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