# capacitive measuring ic that is capable of measuring 0-200pf value

#### NAGARAJ_JP

Joined Aug 21, 2020
17
hi im designing a capacitance measurement system that will measure the capacitance ranging 0- 200 pf i searched for google and i got AD7747,AD7152,FD1004 where all these having the measurable range lesser than 20 pf can any one suggest me a ic or method to measure the capacitance value from 0 - 200 pf

#### Delta prime

Joined Nov 15, 2019
486
Hello there Have you ever heard of Tau?

The time constant of an electronic circuit that contains resistive and capacitive elements is represented by the Greek letter tau (τ). This time constant in seconds is equal to the circuit resistance in ohms times the circuit capacitance in farads, τ = RC. Tau is the time required to charge a capacitor that is in series with a resistor to a level of 63.2% of the initial value, typically 0 V.
digital storage oscilloscope can readily display voltage plotted against time in a capacitor charging or discharging through a resistor. Then the time constant of the circuit can be calculated and, from that, the value of the capacitor can be found.
If you apply a dc voltage to a capacitor in series with a resistor, its charge rises, at first rapidly, then more slowly as it approaches the supply voltage. Plotting voltage against time, the curve seen on an oscilloscope display is known as exponential rise. Conversely, the discharge of a capacitor in series with a resistor is known as exponential decay.
Theoretically, the voltage across a capacitor never becomes equal to the full battery voltage because the rate of change declines as it approaches that level. The time constant by definition is the time in seconds required for the charge as measured at the capacitor terminals to equal 63.2% of the applied voltage.
To determine an unknown capacitance using an oscilloscope, a dc power source such as a 9-V battery, a known resistance, a switch and the capacitor are all connected in series. An oscilloscope probe tip and ground lead are connected across the capacitor. Additionally, you need a short wire jumper to shunt across the capacitor.

When the switch is moved to the on position, the scope display graphs the voltage across the capacitor. Because the instrument is in the time-domain mode, amplitude in volts displays on the Y-axis and elapsed time is on the X-axis. The task before us is to find the time constant of the resistor and capacitor that are in series. To do this, determine the final charge on the capacitor, which should be substantially equal to the nominal battery voltage. Then, multiply that amount by 0.632 because the time constant is by definition based on 63.2% of the maximum charge on the capacitor.

Locate that point on the oscilloscope trace, using a horizontal line from the Y-axis. Next, from that point on the charging curve, drop a vertical line down to the X-axis, which must be calibrated in seconds. (A cursor may be used for this purpose.) This provides the time constant of the RC combination, . With the time constant known, it is a simple matter to find the unknown capacitance.
t= RC
Transposing,
C = t/R
Recalling that in the time constant equation C is expressed in farads, the large value for R, which is known, in the denominator yields a reasonable value for the capacitance expressed in microfarads, millionths of a farad. This is the unit more commonly used.

#### jpanhalt

Joined Jan 18, 2008
10,219

#### MrChips

Joined Oct 2, 2009
21,827
Presumably, you are going to use a microcontroller (MCU) to interface to an IC that measures capacitance.
You do not need an extra IC. You can do it directly at the MCU using a single I/O pin by charging and discharging the capacitor.

#### NAGARAJ_JP

Joined Aug 21, 2020
17
Hello there Have you ever heard of Tau?

The time constant of an electronic circuit that contains resistive and capacitive elements is represented by the Greek letter tau (τ). This time constant in seconds is equal to the circuit resistance in ohms times the circuit capacitance in farads, τ = RC. Tau is the time required to charge a capacitor that is in series with a resistor to a level of 63.2% of the initial value, typically 0 V.
digital storage oscilloscope can readily display voltage plotted against time in a capacitor charging or discharging through a resistor. Then the time constant of the circuit can be calculated and, from that, the value of the capacitor can be found.
If you apply a dc voltage to a capacitor in series with a resistor, its charge rises, at first rapidly, then more slowly as it approaches the supply voltage. Plotting voltage against time, the curve seen on an oscilloscope display is known as exponential rise. Conversely, the discharge of a capacitor in series with a resistor is known as exponential decay.
Theoretically, the voltage across a capacitor never becomes equal to the full battery voltage because the rate of change declines as it approaches that level. The time constant by definition is the time in seconds required for the charge as measured at the capacitor terminals to equal 63.2% of the applied voltage.
To determine an unknown capacitance using an oscilloscope, a dc power source such as a 9-V battery, a known resistance, a switch and the capacitor are all connected in series. An oscilloscope probe tip and ground lead are connected across the capacitor. Additionally, you need a short wire jumper to shunt across the capacitor.

When the switch is moved to the on position, the scope display graphs the voltage across the capacitor. Because the instrument is in the time-domain mode, amplitude in volts displays on the Y-axis and elapsed time is on the X-axis. The task before us is to find the time constant of the resistor and capacitor that are in series. To do this, determine the final charge on the capacitor, which should be substantially equal to the nominal battery voltage. Then, multiply that amount by 0.632 because the time constant is by definition based on 63.2% of the maximum charge on the capacitor.

