Robert Baer wrote:> Phil Hobbs wrote: >> Robert Baer wrote: >>> Phil Hobbs wrote: >>>> Ebrahim wrote: >>>>> Robert Baer wrote: >>>>>> Ebrahim wrote: >>>>>> Do not think BJTs in general are that good; try an LED... >>>>> >>>>> Hello Robert :) >>>>> Thanks for replying to my post. I have not measured leakage current >>>>> of an LED but it makes sense because GaAs Reverse saturation >>>>> current is about 10 times less than Si. Have you used LED as an >>>>> ultra low leakage diode ? have you measured leakage currents of LEDs? >>>>> >>>>> Regards, >>>>> Ebrahim >>>> >>>> It's far lower than that. I've measured ordinary red display LEDs >>>> whose leakage was below 100 fA from -5V to +0.5V bias. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>>> >>> Hah! >>> That shows red LEDs to be far superior to white ones (where i got >>> <3nA @ 4V reverse); the "cost" prolly means that it must be protected >>> from all ambient light. >> >> White LEDs are made of GaN--not the world's greatest semiconductor. >> >> Cheers >> >> Phil Hobbs > Sorry; that is all i had for testing.Not a crisis. I used this trick ten years or so ago in the Footprints system, which reduced the cost of a low-resolution thermal imager by two orders of magnitude while maintaining competitive performance. It took advantage of the photosensitivity of the LEDs as well, to supply a few picoamps of bias current under processor control. Ironically, if it had been only 0.5-1 order of magnitude cheaper instead of two, it would have sold much better. Some people won't let you save them money. :( (I've mentioned it here a few times before--for anyone who isn't tired of hearing about it, the gory details are at http://electrooptical.net/www/footprints/fpspie11.pdf.) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
BJTs as ultra low leakage protection diodes
Started by ●June 12, 2009
Reply by ●June 14, 20092009-06-14
Reply by ●June 14, 20092009-06-14
On Jun 14, 11:44=A0am, John Larkin <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:> On Sun, 14 Jun 2009 03:46:42 -0700 (PDT), Winfield Hill > > <h...@rowland.org> wrote: > >On Jun 14, 12:32=A0am, "Phil Allison" <phi...@tpg.com.au> wrote: > >> "Ebrahim" > > >>> I did not understand what is your mean. will you explain more please =?> > >> ** You are a totally stupid pile of autistic shit, a waste of space a =waster> >> of good people's valuable time. > > >> =A0 =A0Fuck off and DROP DEAD =A0!!! > > > Ahem. =A0Well, in 32 posts, has anyone answered Ebrahim's > > question? > > Yes, I did. > > =A0Which was, does a C-B junction make a good > > > low-leakage high-voltage diode, and my answer is yes. > > Likely as low as 10pA, but I'd want to check specific parts. > > ftp://jjlarkin.lmi.net/BFT25.JPG > > The c-b capacitance of the BFT25 is around 0.5 pF. > > The forward and reverse curves should converge around zero volts; I > think I may have had 20 or 30 fA of offset in my test rig. > > I've used 2N4402's below 1 pA... not sure how much below. > > > > > It's the same question one asks before using a transistor at > > extremely low collector currents. =A0OK, not too many people > > use BJT transistors at 10pA, for one thing they're very slow, > > but it's surprising how well they work down in that territory. > > > Often when doing this trick, one wants very low capacitance, > > and I've been disappointed with the high capacitance of the > > usual candidates, e.g., 5pF. =A0This is what steers people > > back to the old PAD-1 beauties, spec'd at 0.5pF and 0.3pA > > The PAD parts are tiny jfets, so have high channel resistances, a few > K typically. So they're not very hard clamps. > > > > > When one is in this pA territory, PCB leakage can be a big > > problem, and teflon posts and in-air wiring are often used. > > Did you see my fA tester box? > > ftp://jjlarkin.lmi.net/99S260A.JPGftp://jjlarkin.lmi.net/99A260A3.JPGftp:=//jjlarkin.lmi.net/99A260A1.JPG> > ftp://jjlarkin.lmi.net/PAD5.JPG > > JohnVery nice, John. Excellent!
