CMOS analogues switch capacitance
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From: bill.sloman on 14 Oct 2006 19:14 Joerg wrote: > 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 Buying a laser diode assembly for the Pelteir junction probably doesn't represent the optimum procurement strategy. > > 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. -- Bill Sloman, Nijmegen
From: Robert Baer on 15 Oct 2006 00:07 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? > > > Regards > Joe > > > 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.
From: Robert Baer on 15 Oct 2006 00:09 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.
From: Robert Baer on 15 Oct 2006 00:11 mkaras 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. >> >> > > > 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. > > - mkaras > > >>Would you expect input capacitance to be fairly constant over the operating >>temp range? >> >> >>Regards >>Joe > > 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.
From: Jim Thompson on 15 Oct 2006 00:28
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 -- | 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. |