Locate that point on the oscilloscope trace, using a horizontal line from the Y-axis. Next, from that point on the charging curve, drop a vertical line down to the X-axis, which must be calibrated in seconds. (A cursor may be used for this purpose.) This provides the time constant of the RC combination, . With the time constant known, it is a simple matter to find the unknown capacitance.
t= RC
Transposing,
C = t/R
Recalling that in the time constant equation C is expressed in farads, the large value for R, which is known, in the denominator yields a reasonable value for the capacitance expressed in microfarads, millionths of a farad. This is the unit more commonly used.
thankyou very much for the effort i will try this method and update in this forum

#### NAGARAJ_JP

Joined Aug 21, 2020
17

#### Janis59

Joined Aug 21, 2017
1,221
Have no idea of appropriatness of building ANY capacitance meter nowadays when for 5 USD may obtain already done from ebay with built-in component identificating microprocessor able to get out 0,1 pF to 100 000 mkF with accuracy of 0,1 pF or bit more costy one port VNA (120-140 USD) measuring C with accuracy of +/- 0,005 pF at frequency of choice up to 3 GHz with simultaneous acquiring the parasythic L and R (=ESR and ESL) what is capable to draw a graph how those three parameters depend of wide band frequency and where have a resonant peak. If You pay me a 10K USD I would rethink three times is it worth to take such building task, but there it have a price of one my work hour.... Do You are a well hidden underground millionaire if spend a time so bashfully??
P.S.: However when I stucked 16 age at some 1975, I indeed built a capacitance meter (when nowhere nothing was available around) and even it was chosen to exponate at Moscow VDNH exhibition in schoolsters section. So I got my first travel to russia alone without of parents, bought my first foreign cigarettes pack (Los Carribos was the name - oh fool, how much efforts demanded later to get off the hook of death), got my first glass of alcohol (russia have such national refresment drink tradition) etc etc. At least from that point I like to travel, but never are designing any capacitance meters

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#### bob2

Joined Jun 15, 2019
194

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#### MisterBill2

Joined Jan 23, 2018
6,997
Have no idea of appropriatness of building ANY capacitance meter nowadays when for 5 USD may obtain already done from ebay with built-in component identificating microprocessor able to get out 0,1 pF to 100 000 mkF with accuracy of 0,1 pF or bit more costy one port VNA (120-140 USD) measuring C with accuracy of +/- 0,005 pF at frequency of choice up to 3 GHz with simultaneous acquiring the parasythic L and R (=ESR and ESL) what is capable to draw a graph how those three parameters depend of wide band frequency and where have a resonant peak. If You pay me a 10K USD I would rethink three times is it worth to take such building task, but there it have a price of one my work hour.... Do You are a well hidden underground millionaire if spend a time so bashfully??
P.S.: However when I stucked 16 age at some 1975, I indeed built a capacitance meter (when nowhere nothing was available around) and even it was chosen to exponate at Moscow VDNH exhibition in schoolsters section. So I got my first travel to russia alone without of parents, bought my first foreign cigarettes pack (Los Carribos was the name - oh fool, how much efforts demanded later to get off the hook of death), got my first glass of alcohol (russia have such national refresment drink tradition) etc etc. At least from that point I like to travel, but never are designing any capacitance meters
A while back there were plans for a femto-farad meter that was accurate. And that cost far less to build than the commercial equivalent. Certainly you can buy a toy meter that is neither stable nor accurate and not at all durable enough to last a week. And that unit will not have any parts that you can replace except the battery.

#### Janis59

Joined Aug 21, 2017
1,221
Actually accuracy is ca 0,5% and better be other in diagram above but I may agree if the femtofarads are the goal that circuit is no bad. However - nowadays everybody are looking for digitalized output not a reading the galvanometer scale. But any discussion about the fashion is aimless (always, in all contexts).

#### bob2

Joined Jun 15, 2019
194
Have no idea of appropriatness of building ANY capacitance meter nowadays when for 5 USD may obtain already done from ebay with built-in component identificating microprocessor able to get out 0,1 pF to 100 000 mkF with accuracy of 0,1 pF
And do you believe that? Does the change of the wind affect the measurement?

#### DickCappels

Joined Aug 21, 2008
6,580
If it is a zero to 200 pf difference in capacitance that you are looking for, that is a whole lot easier than measuring 0 pf of capacitance.

Build and oscillator that uses a capacitor as one of the frequency determining components, built it but with using a fixed capacitor in parallel with the capacitance you are going to measure.

In the calibration phase measure the oscillator's frequency without the test capacitance connected. Call that the frequency resulting in the capacitive offset.

Then, connect your test value and measure the frequency again and convert that to capacitance.

From the two frequencies you can figure how much capacitance change there was.

You can view these web pages for ideas:
http://www.cappels.org/dproj/EvenBetterLCMeter/Even_Better_LC_Meter.html

#### bob2

Joined Jun 15, 2019
194
Measurement of picofarad units is a fix idea that has no practical application except for some capacitive sensors operating on the appropriate equipment with compensation for probes and other related factors.
The Chinese industry produces primitive crafts such as LC100, which in practice are not even capable of measuring a 1pF capacitor.

#### MisterBill2

Joined Jan 23, 2018
6,997
Measurement of picofarad units is a fix idea that has no practical application except for some capacitive sensors operating on the appropriate equipment with compensation for probes and other related factors.
The Chinese industry produces primitive crafts such as LC100, which in practice are not even capable of measuring a 1pF capacitor.
Clearly this person is not familiar with higher frequency RF designs. Capacity values in the gigahertz bands are quite small.

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#### bob2

Joined Jun 15, 2019
194

#### MisterBill2

Joined Jan 23, 2018
6,997
And the FemtoFarad Meter was created by a person working on some kind of filters. I don't know if the post is still available, that was about 10 years back. I will check.

#### NAGARAJ_JP

Joined Aug 21, 2020
17
If it is a zero to 200 pf difference in capacitance that you are looking for, that is a whole lot easier than measuring 0 pf of capacitance.

Build and oscillator that uses a capacitor as one of the frequency determining components, built it but with using a fixed capacitor in parallel with the capacitance you are going to measure.

In the calibration phase measure the oscillator's frequency without the test capacitance connected. Call that the frequency resulting in the capacitive offset.

Then, connect your test value and measure the frequency again and convert that to capacitance.

From the two frequencies you can figure how much capacitance change there was.

You can view these web pages for ideas: