AD7792: AVDD monitor and temperature sensor channel

Document created by analog-archivist Employee on Feb 23, 2016
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We intend to use AD7792 in a new product and I am implementing the software
that communicates with the AD7792. The only documentation I can find on your
web site is the data sheet. I can not find all the information I need so I
would like to know if you have any application notes that describes: 

-How to use AVdd monitor channel. 
Is calibration necessary and so on? 
Is ít neccessary to calibrate again when switching from the AVdd monitor
channel (Gain is changed since AVdd monitor uses gain 1/6)? 
The AVdd monitor does not have its own calibration registers? 

-How to use the internal temp sensor. 
Necessary to calibrate? 
The temp sensor does not have its own calibration registers? 

 

Q1. AVdd monitor channel is described on page 26 of the datasheet. It is
treated the same as any other channel - only it has a fixed gain and internal
reference.
If you need an accurate voltage measurement then you need to carry out an
internal zero scale and full scale calibration, as with any other channel.
You obviously can't do a zero scale system calibration because the device is
off when VDD is 0.
The AVDD channel has its own calibration registers like any other channel, so
you only need to recalibrate if the temperature changes significantly etc.

Q2. The temperature sensor is an output from the on-chip bandgap reference so
it measures chip temperature and not ambient temperature.
It is not calibrated in production so to take accurate measurements you need to
do your own calibration. To calibrate at room temperature it is probably easier
to calibrate in software because 25degC is not near either end point of the ADC
range (so you can't easily to a zero scale or full scale system calibration).
To have a rough idea of the temperature, you can simply store the reading at
room temperature and then observe any differences which occur later. I don't
think it is worth doing any precision calibration because the basic
repeatability of the measurement is not so good. A typical application would be
to tell if the temperature has changed significantly, in order to carry out a
recalibration of your system etc.

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