Question about Capacitance to frequency conversion

[ssstern]

I am looking into converting 9pF to 22pF capacitance to frequency data. So I started doing research on Oscillator. I have area constraint since I am trying to do it on CMOS technology. The frequency can't be too high. preferable under 100kHz. Inductor is too big for CMOS and due to the low capacitance,low frequency and low area constraint, I am not sure what I should do.

I looked at normal RC oscillator and the Resistor needs to be in Mega ohms range if I want a low frequency. Does anyone has any suggestion?

 

[Skogsgurra]

A schmitt trigger with C between input and ground. A resistor from output to input usually gives adequate results. If you need to go to megohms, then just do it. CMOS input currents are in the picoampere range, so megohms will be OK. Just make sure you don't get too much dirt and humidity on your board.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ssstern]

We were using schmitt trigger with C between input and ground. We would like to know if there is any other possible way avoiding use huge resistor and inductor in our circuits. I am wondering if converting Capacitance to Voltage then use a ring oscillator, or use some sort of VCO that requires cap so depends on the change, we can read out the C.

 

[Skogsgurra]

The other obvious way is to connect a binary divider and get as low a frequency as you want. Why are you using an inductor?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ssstern]

We were not using inductor, I was just reading some papers and most people were using indcutors for LC oscillator. Thanks for the answer, I would see if binary divider will solve our problem.

 

[itsmoked]

Check V to F converter applications notes. Burr Brown makes them.

Consider using something like the CMOS 555 for the converter.

Remember that the C to F converter you are designing must be with the capacitor as you cannot have the wire leads in the picture.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ssstern]

How about replace a switch capacitor to a 1 Meg resistor in the schmitt trigger, would that work?

 

[sreid]

I would get a book such as the following to start with.

http://www.amazon.com/Capacitive-Se...r_1_62?ie=UTF8&s=books&qid=1217018572&sr=1-62

Then I'd start with the circuitry to measure the capacitance (High Frequency Bridge?) and use that value to drive some kind of FM oscillator (which could be all digital).