Measuring femto Farads - Page 1
Forum section: electronics / beginners

author: michaelym
author_rank: Regular Contributor
timestamp: March 12, 2014, 01:17:10 pm
content: What is the simplest way to measure femto farads.Can I do it by using a timer and microcontroller port? Do I need an ADC?

author: EEVblog
author_rank: Unknown
timestamp: March 12, 2014, 01:21:44 pm
content: How many fempto farads? To what accuracy? To what resolution?The Agilent U1733C LCR meter has 1fF resolution on its lowest 20pF range at 100KHz.I demoed this in the breadboard video.Although without proper shielded leads if you fart half away across the room the reading changes.

author: michaelym
author_rank: Regular Contributor
timestamp: March 12, 2014, 01:25:21 pm
content: I want to make a simple pressure sensor using a membrane as one plate and the PCB as the second plate. And I want to measure the capacitance on board without any external gear.

author: rs20
author_rank: Super Contributor
timestamp: March 12, 2014, 02:05:48 pm
content: You still didn't answer Dave's question. Accuracy? Resolution? Range?For example: "What's the simplest way to measure kilograms?" Do you mean weighing cars accurate to a kilogram, or weighing a one kilogram object with higher accuracy? A proper spec is, I want a scale that measure up to 10 kg, with 1 gram resolution, accurate to +/- 10 grams.If you had a 1fF capacitor, 1 microamp divided by 1 femtofarad gives 1 GV per second, which means the cap will charge up to 5V in 5 nanoseconds. I'm personally not a fan of going below one microamp*, and microcontrollers aren't going to routinely measure such short time periods.I've been intrigued by the idea of using a inverting Schmidt trigger to form a simple RC oscillator, even if it whines at 200MHz (1/5ns), you could use a clock divider to get something a uC could understand. Extremely high input impedances will of course be required on the capacitor net to prevent your cap charging currents from being swamped.* I know such blanket rules are dumb, so someone with more experience may be able to help you break this barrier.

author: michaelym
author_rank: Regular Contributor
timestamp: March 12, 2014, 02:52:13 pm
content: Quote from: rs20 on March 12, 2014, 02:05:48 pmSo i'm better off increasing my capacitance range to around 1pF (I can just increase the area of my capacitor).Lets say for example I want to measure 1-50pF with an accuracy of 2 pF, how do I do it?Thanks

author: edavid
author_rank: Super Contributor
timestamp: March 12, 2014, 03:27:07 pm
content: You buy one of these, or more likely an evaluation board for one of these:http://www.analog.com/en/analog-to-digital-converters/capacitance-to-digital-converters/products/index.html

author: Zero999
author_rank: Super Contributor
timestamp: March 12, 2014, 11:00:57 pm
content: Using the capacitor as part of an LC oscillator would provide you with a frequency dependant on the capacitance.

author: tstanford
author_rank: Newbie
timestamp: March 13, 2014, 03:52:32 pm
content: Quote from: Hero999 on March 12, 2014, 11:00:57 pm^This.Ping the tank circuit with a square pulse, pass it through a zero crossing detector comparator, rectify it, bam you can measure period/frequency on a mcu. Just set the rails of the op amp at your +- 3.3/5v. I did this to make a cheap inductance meter to measure some coils we had laying around the lab and i just used a 741, worked alright. Works better if you use a good comparator.

author: edavid
author_rank: Super Contributor
timestamp: March 13, 2014, 05:00:00 pm
content: The oscillator approach seems impractical to me. If the OP builds his capacitor at a practical size, it will be only 10-20pF. Why not use the $2 Analog Devices chip that is designed for this application? (Or better yet, buy a cheap pressure sensor.)

author: Zero999
author_rank: Super Contributor
timestamp: March 14, 2014, 07:12:00 pm
content: The advantage of the oscillator approach is very tiny capacitance values can be measured but you need to run at VHF for decent results with very small capacitances.An oscillator running at 120MHz with an 100nH inductor, will give a frequency deviation of +/-2MHz for a change of +/-100fF.The oscillator can be placed on the same board as the sensor. and the VHF signal can be sent down a co-axial cable, along with the power if need be then heterodyned to a lower frequency before processing.

author: rs20
author_rank: Super Contributor
timestamp: March 14, 2014, 11:54:12 pm
content: Quote from: Hero999 on March 14, 2014, 07:12:00 pmThat's fairly poor performance compared to the CDC chips, which have resolution down in the attofarads.Quote!Just throwing it out there; what's wrong with forming a capacitance divider with another similar capacitor? Feed square wave to the divider, measure the voltage after the divider. Indeed, measuring voltage is more difficult to do accurately than measuring frequency, but I suspect wrangling hundreds of MHz is even harder.Also, why is an LC oscillator better than this:?Code:10M.-----/\/\/\/\------.| || .-----------. || | Inverting | | Square wave out,|---| Schmidtt |o-------- frequency inversely proportional to C| `-----------'------ Capacitor being measured|GND

author: edavid
author_rank: Super Contributor
timestamp: March 15, 2014, 04:01:10 am
content: An RC oscillator is even less stable than an LC oscillator