measure >100pf capacitance

[akkoyun]

Hi how can i measure more than 100pf capacitance ? thanks

[beshaya]

Mehmet,

How much capacitance are you looking to measure? What accuracy do you need?
Could you redesign your sensor to have a smaller capacitance?

If you only need a little more range (<200pF or so), you could use a series
capacitor to reduce the capacitance seen by the FDC1004.

The FDC2112 family has a listed input range of up to 250,000pF, but are
more involved to set up and realistically can only measure a small region
of that range determined by external components.

If you only need to measure capacitors manually, consider a handheld meter.

On Thu, Aug 4, 2016 at 12:38 AM, Mehmet Günce Akkoyun <
notifications@github.com> wrote:

Hi how can i measure more than 100pf capacitance ? thanks

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[akkoyun]

@beshaya ,
Thanks for your response. We want to measure soil moisture with our special design probes. i send design of probe below. We use shield pin to control probe sensitive. How can we calculate the capacitance with technical drawing and how can we measure the capacitance better. Thanks. (small coppers are shield and big coppers are capacitance probes)

Sensör.pdf

[beshaya]

That's a cool-looking device! You geometry makes estimating capacitance by
hand very difficult (concentric rings and parallel plates are about the
only geometries that can easily be calculated). Instead, I'd suggest
getting a meter like this:
https://www.amazon.com/Excelvan-Digital-Capacitance-Capacitor-470000uF/dp/B009CSR8BA/ref=sr_1_2?ie=UTF8&qid=1470690118&sr=8-2-spons&keywords=capacitance+meter&psc=1
to characterize your device.

If you need to reduce the capacitance of the device, you can make the
sensor rings shorter or farther apart.

Best of luck,
Ben

On Mon, Aug 8, 2016 at 1:25 PM, Mehmet Günce Akkoyun <
notifications@github.com> wrote:

@beshaya https://github.com/beshaya ,
Thanks for your response. We want to measure soil moisture with our
special design probes. i send design of probe below. We use shield pin to
control probe sensitive. How can we calculate the capacitance with
technical drawing and how can we measure the capacitance better. Thanks.
(small coppers are shield and big coppers are capacitance probes)

[image: prob prototip 1]
https://cloud.githubusercontent.com/assets/8276073/17494851/31236d3e-5dbf-11e6-80b3-d9cca6c2edb3.JPG

Sensör.pdf https://github.com/beshaya/FDC1004/files/407668/Sensor.pdf

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[akkoyun]

we use an capacitance meter but there is a problem with shield layers. how can we measure capacitance with shield layers :)

[beshaya]

Ooooh, that is a problem... If I'm not mistaken, though, you can just think
of the shield layers as a parallel capacitance. You should be able to get a
pretty decent measurement by measuring the capacitance from shield to
ground (call this Cs), then connecting the shield and sensor lines together
and measuring the capacitance again (call this Ct). The capacitance of the
sensor by itself should be C = Ct - Cs.

Let me know if that works...

On Mon, Aug 8, 2016 at 2:19 PM, Mehmet Günce Akkoyun <
notifications@github.com> wrote:

we use an capacitance meter but there is a problem with shield layers. how
can we measure capacitance with shield layers :)

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[akkoyun]

In parallel capacitance formula is C = Ct+Cs ? m i wrong ?

[akkoyun]

if u look at the pdf file an shield pipe is in the sensor :)
this is a master degree cap. calculation i think :D

[beshaya]

This is a finite element analysis no-closed-form-solution capacitance
calculation (as in, find a program to do the calculation).

The parallel capacitance formula is

Ctotal = Csensor + Cshield

so

Csensor = Ctotal - Cshield

I was being lazy and used Csensor as the capacitance for the sensor.

The reason I am suggesting measuring it this way is that the shield couples
the sense electrode to ground strongly, so it is very difficult to measure
the capacitance of just the sensor on its own (as you so rightly pointed
out). However, the same is not true in reverse (The shield electrode is
coupled directly to the ground electrode much more strongly than than the
sense electrode is). The downside is that the capacitance from shield
electrode to sensor electrode might be too high to measure a difference
when you add the sense electrode to it (note that the FDC1004 has a maxium
shield capacitance that you probably don't want to go over as well).

Anyway, If you want less capacitance, make your electrodes smaller and
farther away.

On Mon, Aug 8, 2016 at 3:13 PM, Mehmet Günce Akkoyun <
notifications@github.com> wrote:

if u look at the pdf file an shield pipe is in the sensor :)
this is a master degree cap. calculation i think :D

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[akkoyun]

i see your cv in linked in and interesting in OTA firmware update can you contact me on Skype ? gunceakkoyun

[akkoyun]

Hi @beshaya ,
I measure the capacitance with the fdc1004. The probe capacitance is 6500 fF in air dielectric. :) I surprised at 6,5pF :) so i m waiting 200pf :)

[beshaya]

Nice! If you need a range of 200pf, then you'll want a 200pf series capacitor (so that when your probe reads 200pf, the total capacitance will be 100pf. Unfortunately, I don't think this will play well with the shield electrode.

Perhaps you can just separate the electrodes a little more to bring down the highest capacitance you will measure.

OTA firmware update is probably a question you should ask your MCU manufacturer (or related forum) as it varies greatly between architectures...