the Stilt :
Thermal margin = Inverted tCTL reading (i.e. distance to thermal management a.k.a HTC activation temperature on tCTL scale).
 
On FM2 & FM2r2 (TN/RL/KV) the thermal management activation temperature (HTC) is 70 units in tCTL scale
 
The formula to calculate the actual die temperature (ROS) from tCTL value on all current AMD designs is tDieMax - (tCTLMax - tCTL).
 
tDieMax = 130°C (Shutdown)
tCTLLimit = 70 Units (HTC Activation)
tCTLMax = 85 Units (Shutdown)
 
The tDieMax, tCTLLimit and tCTLMax temperature vary between the families and models, but these apply to FM2 and FM2r2 TN/RL/KV parts.
The mobile parts (FS1r2, FP2 & FP3) have different scaling.
 
Case tCTL -5 = 130 - (85 - -5) 40°C die temperature
Case tCTL 0 = 130 - (85 - 0) = 45°C die temperature
Case tCTL 70 = 130 - (85 - 70) = 115°C die temperature (Throttling activated)
Case tCTL 85 = 130 - (85 - 85) = 130°C die temperature (Shutdown asserted)
 
This means that on these parts the tCTL offset is -45°C.
So in AOD, 70 - "Thermal Margin temperature" + 45 = The actual die temperature.
 
The external motherboard sensor temperature measures the package temperature via SBI-TSI interface. The package temperature is not the actual die temperature, but a temperature measured from the center of the heatspreader (both AMD and Intel use the same method to determine the tCase, package temperature). Since obviously there is no probe attached on the heatspreader the package temperature is calibrated to match the predefined pattern. Therefore it slighly varies between the ODMs and monitoring chips.