Sub-femtofarad capacitance measurement - Page 1
Forum section: electronics / testgear

author: austere
author_rank: Contributor
timestamp: May 22, 2014, 09:47:35 pm
content: Hi guys, I was wondering if there's an existing bench top or portable device that is capable of measuring capacitance of around 20fF-1pF with a precision of 0.1fF or better? There are a lot of designs floating around the internet for attofarad measurements and a paper on zeptofarad measurements, but they seem like they can only applied with monolithic integration. I'm working on something application specific in that department for my PhD but until that is complete, discrete prototyping would be greatly assisted by a known-working device.I bought the LC200A just to have one more measurement for increased confidence, but sadly 0.01pF just won't cut it.

author: larry42
author_rank: Regular Contributor
timestamp: May 22, 2014, 10:09:36 pm
content: Maybehttp://www.andeen-hagerling.com/?About 20k USD according to the internet

author: IanB
author_rank: Super Contributor
timestamp: May 22, 2014, 10:13:12 pm
content: What is the capacitance of hookup wiring or traces on a PCB? When capacitance is low enough the stray capacitance of the system will be greater than the capacitance you wish to measure.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: May 22, 2014, 10:21:09 pm
content: Hi,I have designed several circuits that have similar parameters.They are used in Electrical Capacitance Tomography systems.Please contact me on priv. and describe Your problem.

author: austere
author_rank: Contributor
timestamp: May 23, 2014, 02:29:33 am
content: Quote from: larry42 on May 22, 2014, 10:09:36 pmThe price is quite reasonable given what this thing is capable of, not sure how it would be operated and the DUT connected to it. Compared to Agilent's (Keysight? lol) top of the line LCR meter it's a bargain.Quote from: IanB on May 22, 2014, 10:13:12 pmYeah that's a given, by manually wire bonding I can get the capacitance down to about 500fF, see attached image. I'd have to lose the connector to get down to that level though, which would be inconvenient. Going to do a measurement the moment the bond wire anneals (not shown in photograph, that was before I stuck the die on). I could lower the thing ever further by fabricating on BK7 but those substrates are pretty expensive.Quote from: doktor pyta on May 22, 2014, 10:21:09 pmThanks for your offer, I'll PM you shortly.

author: georgd
author_rank: Regular Contributor
timestamp: May 23, 2014, 07:14:12 am
content: ACAM Mess Electronic produce capacitance to digital converters with quite impressive resolution.http://www.acam.de/products/picocap/Quote from datasheet:High resolution: up to 15 aF at 2.5 Hzand 10 pF base capacitance or, 17 bitresolution at 5 Hz with 100 pF basecapacitance and 10 pF excitationGeorg

author: edavid
author_rank: Super Contributor
timestamp: May 24, 2014, 05:55:30 am
content: Any reason why you couldn't use one of these?http://www.analog.com/en/analog-to-digital-converters/capacitance-to-digital-converters/ad7745/products/product.html

author: austere
author_rank: Contributor
timestamp: May 24, 2014, 06:49:18 am
content: That's precisely what I will use now, it was kindly suggested by doktor pyta. Good to note for the thread though!

author: branadic
author_rank: Super Contributor
timestamp: May 24, 2014, 11:29:02 am
content: My suggestion is, if you want to keep it simple and have no capabilities in signal processing just go for the chips from AD such as AD7745, AD7746 or AD7747, depending on how you want to measure.If you like programming and signal processing go for the PCap0x. The PCap series includes a DSP so that you can implement signal correction. PCap02 is somewhat worse in resolution, but was designed for capacitive MEMS sensors. PCap03 was announced for last year, engineers samples have been available as far as I now, but they canceled their announcement. The new info is that they jump over to PCap04 with better resolution and some other differents in specs. All these chips are not easy to handle and the standard firmware that comes with them by ACAM is, well... Setting up a working communication between µC and the CDCs is not easy, thanks to the worde datasheet. But if once done you'll be happy with the chips and their features they come with. Writing your own firmware for the DSP is also hard, this is done in Assembler. If you want to go for the evalkit, this is about 250€! The chips are cheaper, but harder to use.