Reply by ●June 14, 20092009-06-14
Ebrahim wrote:> Phil Hobbs wrote:<snip>>> The diode needs almost no breakdown voltage at all--certainly not >> 100V. Something like a BFT25 B-C junction (about 0.3 pF) will work >> great. (WDNNS PAD-1!) >> > *** > That's an interesting idea, you actually bootstrapped the first diode. > Are you sure that any diode can do this? I mean I think I have done > some measurements with 1N4007 and even in ~0 volt, its leakage was high. > I don't remember the exact numbers, though I must find my papers.Rectifiers intended for 60 Hz applications are very slow, both slow to turn off and (interestingly) slow to turn on. They also have lots of capacitance. If you're doing something slow, and working at very low impedance levels, then a 1N4007 could be the right part in this circuit. Personally if it was as slow as that I'd probably be wanting to use a Polyfuse and a Transzorb. A diode has a nonzero conductance at zero bias--for an ideal diode, the zero-bias conductance is Is*kT/e, as you can easily verify by differentiating the diode equation. Bootstrapping will reduce the current, but won't reduce the noise. Pick something sensible--you've had several suggestions--and build something. Then tell us how it worked. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●June 14, 20092009-06-14
On Jun 14, 6:51=A0am, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote:> Ebrahim wrote: > > Winfield Hill wrote: > >> =A0Ahem. =A0Well, in 32 posts, has anyone answered Ebrahim's > >> =A0question? =A0Which was, does a C-B junction make a good > >> =A0low-leakage high-voltage diode, and my answer is yes. > >> =A0Likely as low as 10pA, but I'd want to check specific parts. > > >> =A0It's the same question one asks before using a transistor at > >> =A0extremely low collector currents. =A0OK, not too many people > >> =A0use BJT transistors at 10pA, for one thing they're very slow, > >> =A0but it's surprising how well they work down in that territory. > > >> =A0Often when doing this trick, one wants very low capacitance, > >> =A0and I've been disappointed with the high capacitance of the > >> =A0usual candidates, e.g., 5pF. =A0This is what steers people > >> =A0back to the old PAD-1 beauties, spec'd at 0.5pF and 0.3pA > > >> =A0When one is in this pA territory, PCB leakage can be a big > >> =A0problem, and teflon posts and in-air wiring are often used. > > > ***Hello Winfield :) : > > Thank you very much for your technical and non-political answer. > > So it makes sense to look for such a candidate among high frequency > > transistors that should have low capacitance. > > > Thank you again. > > Best wishes for you, > > Ebrahim > > The part of this that makes zero sense is the 100V part. =A0Almost all op > amps, in fact AFAIK every single one ever sold, has PN junctions between > its inputs and its supply pins.(*) =A0That's why the Absolute Maximum > Ratings section of datasheets specifies that you mustn't take their > inputs more than 0.3V outside the supplies. > > Not everything on an IC maps well onto a schematic--e.g. schematics > don't show the parasitic SCR inherent in junction-isolated CMOS > processes. =A0It's there, though, as you'll find if you try dumping 20 mA > into an input. > > If you want the same sort of function provided by the input protection > diodes, without a big leakage problem, you can use something like this: > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A00 =A0VDD > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0_ > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0A > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| > =A0 =A0 =A0 =A0 =A0 =A0*---Ri------*--------Rf-----------* > =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 =A0 |> =A0 =A0 =A0 =A0 =A0 GGG =A0 =A0 =A0 =A0 =A0_ =A0 =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 |> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0A =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 =A0 |> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 =A0 |> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 |\ ==A0 =A0 =A0 =A0|> 0-------------RRRRR---*---RRRRR---|+\ =A0 =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| ==A0\ =A0 =A0 =A0|> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| ==A0 >---*-*---0> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0*---|- / ==A0 =A0|> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 | / ==A0 =A0 |> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 |/ ==A0 =A0 =A0|> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 ==A0 =A0 =A0 |> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0*---Ri------*---Rf------* > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 GGG > > (Noninverting shown because inverting is reasonably obvious, and > negative polarity left as an exercise for the reader.) =A0You have to > scale the resistances to fit your problem. =A0The idea is that the first > diode has nearly no voltage across it, and hence no leakage or > capacitive current. =A0This isn't a complete solution, because it costs > you SNR eventually, but it's a lot better than nothing. > > The diode needs almost no breakdown voltage at all--certainly not 100V. > =A0 Something like a BFT25 B-C junction (about 0.3 pF) will work great. > (WDNNS PAD-1!) > > Cheers > > Phil Hobbs > > (*) Some chips, e.g. old CMOS->TTL converters use Zeners instead, > because their inputs are intended to be overdriven by volts. =A0Also, som=e> rail-to-rail input op amps use charge pumps to allow them to run their > input stages on more than VDD-VSS. > > -- > Dr Philip C D Hobbs > Principal > ElectroOptical Innovations > 55 Orchard Rd > Briarcliff Manor NY 10510 > 845-480-2058 > hobbs at electrooptical dot nethttp://electrooptical.netI'm not so sure about zener input protection. Generally when you need to go above the rail, you just use a N mosfet with gate tied to source and the drain going to the pad. You depend on the breakdown of the fet to provide input protection. [If you are trying this in layout, antenna rules may apply.] Every time I investigated chip damage after ESD testing, it was always due to over-voltage versus over current. I'm not a fan of high fields (as found in reverse bias) in semiconductors. They seem to find the weak spot in the junction and zap it. Forward biased junction might have hot spots, i.e. current hogging, but in general they are pretty rugged.
Reply by ●June 14, 20092009-06-14
On Jun 13, 9:13=A0pm, Ebrahim <Engineer....@gmail.com> wrote:> m...@sushi.com wrote: > > Getting back to the basic question, breakdown voltages are only > > guaranteed at a minimum. Suppose you get a device that has exceedingly > > good breakdown? > > I did not understand what is your mean. will you explain more please ? > > Regards, > EbrahimA manufacturer sells a device that passes a minimum breakdown. It is a one sided specification. That is, a minimum voltage is specified, but not a maximum. Maybe the breakdown voltage will be much higher than where you want it to be. I see all sorts of ideas in this forum on how to cheat and make a part do something that it is not intended to do. Fine, but you are on your own. I've seen specifications of parts change wildly over the years. Case in point was a SC filter which had a pretty good THD. The THD was not a guaranteed specification, but there was an internal spec to make sure things didn't go wild. Well somebody changed epi vendors, and the THD went about the internal limit. Since the parts didn't have a THD spec, they went out into the public, though the old epi vendor was used again. If you want a bullet proof design, take every line item without limits and blot them out with a sharpie, then go design.
Reply by ●June 14, 20092009-06-14
On Jun 13, 1:54=A0pm, John Larkin <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:> On Sat, 13 Jun 2009 21:48:27 +0430, Ebrahim <Engineer....@gmail.com> > wrote: > > > > > > >Phil Hobbs wrote: > >> Ebrahim wrote: > > >> What op amp have you ever seen that didn't, and how do you know? =A0Su=ch a> >> device might be quite interesting in applications, but I don't know of=one.> > > Even femtoamp leakage devices (e.g. the venerable LMC660) have > > > protection diodes. =A0You can use them for resetting integrators, by > > > briefly inverting the power rails (with suitable current limits). =A0=Win> > > Hill posted a circuit for that in this very group a few years back. > > >*** > >Hello Phil > > >Look at data sheet of OPA627, It has shown equivalent schematic. I do > >not see any protection diodes in inputs. > > An ohmmeter will tell. If it has them, all you need to do is limit the > current. > > If not, use the c-b junctions of a BFT25. > > John- Hide quoted text - > > - Show quoted text -Cool, Thanks John I'm going to have to probe the inputs of some opamps. I've always assumed they had protection diodes on the front end. I should be able to find the series resistance if I'm careful. George H.
