STM32F733 Internal Temperature Sensor: Large deviation (>10°C) vs. Ambient Temp

[MERSI]

Hi everyone,

I am currently trying to measure the CPU temperature using the internal temperature sensor of an STM32F733.

I am testing the device inside a climate chamber (oven) to verify the readings. I am fully aware that the datasheet states the sensor is more suitable for detecting temperature variations rather than absolute measurements. However, the values I am getting seem to have a significantly larger offset/gain error than expected.

The Setup:

• MCU: STM32F733

• Method: To rule out self-heating of the die, the device is kept powered OFF inside the oven until the target ambient temperature is reached and stabilized. I then power it on and immediately take the ADC measurement.

• Vref+: Connected to external reference voltage (2.048V).

The Observations:

• Oven at 60°C -> MCU calculates 68°C

• Oven at 80°C -> MCU calculates >90°C

My Question:

Is an error of this magnitude (>10°C at higher temperatures) considered "normal" for the internal sensor of the F7 series, even when using the factory calibration values stored in the system memory?

Or am I likely missing a configuration step?

• I am using the factory calibration values (TS_CAL1 @ 30°C and TS_CAL2 @ 110°C) for the calculation.

• The ADC sampling time is set to comply with the minimum sampling time required for the internal temp sensor ($10 \mu s$ according to datasheet).

Any hints or experiences with the F7 internal sensor accuracy would be appreciated!

Thanks in advance.

[mbarg.1]

This is junction temperature, not ambient - actual temp depend on acivity

 

[LCE]

As said above, it is quite normal that the MCU "heats up" internally as soon as it starts running.

To verify that, you could play with:

- reducing system & peripheral clock frequencies

- sleep modes

[waclawek.jan]

I presume you've verified independently that the external VREF source is rock stable and precise even at the elevated temepratures.

> I am using the factory calibration values (TS_CAL1 @ 30°C and TS_CAL2 @ 110°C) for the calculation.

Show numbers, i.e. the two calibration values, your ADC readings at various temperatures. Try to measure a known voltage source (perhaps internal VREFINT), too. 

> I then power it on and immediately take the ADC measurement.

Maybe instead of this strategy, try to run the mcu and the measurements continuously with the smallest possible power consumption - at low frequencies, all peripherals off except the vitals (ADC, maybe one timer, maybe one UART to transmit measurements, or whatever is your method), sleep between taking measurements, etc.

JW