The only relatively constant term would be gate capacitance, ESD and MOS body diode capacitances will be all over the place with temperature.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Capacitance measurements may also be influenced by the series ON resistance of the switch. The Ron of a switch can vary depending upon the voltage level of signal so you may also need to take this factor into account.
Yep, I like it! Like a Weatstone Bridge arrangement.....
In Telecom POTS.... to convert 2wire to 4 wire... a Weatstone bridge arrngement is often used to cancel the local Tx audio from going in to the Rx local.
Consider going from coach to business class, for example to a device like the SD5400. They have become expensive and hard to procure but for a test station that doesn't enter mass production might be worth a look.
Heed John's advice. Clamping away drifty capacitance by monitoring one unused section on the same chip is the way to go, just like you'd clamp out Rdson if that were a concern.
Last but not least, if this is a test station where space and power consumption might not be of great concern, how about putting the head assy in a heated box? Doesn't have to be cooking, just far enough above the highest expected temperature. Peltier regulating the chip is another option but now it becomes esoteric. If you wanted to go that route you might be able to scavenge parts from a laser diode arrangement.
There's nothing all that esoteric about Peltier junctions. Analog Devices and Linear Technology both sell chips designed to drive them - ADN8830 and LTC1923. They don't offer great stability (only +/-0.01C) and they don't work all that well over a wide temperature range, but they'd be fine in this application.
For more detail, and some references to the literature, see my comment in Rev.Sci. Instrum.
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The Linear Technology application ote is pretty good, but ti doesn't include the formula for calculating the heat transferred (in joules per amp) of the Peltier junction, which I did include in my comment. Jim Williams claimed that every equation in an application note halves the number of readers - I not so sure that this applies to application notes.
I know but you'd have to get the peltiers which means that you might have to butcher a laser diode assembly. Nothing wrong with that but this tends to be expensive.
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Nah, but the number of multiple integrals with lots of summation terms behind them does. Because usually those articles remain too far in the theoretical realms. Or are the usual products of the publish-or-perish folks. Same for invoking old Maxwell too many times ;-)
How did you arrange your measuring setup. It's very possible to make it insensitive to parasitics to ground.
Still, if your setup is sensitive to switch parasitics, then it is also to your cable capacitance, which will also vary... So your best bet is to use a setup free from parasitics dependency.
Keep in mind that PCB material also changes dielectric constant with both temperature and humidity. So, if possible, you might want to mimic a typical layout for the test switch , so you get a change to compensate that, also. To get really fancy, you could have one test switch that has minimal trace area, and one with lots, and by test in a temperature and humidity chamber, calculate the optimum correction factors for each other switch as a combination of these two.
Farnell stocks a number of parts from the Marlow range of Peltier junctions - 6W to 71W capacity, for from about $50 to $75 in small numbers. You would be able to get them via Farnell.
There is at least one website that offers direct ordering
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Buying a laser diode assembly for the Pelteir junction probably doesn't represent the optimum procurement strategy.
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The formula in my comment doesn't include any integrals - it is just a simple quadratic. It isn't all that accurate, but it worked well enough in the project for which I had to work it out, and I think we ended up shipping a fair number - of the order of hundreds rather than tens - of that system.
It's written up in Measurement Science and Technology - volume 7, pages
1653-1664 in the November 1996 issue.
There isn't a lot of theory in the article - no more than I needed to explain why we designed the system the way we did - and I published it because I didn't have enough to do at the time. My chance of getting work doesn't seem to be dependent on my publication record - and in fact the Dutch organisation for space research (Stichting Ruimte-Onderzoek Nederland,
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who regularly advertise for electronic engineers to do instrument development, actually refused to interview me on the grounds that the sort of hands-on engineers that they wanted didn't publish papers in refereed journals. They can't have read the papers.
I suspect that what this really meant was that they want younger, cheaper and more biddable engineers, but when pressed they claimed to have a 58-year-old on their staff ...
If you want a copy of the article, e-mail me - my address is real.
Absolutely *not*. Use the system to measure its own capacitance (at a given temperature) before using it to measure attached unknowns, and use software to calculate the difference. That capacitance should be reasonably repeatable (ie little hysteresis) as long as the ICs in the instrument are not changed.
Not a significant factor; what is worse in using a FET as a switch for commutating an input to a mesuring system, is the fact that the applied gate voltage will show up (attenuated by capacitive division) in the source and drain.
Duh! The gate oxide is GLASS... so its changes are quite small.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
I did a 128-channel capacitor measurement test set, not in the pF range but it doesn't matter.
What proved most useful was dedicating two channels to reading back Cstray and (Cref+Cstray), (where Cref was about 50% of full scale). This allowed an autocalibration immediately before each scan of the C-unknowns.
We have walked the same 100-odd channel UUT around the factory, across the 5 test sets. I always have a little gloat at the consistency between test sets.
If the design is ever revisited I might look at changing the two reference channels to (Cref+Cstray) and (2*Cref+Cstray), where 2*Cref would be about 60% of full scale.
No, but the gate capacitance varies trivially compared to the surrounding junctions.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Pretty steep pricing. When having to stabilize oscillators or phase shifters (usually also C-drift issues) I mostly opted for the heating-only method. An insulated housing, heat sensing and a big resistor would keep things rock stable for a Dollar or two.
Only for experiments. Not for production, of course.
Sometimes when looking for a job people over here leave achievements off their resume. It's sad but it might be the only way to avoid being weeded out as "over-qualified". In general age discimination is happening and can't be legislated away, doesn't work. However, some companies have not fared well retaining only the young ones. They just don't have the experience that is often needed.
Another thing to watch is the substrate, for chips where it is brought out to a pin. During many of my consulting trips I found that much of the grief was rooted there. The substrate was connected to "somewhere" and in one case the pin was left open...
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