Reply by ●June 14, 20092009-06-14
On Jun 14, 9:51=A0am, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote:> Ebrahim wrote: > > Winfield Hill wrote: > >> =A0Ahem. =A0Well, in 32 posts, has anyone answered Ebrahim's > >> =A0question? =A0Which was, does a C-B junction make a good > >> =A0low-leakage high-voltage diode, and my answer is yes. > >> =A0Likely as low as 10pA, but I'd want to check specific parts. > > >> =A0It's the same question one asks before using a transistor at > >> =A0extremely low collector currents. =A0OK, not too many people > >> =A0use BJT transistors at 10pA, for one thing they're very slow, > >> =A0but it's surprising how well they work down in that territory. > > >> =A0Often when doing this trick, one wants very low capacitance, > >> =A0and I've been disappointed with the high capacitance of the > >> =A0usual candidates, e.g., 5pF. =A0This is what steers people > >> =A0back to the old PAD-1 beauties, spec'd at 0.5pF and 0.3pA > > >> =A0When one is in this pA territory, PCB leakage can be a big > >> =A0problem, and teflon posts and in-air wiring are often used. > > > ***Hello Winfield :) : > > Thank you very much for your technical and non-political answer. > > So it makes sense to look for such a candidate among high frequency > > transistors that should have low capacitance. > > > Thank you again. > > Best wishes for you, > > Ebrahim > > The part of this that makes zero sense is the 100V part. =A0Almost all op > amps, in fact AFAIK every single one ever sold, has PN junctions between > its inputs and its supply pins.(*) =A0That's why the Absolute Maximum > Ratings section of datasheets specifies that you mustn't take their > inputs more than 0.3V outside the supplies. > > Not everything on an IC maps well onto a schematic--e.g. schematics > don't show the parasitic SCR inherent in junction-isolated CMOS > processes. =A0It's there, though, as you'll find if you try dumping 20 mA > into an input. > > If you want the same sort of function provided by the input protection > diodes, without a big leakage problem, you can use something like this: > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A00 =A0VDD > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0_ > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0A > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| > =A0 =A0 =A0 =A0 =A0 =A0*---Ri------*--------Rf-----------* > =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 =A0 |> =A0 =A0 =A0 =A0 =A0 GGG =A0 =A0 =A0 =A0 =A0_ =A0 =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 |> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0A =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 =A0 |> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 =A0 |> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 |\ ==A0 =A0 =A0 =A0|> 0-------------RRRRR---*---RRRRR---|+\ =A0 =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| ==A0\ =A0 =A0 =A0|> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| ==A0 >---*-*---0> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0*---|- / ==A0 =A0|> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 | / ==A0 =A0 |> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 |/ ==A0 =A0 =A0|> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 ==A0 =A0 =A0 |> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0*---Ri------*---Rf------* > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 GGG > > (Noninverting shown because inverting is reasonably obvious, and > negative polarity left as an exercise for the reader.) =A0You have to > scale the resistances to fit your problem. =A0The idea is that the first > diode has nearly no voltage across it, and hence no leakage or > capacitive current. =A0This isn't a complete solution, because it costs > you SNR eventually, but it's a lot better than nothing. > > The diode needs almost no breakdown voltage at all--certainly not 100V. > =A0 Something like a BFT25 B-C junction (about 0.3 pF) will work great. > (WDNNS PAD-1!) > > Cheers > > Phil Hobbs > > (*) Some chips, e.g. old CMOS->TTL converters use Zeners instead, > because their inputs are intended to be overdriven by volts. =A0Also, som=e> rail-to-rail input op amps use charge pumps to allow them to run their > input stages on more than VDD-VSS. > > -- > Dr Philip C D Hobbs > Principal > ElectroOptical Innovations > 55 Orchard Rd > Briarcliff Manor NY 10510 > 845-480-2058 > hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted tex=t -> > - Show quoted text -Wow, Thanks Phil, I've scribbled the circuit down in my notebook and I'll analyze it if I ever need low leakage protection. George H.
