hi guys,
i want to measure capacitance in the range 150pf to 600pf. In addition , i have around 5 to 10 meters of cable. so i need to nullify the cable capacitance also. i searched the web, could not find a suitable capacitance to voltage or capacitance to digital converter.Can anybody plz help me on this?
The following IC measures capacitance from 50 pF to 100 uF:
Universal Sensors & Transducers Interface (USTI)
For the easyest method You can use 555 timer as Astyble conection and on the RC circuit you can place an unknown capacitor thus you are actualy converting capacitance to frequency , then you should measure that frequency outputing the 555 timer by microcontroller and convert that frequency to capacitance by simple calculation thats it
a great theoratical tutorial (not giving source code, just hex) here is a link chk it out , it will help you a lot
www.romanblack.com/onesec/capmeter.htm
---------- Post added at 10:20 ---------- Previous post was at 10:18 ----------
High-res Cap meter by RomanBlack
Yes, it is. And after, the frequency can be easy converted to digital by the same USTI IC.
you want to measure mutual capacitance between 2 pins?
in case of measuring mutual capacitance the GND is not a big problem.
The easiest way is to drive one electrode with some pulse and to integrate the trasnferred charge on the other line. In all cases the X and Y line should be separated (for example Y line is shielded).
what is the construction of the electrodes? What kind of dielectric you have on top?
Roman Black's design is very good, it works on the same principle as many other designs you will find on the Internet that use LM311 comparators but he cleverly uses the ones internal to the PIC instead.
You might be able to reduce the effects of the long cables by using a guarding technique. You will have to experiment to see if it is effective enough. The principle is to use screened wires to the capacitance you are measuring, thin RF or even audio cable may work. You use the inner wires to connect to the capacitance. The screen is connected to the inner wire at the PIC end only and through a unity gain buffer. The idea is the buffer presents only a small extra capcitance at the measurement point and this can be nulled by the software. The screen now has an independent duplicate of the inner wire on it so no current will flow between the inner and screen, effectively isolating it. External effects, such as touching the wires will only effect the screen which is not part of the measurement.
Brian.
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