When comparing the Temperature measured by the System Monitor and a thermal probe, large differences can be seen. Why is this the case?
When looking at the temperature measured via the System Monitor or XADC, discrepancies might be seen between the package temperature (T-top / Tc) and the Die temperature or System Monitor (XADC) readings (Tj).
The reason for these differences is not usually an issue with the System Monitor / XADC readings, but rather errors in how the temperature is measured.
In a customer system, the following factors can affect the accuracy of temperature measurements:
1) Measuring T-top (temperature of the package) in an accurate and repeatable way is always a challenge, and when measured it will usually be slightly lower than the actual T-top.
The factors that influence the accuracy of these measurements are the location and size of the thermocouple, both of which are crucial.
Even when the location and size of the thermocouple are correct, a difference of between two and five degrees C can be seen between the T-top measured and the actual T-top.
There are also additional factors that need to be correct in order to get the T-top measurements as accurate as possible:
You can use an infrared sensor to measure the temperature. While this removes the variable of the contact media and pressure required, it brings its own measurement error. The area around the dead center is averaged, so an error of 2 to 3 degrees C can be seen. Improving the area resolution and surface compatibility can improve this.
2) The Tj measured through the System Monitor (XADC) is done through a directly probed passive diode. This can result in approximately +3 degrees C of difference with the diode if the reference voltage is properly stabilized. However, if the reference voltage is not regulated correctly, this can cause an increase in the measurement error of the System Monitor in excess of +3 degrees C, further exacerbating the delta between T-Top and the System Monitor Tj.
These potential sources of measurement error in both T-Top and Tj from the system monitor explain why you might measure a larger than expected delta between T-top and Tj.