Reply by ●June 14, 20092009-06-14
miso@sushi.com wrote:> On Jun 14, 6:51 am, Phil Hobbs > <pcdhSpamMeSensel...@electrooptical.net> wrote: >> Ebrahim wrote: >>> Winfield Hill wrote: >>>> Ahem. Well, in 32 posts, has anyone answered Ebrahim's >>>> question? Which was, does a C-B junction make a good >>>> low-leakage high-voltage diode, and my answer is yes. >>>> Likely as low as 10pA, but I'd want to check specific parts. >>>> It's the same question one asks before using a transistor at >>>> extremely low collector currents. OK, not too many people >>>> use BJT transistors at 10pA, for one thing they're very slow, >>>> but it's surprising how well they work down in that territory. >>>> Often when doing this trick, one wants very low capacitance, >>>> and I've been disappointed with the high capacitance of the >>>> usual candidates, e.g., 5pF. This is what steers people >>>> back to the old PAD-1 beauties, spec'd at 0.5pF and 0.3pA >>>> When one is in this pA territory, PCB leakage can be a big >>>> problem, and teflon posts and in-air wiring are often used. >>> ***Hello Winfield :) : >>> Thank you very much for your technical and non-political answer. >>> So it makes sense to look for such a candidate among high frequency >>> transistors that should have low capacitance. >>> Thank you again. >>> Best wishes for you, >>> Ebrahim >> The part of this that makes zero sense is the 100V part. Almost all op >> amps, in fact AFAIK every single one ever sold, has PN junctions between >> its inputs and its supply pins.(*) That's why the Absolute Maximum >> Ratings section of datasheets specifies that you mustn't take their >> inputs more than 0.3V outside the supplies. >> >> Not everything on an IC maps well onto a schematic--e.g. schematics >> don't show the parasitic SCR inherent in junction-isolated CMOS >> processes. It's there, though, as you'll find if you try dumping 20 mA >> into an input. >> >> If you want the same sort of function provided by the input protection >> diodes, without a big leakage problem, you can use something like this: >> >> 0 VDD >> | >> _ >> A >> | >> *---Ri------*--------Rf-----------* >> | | | >> GGG _ | >> A | >> | | >> | |\ | >> 0-------------RRRRR---*---RRRRR---|+\ | >> | \ | >> | >---*-*---0 >> *---|- / | >> | | / | >> | |/ | >> | | >> *---Ri------*---Rf------* >> | >> GGG >> >> (Noninverting shown because inverting is reasonably obvious, and >> negative polarity left as an exercise for the reader.) You have to >> scale the resistances to fit your problem. The idea is that the first >> diode has nearly no voltage across it, and hence no leakage or >> capacitive current. This isn't a complete solution, because it costs >> you SNR eventually, but it's a lot better than nothing. >> >> The diode needs almost no breakdown voltage at all--certainly not 100V. >> Something like a BFT25 B-C junction (about 0.3 pF) will work great. >> (WDNNS PAD-1!) >> >> Cheers >> >> Phil Hobbs >> >> (*) Some chips, e.g. old CMOS->TTL converters use Zeners instead, >> because their inputs are intended to be overdriven by volts. Also, some >> rail-to-rail input op amps use charge pumps to allow them to run their >> input stages on more than VDD-VSS. >> >> -- >> Dr Philip C D Hobbs >> Principal >> ElectroOptical Innovations >> 55 Orchard Rd >> Briarcliff Manor NY 10510 >> 845-480-2058 >> hobbs at electrooptical dot nethttp://electrooptical.net > > I'm not so sure about zener input protection. Generally when you need > to go above the rail, you just use a N mosfet with gate tied to source > and the drain going to the pad. You depend on the breakdown of the fet > to provide input protection. [If you are trying this in layout, > antenna rules may apply.] > > Every time I investigated chip damage after ESD testing, it was always > due to over-voltage versus over current. I'm not a fan of high fields > (as found in reverse bias) in semiconductors. They seem to find the > weak spot in the junction and zap it. Forward biased junction might > have hot spots, i.e. current hogging, but in general they are pretty > rugged. >Transzorbs are pretty bulletproof--its job would be just to limit the overvoltage across the series protection resistor long enough for the polyfuse to switch. The resistor and the input diodes would protect the input devices, at least if it were done right. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●June 15, 20092009-06-15
