Author Topic: How to measure a capacitor with an oscilloscope  (Read 3823 times)

FOX

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How to measure a capacitor with an oscilloscope
« on: June 21, 2013, 12:24:29 PM »
Hi Martin,

when measuring such low capacitance's down in the pF range one must consider the additional capacitance's in the measurement setup.
First the input capacitance of your scope is 11pF, so you measuring 15.7pF instead of joust 4.7pF,
second the capacitance in your wiring can easily reach 10s of pF.
Just put some wires on an LCR meter and you will get  some noticeable capacitance.
The problem with unwanted stray capacitance will get worse the higher the frequency will get.
And as someone mentioned in the youtube comments the actual measurement of the time and Voltage on the scope is critical.
To get a better measurement make your waveform as big as possible on the screen, and keep in mind that the scope is not that precise in its Volts measurements (it has only an 8bit DAC) 
A closed Switch should have 0 Ohm or less

steve30

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Re: How to measure a capacitor with an oscilloscope
« Reply #1 on: June 21, 2013, 05:04:03 PM »
Useful video, thanks Martin.

I recommend the usage of engineering notation though for expressing micros, nanos and picos :).

valentinc

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Re: How to measure a capacitor with an oscilloscope
« Reply #2 on: June 21, 2013, 08:20:29 PM »
Martin, what attenuation factor were you using in your probe... 10X or 1X ? I just made a measurement at 300 Khz, with a 10K resistor and just the probe itself as a capacitor... And this is the result: http://postimg.org/image/qtbupukdr/ (500mVpp signal, 63.2% is 300mVpp, that's 3 divisions) and the calculated capacitance (10X probe) that I obtain is 14.4 pF... Which is very close to the actual input capacitance of the probe in 10X mode...

If you were using 10X, you should have obtained about 20pF... 56pF is way out... There's a problem somewhere...

Quote
The problem with unwanted stray capacitance will get worse the higher the frequency will get.

And the higher the frequency, the more your coaxial cable act like a transmission like... (beginning when 1/10 wavelength of the signal becomes comparable with the cable length) And then you want to set the output impedance of your function generator to 50 ohms, have a cable with a characteristic impedance of 50 ohms and also have a termination resistor of 50 ohms...

Keep in mind that a squarewave is composed by a sum o sinewaves at different frequencies (http://www.youtube.com/watch?v=eC36AqL5mw8) and you must take the harmonics into consideration as well...

Of course, this method isn't an accurate method of measuring capacitance anyway... A more proper method is to to make use of the relationship between the capacitor voltage and current: (I=C dV/dt)... C=I x dt/dV ... For example, you supply the capacitor with a 1ms pulse of current, you measure the increase in voltage on the capacitor and calculate the value of the capacitance with the forumla above...
Valentin

iloveelectronics

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Re: How to measure a capacitor with an oscilloscope
« Reply #3 on: June 22, 2013, 05:44:55 AM »
I recommend the usage of engineering notation though for expressing micros, nanos and picos :).

Did you mean ball hairs and bee's dicks were not engineering units??  ;D
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steve30

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Re: How to measure a capacitor with an oscilloscope
« Reply #4 on: June 22, 2013, 04:40:31 PM »
I recommend the usage of engineering notation though for expressing micros, nanos and picos :).

Did you mean ball hairs and bee's dicks were not engineering units??  ;D

Nah, nowt wrong with ball hairs and bees' dicks :D.

MJLorton

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Re: How to measure a capacitor with an oscilloscope
« Reply #5 on: June 24, 2013, 02:19:22 PM »
Thanks to FOX, Steve30 and valentinc!

Yup...I certainly did not take into account how the probes (x10), function generator, etc, etc, would affect the signal on the scope. I have received some very useful comments and emails on the topic. I hope to revisit once I have it all ironed out and can post a "concise" follow-up on the topic.

In terms of the engineering mode on the calculator...I laughed at myself while editing and did add an annotation to the video about that. I plan to cover that as a tutorial on my other channel at some point.

Thanks for pointing it out ;).

Cheers, Martin.
Play, discover, learn and enjoy! (and don't be scared to make mistakes along the way!)