[i2c] problem reading LM92 and AD7416

[pierre demanax]

Hi all,
 
I have some trouble in reading temperature value from LM and AD sensors.
I use different i2c bus setup and with all of them i have different results.
As interface i use the followings one:
 

paralelle port ( ELV) one with pull ups of 3k3
i2c-tiny-usb from till harbaum and same pull up value
self made interface using atmega8 (TWI driver) and USB.
 
I manage having a connection with all of the interfaces, (i2cdetect works and detect the different sensors), i can read the default register ( nl hyst, temp min, temp max...), but once i read the actual temperature i have wierd results:
 
i used the same sensors with same cables for all of the interfaces : 
 

PP : i measure 25°
tiny : i measure 27°
self made : i measure 30°
( my personnal thermometer is showing me 22°, i come always to the same value when i redo the test)
 
I have those behaviours with all of my sensors and if i change the length of the cables, the temperature also change ( actually it decreases with the length).
 
So as i2c is a digital communication i can't understand why the temperature is so sensitive to interfaces or cable length, as the bus has nothing to do with the sensors ?.
I am stuk, and i don't see what i could do to find the problem.
 
has someone of you ever been experimenting such a strange behaviour?
 
what could be the cause of it ?
 
regards
vincent
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