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Help needed in developing Capacitive sensor using AD7746

Question asked by shantanu142 on Jul 28, 2017
Latest reply on Aug 18, 2017 by shantanu142

Hi,

I am planning to use the capacitive sensing technique to measure the variation in the surface roughness of an insulating test object and  also to identify the presence of smaller surface features on the surface.  At present I am using  AD7746 evaluation board for this purpose. The basic scheme of the measure setup is shown in the attached picture. Now I am able to identify smaller surface structures/features  up to 500 µm  dimension, which corresponds to a capacitance change up to 20 fF. But, in this case the sensor PCB at a working distance d - 0.5 mm from the surface of the test object, since the excitation voltage across the capacitance plates is very small (~ ± VDD/2).

My questions are:

(1) How to increase the voltage across the capacitance plates (up to 50 V - 100 V) so that the generated signal will have much higher amplitude and similar measurement can be done from a large working distance (working distance, d~ several cm)  ?

(2) At present a single oscillation frequency is available on the board (32 KHz), instead of a fixed oscillation frequency how to use an input signal over a range of frequencies (e.g. 10 KHZ-10MHz) to find out at what oscillation frequency the system responds most efficiently ?

I am not sure whether it is possible to achieve all these optimization using AD7746 chip. If not, is there any single chip or any combination of chips which can be used together to perform the same task as it has been described above ? If chips are available kindly give some details about those chips.

Also, if developing the above mentioned circuit using discrete components is the only possible option, kindly help me to select suitable components or parts with their numbers.

 I am still in the learning stage, any help in this regard will be highly appreciated.

 

Basic Scheme of Capacitive Sensor System using AD7746

 

with thanks and regards,

Shantanu Pal.

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