CMOS analogues switch capacitance
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From: Robert Baer on 15 Oct 2006 04:47 Jim Thompson wrote: > On Sun, 15 Oct 2006 04:09:17 GMT, Robert Baer > <robertbaer(a)earthlink.net> wrote: > > >>Jim Thompson wrote: >> >> >>>On Sat, 14 Oct 2006 11:28:57 -0400, John Popelish <jpopelish(a)rica.net> >>>wrote: >>> >>> >>> >>>>Joe G (Home) wrote: >>>> >>>> >>>>>Hi All, >>>>> >>>>>I am measuring capacitance in the pF range and I need to switch monitoring >>>>>inputs. >>>>> >>>>>We can calibrate out any constant capacitance.. However >>>>> >>>>>The 64,000 dollar question is would you expect the input capacitances of the >>>>>CMOS switch to be constant over tempreature? >>>>> >>>>>The devices I have looked at are the better versions of the CMOS 4053 etc >>>>>... a few from NXP (formerly Philips) >>>>>http://www.standardics.nxp.com/products/switches/ >>>>> >>>>> >>>>>While there are input capacitance specifications..... there is no spec on >>>>>the relationship between input capacitance vs Temp. >>>>> >>>>> >>>>>Would you expect input capacitance to be fairly constant over the operating >>>>>temp range? >>>> >>>>The short answer is that, unless someone has taken great >>>>pains to suppress it, everything changes with temperature. >>>> >>>>I would use an extra section of switch to measure the >>>>capacitance of a switch, and subtract the changes of that >>>>that from all the other measurements. >>> >>> >>>John, _Very_good_suggestion_! >>> >>>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 >> >> DEfine "gate capacitance"; i think it is subject to changes over >>temperature. > > > Duh! The gate oxide is GLASS... so its changes are quite small. > > ...Jim Thompson Are you trying to tell me that glass does not expand and contract over temperature???
From: Tony Williams on 15 Oct 2006 05:24 In article <4530f567$0$11972$afc38c87(a)news.optusnet.com.au>, Joe G \(Home\) <joe.g(a)optusnet.com.au> wrote: > I am measuring capacitance in the pF range and I need to switch > monitoring inputs. > We can calibrate out any constant capacitance.. However 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. -- Tony Williams.
From: Jim Thompson on 15 Oct 2006 10:51 On Sun, 15 Oct 2006 08:47:27 GMT, Robert Baer <robertbaer(a)earthlink.net> wrote: >Jim Thompson wrote: >> On Sun, 15 Oct 2006 04:09:17 GMT, Robert Baer >> <robertbaer(a)earthlink.net> wrote: >> >> >>>Jim Thompson wrote: >>> >>> >>>>On Sat, 14 Oct 2006 11:28:57 -0400, John Popelish <jpopelish(a)rica.net> >>>>wrote: >>>> >>>> >>>> >>>>>Joe G (Home) wrote: >>>>> >>>>> >>>>>>Hi All, >>>>>> >>>>>>I am measuring capacitance in the pF range and I need to switch monitoring >>>>>>inputs. >>>>>> >>>>>>We can calibrate out any constant capacitance.. However >>>>>> >>>>>>The 64,000 dollar question is would you expect the input capacitances of the >>>>>>CMOS switch to be constant over tempreature? >>>>>> >>>>>>The devices I have looked at are the better versions of the CMOS 4053 etc >>>>>>... a few from NXP (formerly Philips) >>>>>>http://www.standardics.nxp.com/products/switches/ >>>>>> >>>>>> >>>>>>While there are input capacitance specifications..... there is no spec on >>>>>>the relationship between input capacitance vs Temp. >>>>>> >>>>>> >>>>>>Would you expect input capacitance to be fairly constant over the operating >>>>>>temp range? >>>>> >>>>>The short answer is that, unless someone has taken great >>>>>pains to suppress it, everything changes with temperature. >>>>> >>>>>I would use an extra section of switch to measure the >>>>>capacitance of a switch, and subtract the changes of that >>>>>that from all the other measurements. >>>> >>>> >>>>John, _Very_good_suggestion_! >>>> >>>>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 >>> >>> DEfine "gate capacitance"; i think it is subject to changes over >>>temperature. >> >> >> Duh! The gate oxide is GLASS... so its changes are quite small. >> >> ...Jim Thompson > Are you trying to tell me that glass does not expand and contract >over temperature??? 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.
From: Joerg on 15 Oct 2006 16:57 Hello Bill, >> >>>>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. >>> >> >>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. > > > 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 > > http://www.quick-cool.com/shop/formular.php > 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. > Buying a laser diode assembly for the Pelteir junction probably doesn't > represent the optimum procurement strategy. > Only for experiments. Not for production, of course. > >>>For more detail, and some references to the literature, see my comment >>>in Rev.Sci. Instrum. >>> >>>http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=RSINAK000075000003000788000001&idtype=cvips&prog=normal >>> >>>The Linear Technology application note is pretty good, but it 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. >>> >> >>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 ;-) > > > 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, http://www.nnv.nl/NNV/CORE/00/00/0.HTML) > 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. > 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. -- Regards, Joerg http://www.analogconsultants.com
From: Chris Jones on 15 Oct 2006 18:02
Jim Thompson wrote: > On Sat, 14 Oct 2006 11:28:57 -0400, John Popelish <jpopelish(a)rica.net> > wrote: > >>Joe G (Home) wrote: >>> Hi All, >>> >>> I am measuring capacitance in the pF range and I need to switch >>> monitoring inputs. >>> >>> We can calibrate out any constant capacitance.. However >>> >>> The 64,000 dollar question is would you expect the input capacitances of >>> the CMOS switch to be constant over tempreature? >>> >>> The devices I have looked at are the better versions of the CMOS 4053 >>> etc >>> ... a few from NXP (formerly Philips) >>> http://www.standardics.nxp.com/products/switches/ >>> >>> >>> While there are input capacitance specifications..... there is no spec >>> on the relationship between input capacitance vs Temp. >>> >>> >>> Would you expect input capacitance to be fairly constant over the >>> operating temp range? >> >>The short answer is that, unless someone has taken great >>pains to suppress it, everything changes with temperature. >> >>I would use an extra section of switch to measure the >>capacitance of a switch, and subtract the changes of that >>that from all the other measurements. > > John, _Very_good_suggestion_! > > 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 One other thing (that you would know well) is that the gate capacitance of the MOS devices will depend strongly on the bias (VGS) so it is important to measure the calibration channel and the measurement channel with the same DC bias and about the same signal amplitude on the switch pins. Chris |