On Jun 14, 7:32=A0pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote:> m...@sushi.com wrote: > > On Jun 14, 6:51 am, Phil Hobbs > > <pcdhSpamMeSensel...@electrooptical.net> wrote: > >> Ebrahim wrote: > >>> Winfield Hill wrote: > >>>> =A0Ahem. =A0Well, in 32 posts, has anyone answered Ebrahim's > >>>> =A0question? =A0Which was, does a C-B junction make a good > >>>> =A0low-leakage high-voltage diode, and my answer is yes. > >>>> =A0Likely as low as 10pA, but I'd want to check specific parts. > >>>> =A0It's the same question one asks before using a transistor at > >>>> =A0extremely low collector currents. =A0OK, not too many people > >>>> =A0use BJT transistors at 10pA, for one thing they're very slow, > >>>> =A0but it's surprising how well they work down in that territory. > >>>> =A0Often when doing this trick, one wants very low capacitance, > >>>> =A0and I've been disappointed with the high capacitance of the > >>>> =A0usual candidates, e.g., 5pF. =A0This is what steers people > >>>> =A0back to the old PAD-1 beauties, spec'd at 0.5pF and 0.3pA > >>>> =A0When one is in this pA territory, PCB leakage can be a big > >>>> =A0problem, and teflon posts and in-air wiring are often used. > >>> ***Hello Winfield :) : > >>> Thank you very much for your technical and non-political answer. > >>> So it makes sense to look for such a candidate among high frequency > >>> transistors that should have low capacitance. > >>> Thank you again. > >>> Best wishes for you, > >>> Ebrahim > >> The part of this that makes zero sense is the 100V part. =A0Almost all=op> >> amps, in fact AFAIK every single one ever sold, has PN junctions betwe=en> >> its inputs and its supply pins.(*) =A0That's why the Absolute Maximum > >> Ratings section of datasheets specifies that you mustn't take their > >> inputs more than 0.3V outside the supplies. > > >> Not everything on an IC maps well onto a schematic--e.g. schematics > >> don't show the parasitic SCR inherent in junction-isolated CMOS > >> processes. =A0It's there, though, as you'll find if you try dumping 20=mA> >> into an input. > > >> If you want the same sort of function provided by the input protection > >> diodes, without a big leakage problem, you can use something like this=:> > >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A00 =A0VDD > >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| > >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0_ > >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0A > >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| > >> =A0 =A0 =A0 =A0 =A0 =A0*---Ri------*--------Rf-----------* > >> =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0 =A0 =A0 |> >> =A0 =A0 =A0 =A0 =A0 GGG =A0 =A0 =A0 =A0 =A0_ =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 =A0 |> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0A =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 =A0 =A0 |> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 =A0 =A0 |> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 |=\ =A0 =A0 =A0 =A0|> >> 0-------------RRRRR---*---RRRRR---|+\ =A0 =A0 =A0 | > >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ==A0| =A0\ =A0 =A0 =A0|> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ==A0| =A0 >---*-*---0> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0*---|- =/ =A0 =A0|> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 |=/ =A0 =A0 |> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 |=/ =A0 =A0 =A0|> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 ==A0 =A0 =A0 =A0 |> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0*---Ri------*---Rf------* > >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| > >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 GGG > > >> (Noninverting shown because inverting is reasonably obvious, and > >> negative polarity left as an exercise for the reader.) =A0You have to > >> scale the resistances to fit your problem. =A0The idea is that the fir=st> >> diode has nearly no voltage across it, and hence no leakage or > >> capacitive current. =A0This isn't a complete solution, because it cost=s> >> you SNR eventually, but it's a lot better than nothing. > > >> The diode needs almost no breakdown voltage at all--certainly not 100V=.> >> =A0 Something like a BFT25 B-C junction (about 0.3 pF) will work great=.> >> (WDNNS PAD-1!) > > >> Cheers > > >> Phil Hobbs > > >> (*) Some chips, e.g. old CMOS->TTL converters use Zeners instead, > >> because their inputs are intended to be overdriven by volts. =A0Also, =some> >> rail-to-rail input op amps use charge pumps to allow them to run their > >> input stages on more than VDD-VSS. > > >> -- > >> Dr Philip C D Hobbs > >> Principal > >> ElectroOptical Innovations > >> 55 Orchard Rd > >> Briarcliff Manor NY 10510 > >> 845-480-2058 > >> hobbs at electrooptical dot nethttp://electrooptical.net > > > I'm not so sure about zener input protection. Generally when you need > > to go above the rail, you just use a N mosfet with gate tied to source > > and the drain going to the pad. You depend on the breakdown of the fet > > to provide input protection. [If you are trying this in layout, > > antenna rules may apply.] > > > Every time I investigated chip damage after ESD testing, it was always > > due to over-voltage versus over current. I'm not a fan of high fields > > (as found in reverse bias) in semiconductors. They seem to find the > > weak spot in the junction and zap it. Forward biased junction might > > have hot spots, i.e. current hogging, but in general they are pretty > > rugged. > > Transzorbs are pretty bulletproof--its job would be just to limit the > overvoltage across the series protection resistor long enough for the > polyfuse to switch. =A0The resistor and the input diodes would protect th=e> input devices, at least if it were done right. > > Cheers > > Phil Hobbs > > -- > Dr Philip C D Hobbs > Principal > ElectroOptical Innovations > 55 Orchard Rd > Briarcliff Manor NY 10510 > 845-480-2058 > hobbs at electrooptical dot nethttp://electrooptical.netI am referring to on-chip protection and failure analysis I've done. I've used transorbs, but only on power rails. In any event, putting zeners on chip isn't done to my knowledge, at least in MOS/BiCMOS. See the slide "ground gate NMOS" http://www.icims.csl.uiuc.edu/icap00/sjoshi_icap00.pdf
Reply by ●June 15, 20092009-06-15
miso@sushi.com wrote:> On Jun 14, 7:32 pm, Phil Hobbs > <pcdhSpamMeSensel...@electrooptical.net> wrote: >> m...@sushi.com wrote: >>> On Jun 14, 6:51 am, Phil Hobbs >>> <pcdhSpamMeSensel...@electrooptical.net> wrote: >>>> Ebrahim wrote: >>>>> Winfield Hill wrote: >>>>>> Ahem. Well, in 32 posts, has anyone answered Ebrahim's >>>>>> question? Which was, does a C-B junction make a good >>>>>> low-leakage high-voltage diode, and my answer is yes. >>>>>> Likely as low as 10pA, but I'd want to check specific parts. >>>>>> It's the same question one asks before using a transistor at >>>>>> extremely low collector currents. OK, not too many people >>>>>> use BJT transistors at 10pA, for one thing they're very slow, >>>>>> but it's surprising how well they work down in that territory. >>>>>> Often when doing this trick, one wants very low capacitance, >>>>>> and I've been disappointed with the high capacitance of the >>>>>> usual candidates, e.g., 5pF. This is what steers people >>>>>> back to the old PAD-1 beauties, spec'd at 0.5pF and 0.3pA >>>>>> When one is in this pA territory, PCB leakage can be a big >>>>>> problem, and teflon posts and in-air wiring are often used. >>>>> ***Hello Winfield :) : >>>>> Thank you very much for your technical and non-political answer. >>>>> So it makes sense to look for such a candidate among high frequency >>>>> transistors that should have low capacitance. >>>>> Thank you again. >>>>> Best wishes for you, >>>>> Ebrahim >>>> The part of this that makes zero sense is the 100V part. Almost all op >>>> amps, in fact AFAIK every single one ever sold, has PN junctions between >>>> its inputs and its supply pins.