esd diodes as diodes
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From: linnix on 3 Aug 2010 17:37 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=497-6636-1-ND give you 18 channels for same size and same cents per channel. > > I was just wondering if anybody did this and knew of gotchas. Or has > other suggestions for clamping a lot of analog signals without having > to pick-and-place a lot of parts. I'm thinking about doing a cheapish > 64-channel ADC board and every part hurts. > > John
From: miso on 3 Aug 2010 17:53 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. 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.
From: linnix on 3 Aug 2010 18:12 On Aug 3, 2:53 pm, "m...(a)sushi.com" <m...(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. > > 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. You can add a zener to the supply rail. TVS diodes are just fast zeners. Regular zeners are too slow for signals but not a problem for the power rail. I proposed adding 26 TVS diodes to our board, but they saw the cost and said "to hack with it, do a BP" (risk the company on it).
From: Joerg on 3 Aug 2010 19:00 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=497-6636-1-ND > 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. > >> I was just wondering if anybody did this and knew of gotchas. Or has >> other suggestions for clamping a lot of analog signals without having >> to pick-and-place a lot of parts. I'm thinking about doing a cheapish >> 64-channel ADC board and every part hurts. >> The gotcha is that you have to guestimate the total simultaneous spike energy, when it hits nearly all inputs. For example, at some point the bond wire to GND or VCC is going to go ... plink ... *PHUT* -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: mike742 on 3 Aug 2010 19:28
On Aug 3, 2:09 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > 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? I remember seeing a S100 dynamic memory board with something like a 74ls30 used as damping diodes... |