Search FAQs on the left to see if your question has been answered. Click on the dropdown to view all of the documents associated with the product. If you can't find your question, click on Ask a Question.
I've got the latest datasheet but there are still two unresolved problems: question 1: 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 ? question 2: 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.