Question
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 have better linearity with my capacitive sensor
element.
Answer
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. The AD7746
is tolerant to parasitic capacitances to GND (within limits of course). This
allows you to place a passive filter structure in to the frontend of the
capacitive input channel of the AD7746. The filter eliminates 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).
Furthermore, 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.