external excitation to AD7746

Document created by analog-archivist Employee on Feb 23, 2016
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Is it possible to use an external excitation device to produce a higher
excitation frequency i.e. 1Mhz with the AD7746 device? If yes, what would be
the setup/setting/precautions needed to take note of? How would I calculate the
CAPDAC value that needed to be set in the digital interface? My target here is
to use a high frequency excitation together with the AD7746 in order to get rid
of external interference and better linearity with my capacitive sensor
element.

If this is not possible, is there an alternate device that could achieve this?

 

All the timing internally derives from the excitation frequency hence no
external excitation source can be used. 

in order to eliminate interference there are several possibilities to do that.
The AD7746 is tolerant to parasitic capacitances to GND (within limits of
course). This allows to place a passive filter structure in to the frontend of
the capacitive input channel of the AD7746. The filter allows to eliminate high
frequency noise coupled into the sensor element. Ground planes around the CIN
and EXC signal lines allow to avoid potential coupling on the PCB.
(See AN-1055 EMC Protection of the AD7746 and General Layout Guide).

Further it is important to provide sufficient filtering on the supply line
(2-pole passive filter if possible) as the excitation voltage derives directly
from VDD. Noise on VDD can result in elevated noise levels seen in the
conversion results.

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