AD7792: Communication problems

Document created by analog-archivist Employee on Feb 23, 2016
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I have the problem that the AD7792 doesn't talk to me over SPI. I try to read
out the ID register as a start. The /CS is correctly low, the read-command is
transmitted correctly and there are clock pulses for the response but no data
is coming out. What could be the problem ?

 

If you don't manage to get any reaction from the AD7792, you might like to
consider a few possible reasons for the breakdown in communications.

Power supply to AD7792
If the power supply to the AD7792 comes from the CPU board and passes throught
the cable then you will have a lot of inductance between the
AD7792 and the power supply/ground connections. You need to decouple the
AD7792 carefully as recommended in the datasheet. Its also a good idea to use
several of the lines in the flat cable as power and several as ground. In this
way you reduce the parasitic inductance. If this is the problem then you can
easily see the heavily distorted signals on the SPI bus.

- Crosstalk.
Capacitive cross talk from one signal to the other in the flat cable can be
avoided by alternating signal lines with ground lines and or power suppy lines.
This avoids the SCLK coupling onto the other signals.

- different common mode voltage at either end of the cable You need to make
sure that the AD7792 and the micro ground references have some common
connection.

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