TMS320F28388D: Internal temperature sensor acquisition

Hi Cherry,

Cherry Zhou said:

1) How does the internal temperature sensor acquisition function be calculated? As noted, the product test was referenced to 2.5 V, but according to the formula, it's hard to understand how the algorithm corresponds to a 3.0-V reference. Because the vref parameter is 3.0 when this function is called. 

Internal temp sensor slope and offset (more on this later) are derived at the factory with VREF of 2.5V.  In order for the temperature readings to scale correctly, at the customer's end final VREF, the correct VREF has to be entered in function ADC_getTemperatureC().  For instance, at customer's end, the board is set for 3.0VREF, the scaling factor would be 3.0V/2.5V or 1.2.  The typical temperature sensor step per degC is around 3.6mv and for a temperature change from 30C to 125C, this would be around 0.342v increase in temperature sensor output.  With 2.5V ADC reference, this is around 560 codes ([0.342/2.5]*4095) but with 3.0V ADC reference, this would be around 467 codes ([0.342/3.0]*4095).  So to have the equivalent steps for 3.0v VREF with the measurements made with the factory 2.5v testing, the scaling factor has to be used.  You can check with the example I have:  divide 560 codes (2.5v) with scaling factor of 1.2 and you will get 467 codes for 3.0v VREF.

Cherry Zhou said:

2) No definitions or descriptions of the following parameters were found in the manual and TRM, could you help elaborate on this?

At factory testing, there are two parameters logged for temperature sensor: 1) ADC output of tempsensor and 2) actual temperature where measurement is made.  Let us assign these pair in the form of (x,y) or (adc,temp). Temperature sensor is also measured at a) low temperature and b) high temperature so the data pairs will be: (adc,temp)a for low temp and (adc,temp)b for high temp.  Temperature sensor response is linear across temperature so with this characteristic, we can just plot these 2 points as straignt line so it would form the linear equation y=mx + b; where y is the temperature, m is the slope, x is the adc reading (from tempsensor) and m is the offset.  Same principle is done on tempsensor readings.  TSSLOPE is the slope derived using the linear equation on the 2 temp sensor readings logged at factory testing and TSOFFSET is the offset.

Let me know if this answers your queries.

Regards,

Joseph