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AD7689 internal temperature conversion

Question asked by yparadis on Dec 20, 2016
Latest reply on Dec 21, 2016 by yparadis

Dear All,
Could you help me to convert the digital value of the internal temperature coming out of the AD7689 into temperature readable in deg C ?

The configuration register is set as the following :
- unipolar INx, referenced to COM = gnd +/- 0.1V

- temperature channel

- internal reference ref=4.096V temperature enabled
- the sequencer disabled (CFG = 0x2C49)


I found in the datasheet that  for the temperature sensor the output is 0.283 V @ 25 deg C with a sensitivity of 0.001 V/deg C.

Up to now, I used the following equation Vt = V0 * (1+alpha*(T-T0)) with V0 = 283 mV, alpha 1mV/deg C, T0 = 25 deg C and Vt = the digital value /2^16. Reordering this equation to get T as a function of Vt, gave me completely wrong results ; at about 30 deg C, I get from the AD7689 a pretty stable digital value of 0x10e7 and after conversion -19.4 deg C which is not true.

If I cool the chip the digital value decrease and if I heat it up, the value increase. I came to the conclusion that my equation is not the one i have to use. What is the right way to convert AD7689 digital temperature value into deg C ?
Many thanks in advance
Best regards