esd diodes as diodes
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From: John Larkin on 3 Aug 2010 14:09 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
From: Tim Williams on 3 Aug 2010 15:08 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 -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:qimg56180l4mvssq5ukjcdha7i4ur4e86m(a)4ax.com... >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 >
From: linnix on 3 Aug 2010 15:20 On Aug 3, 11:09 am, 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 As suggested by someone here before, I am using BAV99W (actually, should be BAV99N, since it is narrower than BAV99). 2pF C and 50uA Ir. ESD diodes are either too high in C or too expensive. BAV99W is less than a 5 cents and smaller than SOT-23.
From: Jim Thompson on 3 Aug 2010 15:29 On Tue, 03 Aug 2010 11:09:10 -0700, John Larkin <jjlarkin(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 at http://www.analog-innovations.com | 1962 | Spice is like a sports car... Performance only as good as the person behind the wheel.
From: John Larkin on 3 Aug 2010 16:35
On Tue, 3 Aug 2010 14:08:40 -0500, "Tim Williams" <tmoranwms(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. 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 |