AD7745/46/47 external temperature sensor conversion formula

When using external temperature sensor with AD7745/46/47 CDCs, what is the formula to use in order to convert the reading into temperature?

I've seen in "EXTERNAL TEMPERATURE SENSOR" section of datasheet that "The AD7745/AD7746 are factory calibrated for Transistor 2N3906 with the ideality factor nf = 1.008", but I just couldn't get any right temperature using external 2N3906 and the formula "Temperature(°C)= (Code/2048) − 4096", even trying to compensate by nf = 1.008. Maybe I didn't find the right way to use nf in the formula.

For instance, I've got a reading of 8478826 using external 2N3906, which is about 44˚C by the formula above, while internal sensor gives 8459790 that indicates  34.75˚C. A reference thermometer indicates ambient temperature around 29˚C.

Is it necessary to re-calibrate the part to get an external 2N3906 reading?


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