Capacitance measurement

Thread Starter

karthi keyan 9

Joined Apr 17, 2018
30
I want to measure the capacitance in the form of femto farads. at present i am using eval ad7156 evaluation board. But the capacitance which i am measuring through this device is not stable.
For example i measuring 1500fF it is varying between 1480fF to 1530fF continously.

Please suggest some thing so that i can meausre fF capacitance in stable value.

Thank you!
 

danadak

Joined Mar 10, 2018
4,057
Signal averaging (numerical processing of many samples) can help.

If you do FFT on sample set what is the fundamental and first 2-3 harmonics ?

Is this AC line induced noise ? 50/60 Hz ?

Regards, Dana.
 

Hymie

Joined Mar 30, 2018
846
I want to measure the capacitance in the form of femto farads. at present i am using eval ad7156 evaluation board. But the capacitance which i am measuring through this device is not stable.
For example i measuring 1500fF it is varying between 1480fF to 1530fF continously.

Please suggest some thing so that i can meausre fF capacitance in stable value.

Thank you!
The data sheet for the eval ad7156 shows that within the software options is a setting for the ‘average time constant’ – setting this at a higher value may eliminate/reduce the varying output.

If you are using the USB input to power the board, make sure it is stable and not the cause of the problem – with the on board regulator having a minimum input voltage of 4V.
 

MrChips

Joined Oct 2, 2009
21,666
Thank you for the replies.

First i want to know is it possible to measure femto Farads accurately??
femto is 10^-15 and would be very difficult to measure but not impossible.

You will not be able to measure it with traditional probes. You will have to measure the effects of it in an active circuit such as an oscillator in 100GHz region.
 

rtoth

Joined Oct 5, 2017
4
I want to measure the capacitance in the form of femto farads. at present i am using eval ad7156 evaluation board. But the capacitance which i am measuring through this device is not stable.
For example i measuring 1500fF it is varying between 1480fF to 1530fF continously.

Please suggest some thing so that i can meausre fF capacitance in stable value.

Thank you!
femto is 10^-15 and would be very difficult to measure but not impossible.

You will not be able to measure it with traditional probes. You will have to measure the effects of it in an active circuit such as an oscillator in 100GHz region.
To add to MrChips, it would have to be high frequency or high impedance and almost everything will affect your measurement. Tuff Call!
 

BR-549

Joined Sep 22, 2013
4,938
In theory, you should be able to make one with just calculated distance between plates. But then...you probably couldn't shield it. I have heard some electric fields can not be shielded.
 

rtoth

Joined Oct 5, 2017
4
The only way I can think to actually do this is:
1. Set up the highest frequency source you can find, 3 GHZ is about 1PF or 1000fF into 50 ohms.
2. Make a circuit with say 10 pF, find the -3DB (about 300Mhz) point than add your cap and note the difference by readjusting for -3DB. A 1 to 2 pF cap will shift your readings by 10% to 20%. it is just the opposite way of making an accurate measurement, but it is pretty much all you have!
This all needs to be free air, point to point wiring.
 
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