(*) That's why the Absolute Maximum >>>> Ratings section of datasheets specifies that you mustn't take their >>>> inputs more than 0.3V outside the supplies. >>>> Not everything on an IC maps well onto a schematic--e.g. schematics >>>> don't show the parasitic SCR inherent in junction-isolated CMOS >>>> processes. It's there, though, as you'll find if you try dumping 20 mA >>>> into an input. >>>> If you want the same sort of function provided by the input protection >>>> diodes, without a big leakage problem, you can use something like this: >>>> 0 VDD >>>> | >>>> _ >>>> A >>>> | >>>> *---Ri------*--------Rf-----------* >>>> | | | >>>> GGG _ | >>>> A | >>>> | | >>>> | |\ | >>>> 0-------------RRRRR---*---RRRRR---|+\ | >>>> | \ | >>>> | >---*-*---0 >>>> *---|- / | >>>> | | / | >>>> | |/ | >>>> | | >>>> *---Ri------*---Rf------* >>>> | >>>> GGG >>>> (Noninverting shown because inverting is reasonably obvious, and >>>> negative polarity left as an exercise for the reader.) You have to >>>> scale the resistances to fit your problem. The idea is that the first >>>> diode has nearly no voltage across it, and hence no leakage or >>>> capacitive current. This isn't a complete solution, because it costs >>>> you SNR eventually, but it's a lot better than nothing. >>>> The diode needs almost no breakdown voltage at all--certainly not 100V.. >>>> Something like a BFT25 B-C junction (about 0.3 pF) will work great.. >>>> (WDNNS PAD-1!) >>>> Cheers >>>> Phil Hobbs >>>> (*) Some chips, e.g. old CMOS->TTL converters use Zeners instead, >>>> because their inputs are intended to be overdriven by volts. Also, some >>>> rail-to-rail input op amps use charge pumps to allow them to run their >>>> input stages on more than VDD-VSS. >>>> -- >>>> Dr Philip C D Hobbs >>>> Principal >>>> ElectroOptical Innovations >>>> 55 Orchard Rd >>>> Briarcliff Manor NY 10510 >>>> 845-480-2058 >>>> hobbs at electrooptical dot nethttp://electrooptical.net >>> I'm not so sure about zener input protection. Generally when you need >>> to go above the rail, you just use a N mosfet with gate tied to source >>> and the drain going to the pad. You depend on the breakdown of the fet >>> to provide input protection. [If you are trying this in layout, >>> antenna rules may apply.] >>> Every time I investigated chip damage after ESD testing, it was always >>> due to over-voltage versus over current. I'm not a fan of high fields >>> (as found in reverse bias) in semiconductors. They seem to find the >>> weak spot in the junction and zap it. Forward biased junction might >>> have hot spots, i.e. current hogging, but in general they are pretty >>> rugged. >> Transzorbs are pretty bulletproof--its job would be just to limit the >> overvoltage across the series protection resistor long enough for the >> polyfuse to switch. The resistor and the input diodes would protect the >> input devices, at least if it were done right. >> >> Cheers >> >> Phil Hobbs >> >> -- >> Dr Philip C D Hobbs >> Principal >> ElectroOptical Innovations >> 55 Orchard Rd >> Briarcliff Manor NY 10510 >> 845-480-2058 >> hobbs at electrooptical dot nethttp://electrooptical.net > > I am referring to on-chip protection and failure analysis I've done. > I've used transorbs, but only on power rails. In any event, putting > zeners on chip isn't done to my knowledge, at least in MOS/BiCMOS. > > See the slide "ground gate NMOS" > http://www.icims.csl.uiuc.edu/icap00/sjoshi_icap00.pdf >I'm sure it isn't done anymore, but check out the CD4049UB for an example. http://www.fairchildsemi.com/ds/CD%2FCD4049UBC.pdf, page 2. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
