I've got the latest datasheet but there are still two unresolved problems:
page 5 says, NOXREF is active when Vref is less than 0.3 V but page 20 tells me
0.5 V. What is the real low limit for missing reference detection ?
Internal Temp Sensor, how to calculate T from the voltage (I've seen a value of
0.81 mV per degree) Must I calibrate this measurement overwriting the offset
resp. zero registers used for channel 4-6 (AIN 4 is already in use with a
different external reference and gain = 1) ?
Question 1: Page 20 is correct, NOXREF becomes active when the reference
voltage goes below 0.5V.
Question 2: The 0.81mV/degC is a sensitivity specification so you need to
calibrate the sensor at some known temperature. To calibrate the temp sensor,
you need to read the conversions from the device at a known temperature, for
example, 0 deg C. At 0deg C, the temp sensor should generate a code of
000000. You can then record the value generated by the temp sensor at 0 deg
C. This value can be held in the uC and subtracted from each conversion read
back from the AD7794.
Channels 4-6 share the same offset and gain registers. You can select each
channel in turn and perform a calibration. The offset and gain values could
then be stored in the uC. Each time the channel is changed, the appropriate
offset and gain values could be written to the offset and gain registers on the
AD7794. The other alternative is to perform a calibration each time the
channel is changed. However, this would require more time.