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capacitance measurement

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mbna

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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 capaciatnce to digital converter.Can anybody plz help me on this?
 

Re: capaciatnce measurement

i didn't measure the capacitance till now however a range for measurement of capacitance is given in Analog multimeters. i will chek it and let you know. however you can test the capacitors for their leakage, short, open by multimeter.
 

Re: capaciatnce measurement

There are many methods to measure a capacitance: f.i: by mean of an AC bridge similar to the Wheaston's bridge for measuring resistors, measuring the discharge time on a known resistor, making it oscillate coupled with a known inductor so that measuring the frequency you can calculate the capacitance. You can find many circuits in internet.
You can measure the cable capacitance just living it open on one side. Then when you will measure the total cable+C capacitance just subtracting the cable capacitance measured before.
You could experience some resolution problem since your cable is quite long; supposing its capacitance is 100 pF/m, then over 10 m you will have 1 nF that is roughly 10 time higher than the minimum capacitance you want to measure.
 

i want to measure means i want to convert capacitance to voltage or capacitance to digital conversion.i.e. signal conditioning. i have seen the product from ZMDI 31210 analog sensor signal conditioner but the capacitance range is 2pF to 260pF.
 

mbna said:
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?
Click to expand...

The following IC measures capacitance from 50 pF to 100 uF:
Universal Sensors & Transducers Interface (USTI)
 
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    mbna

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syurish said:
The following IC measures capacitance from 50 pF to 100 uF:
Universal Sensors & Transducers Interface (USTI)
Click to expand...

Thank you sir but i have sensor with cable length of 5 to 10 mtrs. whether it is possible to nullify cable capacitance? i have gone through the datasheet.IC seems to be very complicated.whther it is possible to get analog output? bcz first i have to demo with analog output only.
 

yes, you can completely suppress the capacitance of the cable by connecting the shield of the cable to OpAmp follower and make it equipotential to the wire (if the wire and the shiled have the same potential it's not possible currents to flow between them and hence there is virtually no capacitance between them). Google for "actve shielding".

Atmel have a bunch of capacitive sensors. If you get any of the Atmel uP and you add QTouch library into your code you'll turn it into capacitive sensor.
 

bubt sir,
here USTI gives a digial output. where i will connect the shield there? i want to know the suitability of the USTI for my application? or whether u can tell em a suitable signal conditioner whose input cap range is 40pF to 600pF?
 

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
 
supermind2002 said:
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
Click to expand...

Yes, it is. And after, the frequency can be easy converted to digital by the same USTI IC.
 

syurish said:
Yes, it is. And after, the frequency can be easy converted to digital by the same USTI IC.
Click to expand...

with 555, the capacitance to be measured, is grounded.i want to measure floating i.e. ungrounded capacitance.
 

you want to measure mutual capacitance between 2 pins?
 

luben111 said:
you want to measure mutual capacitance between 2 pins?
Click to expand...

Ya i want to measure the capacitance between two electrodes & the cable length is minimum 10 to 15 mrts
 

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?
 

luben111 said:
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?
Click to expand...

my dielectric is changing hence capaciatnce
 

i have already gone trought he papaers.
 

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.
 

betwixt said:
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.
Click to expand...

can u plz send me the link for circuit & in my case i am very new for capacitance measurement. In my case i have single core shielded cables for electrical connection. so plz let me know. S yurish Sir plz tell me how much the USTI is useful for me bcz i want to measure capacitance at a very low rate may be max 100Hz or even less.i have seen the circuit for capacitance measurement in USTI application notebut there not much details are given. whether Cref should be minimum or maximum value of of Cmeasure? how to design let me know some literature?
 

Please check the following references about the USTI IC applications:

1) Yurish S. Y., Universal Capacitive Sensors and Transducers Interface, in Proceedings of the 23rd International Conference EUROSENSORS XXIII, Procedia Chemistry 1, Lausanne, Switzerland,6-9 September 2009, pp.441-444;

2) Yurish S. Y., Digital Sensors and Sensor Systems: Practical Design, IFSA Publishing, 2011:
Digital Sensors and Sensor Systems: Practical Design

Good luck.
 

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