulators are a significant source of heat. Keep an eye on that one.

author: AndyC_772
author_rank: Super Contributor
timestamp: January 14, 2014, 06:58:47 am
content: Quote from: king.oslo on January 14, 2014, 01:09:47 amThose devices could easily raise the temperature of a small board by 2 deg C.Without any air flow, even a small heat sink isn't really going to be effective. Heat sinks rely on air movement around them in order to work, which means either a fan, or a temperature difference which is high enough to start setting up convection currents. In this case, you have a physically small sensor in still air, and that means a lot of thermal resistance between the sensor and what you're trying to measure.I think it's the 0.1" header that's your problem. They're short, fat, and thermally conductive. Heat is being generated by your board, conducted through the FR4 laminate (not just the tracks), up the header, and into your sensor.The best insulator you'll find is physical separation. As a test, try mounting your sensor on thin wires rather than using a header. If that helps, then try supporting it mechanically with a plastic stand-off, which should be less thermally conductive than a connector.

author: Psi
author_rank: Super Contributor
timestamp: January 14, 2014, 07:20:32 am
content: The linear reg is the likely heat source.Maybe remove it and temporarily power the board through some wires externally to prove/disprove the idea.

author: jmaja
author_rank: Frequent Contributor
timestamp: January 14, 2014, 08:20:57 am
content: Quote from: king.oslo on January 14, 2014, 01:09:47 amThose can easily take 50 mA (even backlight alone). If you have e.g. 10 V supply to the regulator that is 0.5 W, which is enough to warm quite a large PCB 2 C. You can measure the PCB temperature with your Fluke.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 14, 2014, 09:38:56 am
content: Ok. I finally feel confident that it is the LDO. The temperature output shifts as a function of the drop across the regulator. Thank you for suggesting that.Moreover, the sensor is mounted straight above the device. I should probably start by moving the sensor to another location, using ultra-thin wires (as few as possible) to connct the stand-off board to the MCU.Thank you for your time.Kind regards,Marius

author: cosmos
author_rank: Regular Contributor
timestamp: January 14, 2014, 09:56:34 am
content: slots in the PCB around the sensor might also be on your list for increased isolation, 3 narrow bridges should be enough.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 14, 2014, 12:10:29 pm
content: Thanks for all your suggestions.I moved the ADT7420 away from the LDO, and now my fluke and sensor agrees exactly. I used 30AWG wire (only four connections), and plastic mounting bracket. Looks good!Thank you for your time.Kind regards,Marius