CMOS analogues switch capacitance
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From: Joerg on 14 Oct 2006 13:16 Hello Joe, > 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? > No :-( 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. -- Regards, Joerg http://www.analogconsultants.com
From: bill.sloman on 14 Oct 2006 14:07 Joerg wrote: > Hello Joe, > > > > 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? > > > > No :-( > > 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. http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=RSINAK000075000003000788000001&idtype=cvips&prog=normal 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. -- Bill Sloman, Nijmegen
From: Joerg on 14 Oct 2006 14:46 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. > 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 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. > 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 ;-) -- Regards, Joerg http://www.analogconsultants.com
From: Fred Bartoli on 14 Oct 2006 16:24 Joe G (Home) a ?crit : > 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 > 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. -- Thanks, Fred.
From: John Popelish on 14 Oct 2006 17:01
Joe G (Home) wrote: > <snip>> > >>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. > > > 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. > > http://en.wikipedia.org/wiki/Wheatstone_bridge 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. |