esd diodes as diodes
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From: John Larkin on 3 Aug 2010 19:48 On Tue, 3 Aug 2010 14:53:18 -0700 (PDT), "miso(a)sushi.com" <miso(a)sushi.com> wrote: >On Aug 3, 12:29�pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)On-My- >Web-Site.com> wrote: >> On Tue, 03 Aug 2010 11:09:10 -0700, John Larkin >> >> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >I want to clamp some signals before applying them to an analog mux, so >> >that customer overloads don't blow through the mux and trash other >> >channels. One obvious way is series resistors and clamp diodes. >> >> >It occurred to me that the cheapest way to get pairs of low-leakage >> >clamp diodes is to use the esd diodes on some really cheap IC, like a >> >cmos AND gate or something. Has anybody done this? >> >> >John >> >> I presume the MUX is also CMOS? �Thus it has its own ESD diodes. �If >> you "use" ESD diodes from another chip it's likely all you will get is >> current sharing and still inject substrate current into the MUX. >> >> Only Germanium or some Schottky's will give you some margin. >> >> How many channels do you need to protect? >> >> Transistor arrays (bipolar) would allow semi-precise clamping right at >> rail potential. >> >> � � � � � � � � � � � � � � � � � � � � ...Jim Thompson >> -- >> | James E.Thompson, CTO � � � � � � � � � � � � � �| � �mens � � | >> | Analog Innovations, Inc. � � � � � � � � � � � � | � � et � � �| >> | Analog/Mixed-Signal ASIC's and Discrete Systems �| � �manus � �| >> | Phoenix, Arizona �85048 � �Skype: Contacts Only �| � � � � � � | >> | Voice:(480)460-2350 �Fax: Available upon request | �Brass Rat �| >> | E-mail Icon athttp://www.analog-innovations.com| � �1962 � � | >> >> � � � � � � � � � �Spice is like a sports car... >> � � �Performance only as good as the person behind the wheel. > >You could use a series resistor before hitting the "protection" chip, >then another resistor before feeding the chip you want to protect. >That would prevent current hogging. I had in mind using lower rails for the clamp, so the esd diodes in the analog mux can never get forward biased. In most analog muxes, once you go beyond the rails enough for the diodes to conduct, the series switches start to turn on and one over-range channel wrecks all the rest. > >Many CMOS chips these days just use an Nfet for ESD. You get a diode >clamp to the ground rail but a breakdown (impact ionization snap back) >clamp for positive hits. > >Stating the obvious, there needs to be some limiting on the positive >rail if you put a protection diode to it, otherwise the external event >will lift the positive supply rail for the whole board. Most >regulators only regulate when sourcing, but not sinking. Sure, I can handle that part. John
From: linnix on 3 Aug 2010 21:06 On Aug 3, 4:00 pm, Joerg <inva...(a)invalid.invalid> wrote: > linnix wrote: > > On Aug 3, 1:35 pm, John Larkin > > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Tue, 3 Aug 2010 14:08:40 -0500, "Tim Williams" > > >> <tmoran...(a)charter.net> wrote: > >>> Isn't MMBT3904 cheaper than your other low-leakage friend? > >>> Can't imagine CMOS gates are healthy for more than 10mA, even if you don't > >>> care about latching (e.g. use a whole chip for positive clamps only, leave > >>> Vss open). Huh, latching would cause the chip to short all other inputs > >>> to the same rail... not pretty. > >>> Tim > >> Lots of chips have latchup current ratings, often 50 mA or some such. > >> My series resistor could be 5 or 10K, so I wouldn't expect much clamp > >> current. Something like an HC240 makes 16 dual clamps, 32 low-leakage > >> diodes, for 60 cents or some such. > > > HC240 give you 8 channels. > > >http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-.... > > give you 18 channels for same size and same cents per channel. > > At over a buck a pop? Nah. A 74HC240 costs around 15 cents. > Yes, cost is a problem. But this little buggy takes 200W peak power, it can certainly beat the HC240 in protections. It does not need to waste silicon for logics.
From: Jim Thompson on 4 Aug 2010 15:02 On Wed, 04 Aug 2010 11:50:12 -0700, Joerg <invalid(a)invalid.invalid> wrote: >Grant wrote: [snip] >> >> I'm sure the pick'n'placer wont complain too much ;) >> >> You can run the mux chip at 6V? Clamp to 5V at the front for mux, >> look after the ADC separately on other side of mux? >> > >Or when using a cheap CMOS chip of the 74HC type, run its GND a diode >drop above GND and its VCC a diode drop below the mux supply. Bypass >both independently and clamp those artificial rails somewhere so they >can't be pumped up. That way the mux substrate diodes shall never see >any significant currents going into them. > [snip] Interesting idea. I'll see if it's twistable into an active, lo-Z, clamp ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Spice is like a sports car... Performance only as good as the person behind the wheel.
From: Joerg on 4 Aug 2010 17:18 Jim Thompson wrote: > On Wed, 04 Aug 2010 11:50:12 -0700, Joerg <invalid(a)invalid.invalid> > wrote: > >> Grant wrote: > [snip] >>> I'm sure the pick'n'placer wont complain too much ;) >>> >>> You can run the mux chip at 6V? Clamp to 5V at the front for mux, >>> look after the ADC separately on other side of mux? >>> >> Or when using a cheap CMOS chip of the 74HC type, run its GND a diode >> drop above GND and its VCC a diode drop below the mux supply. Bypass >> both independently and clamp those artificial rails somewhere so they >> can't be pumped up. That way the mux substrate diodes shall never see >> any significant currents going into them. >> > [snip] > > Interesting idea. I'll see if it's twistable into an active, lo-Z, > clamp > Hey, I wanted to patent that ... oh, too late ... -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Phil Hobbs on 4 Aug 2010 17:30
On 8/4/2010 5:18 PM, Joerg wrote: > Jim Thompson wrote: >> On Wed, 04 Aug 2010 11:50:12 -0700, Joerg<invalid(a)invalid.invalid> >> wrote: >> >>> Grant wrote: >> [snip] >>>> I'm sure the pick'n'placer wont complain too much ;) >>>> >>>> You can run the mux chip at 6V? Clamp to 5V at the front for mux, >>>> look after the ADC separately on other side of mux? >>>> >>> Or when using a cheap CMOS chip of the 74HC type, run its GND a diode >>> drop above GND and its VCC a diode drop below the mux supply. Bypass >>> both independently and clamp those artificial rails somewhere so they >>> can't be pumped up. That way the mux substrate diodes shall never see >>> any significant currents going into them. >>> >> [snip] >> >> Interesting idea. I'll see if it's twistable into an active, lo-Z, >> clamp >> > > Hey, I wanted to patent that ... oh, too late ... > Nah, you still have a year, at least in the US. Folks have been selling those 14-in-one input protection gizmos forever--the first I knew of was the Harris SP720, back in about 1990. I still have some--they were pretty bullet-resistant. 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 |