esd diodes as diodes
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From: keithw86 on 6 Aug 2010 10:31 On Aug 6, 8:57 am, Didi <d...(a)tgi-sci.com> wrote: > On Aug 6, 3:13 pm, John Larkin > > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > > ... > > Most of our VME modules have a calibration connector and a relay per > > channel, so our customer can switch every channel to a traceable > > dvm/source and verify calibration before and after every test run, > > without disconnecting field wiring. That works well with all the gear > > in one rack, but would be very messy to attempt with distributed i/o. > > Why do you say it would be messy? Replacing say 32 analog cables with > a 10/100 Ethernet link should only make it easier from where I look > at it (clearly not from the same point as you). I ask because I was > asked recently about a tiny (50x100mm) ADC board with 16 inputs, > to etherner, the motivation being cabling. > [Nothing came out of it but then the inquiry was from Pakistan, either > the floods got them or it was one of the so many "first ask then > think" > inquiries coming from these parts of the world :-) ]. It's messy because his traceable calibration reference is no longer where the I/O is.
From: langwadt on 6 Aug 2010 10:41 On 6 Aug., 03:11, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 5 Aug 2010 04:39:59 -0500, "Tim Williams" > > <tmoran...(a)charter.net> wrote: > ><m...(a)sushi.com> wrote in message > >news:37c28984-3697-42e3-b1e9-30edbfe39f3b(a)j8g2000yqd.googlegroups.com... > >> But you have to provide two new power rails? This isn't a passive > >> solution. At some point you are just better off buying the protection > >> diodes, probably from both a cost and reliability standpoint. > > >What's hard about this? I've been doing this for years, I hope it's not > >novel? > >http://myweb.msoe.edu/williamstm/Images/Input_Protection.png > >Clamp diodes unmarked, substitute as desired. > > What's hard is that I'd need 256 low-leakage diodes. That's a lot of > parts. > > The clamp rails are easy. One way to make them is to use regular > 3-terminal voltage regulators "upside down", dumping current into > ground. There are lots of ways to make the sink rails; I'd only need > two for the entire board. > > John how about something like this?: http://focus.ti.com/docs/prod/folders/print/tpd4e004.html 4 channels, 1.6pf, 1nA -Lasse
From: John Larkin on 6 Aug 2010 13:01 On Fri, 6 Aug 2010 07:41:22 -0700 (PDT), "langwadt(a)fonz.dk" <langwadt(a)fonz.dk> wrote: >On 6 Aug., 03:11, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Thu, 5 Aug 2010 04:39:59 -0500, "Tim Williams" >> >> <tmoran...(a)charter.net> wrote: >> ><m...(a)sushi.com> wrote in message >> >news:37c28984-3697-42e3-b1e9-30edbfe39f3b(a)j8g2000yqd.googlegroups.com... >> >> But you have to provide two new power rails? This isn't a passive >> >> solution. At some point you are just better off buying the protection >> >> diodes, probably from both a cost and reliability standpoint. >> >> >What's hard about this? �I've been doing this for years, I hope it's not >> >novel? >> >http://myweb.msoe.edu/williamstm/Images/Input_Protection.png >> >Clamp diodes unmarked, substitute as desired. >> >> What's hard is that I'd need 256 low-leakage diodes. That's a lot of >> parts. >> >> The clamp rails are easy. One way to make them is to use regular >> 3-terminal voltage regulators "upside down", dumping current into >> ground. There are lots of ways to make the sink rails; I'd only need >> two for the entire board. >> >> John > >how about something like this?: http://focus.ti.com/docs/prod/folders/print/tpd4e004.html >4 channels, 1.6pf, 1nA > >-Lasse That's nice. 1 nA typical leakage I could maybe tolerate. It would constrain me to 5 volts p-p swing, but that might work, too. Thanks. John
From: Paul Keinanen on 6 Aug 2010 13:18 On Thu, 05 Aug 2010 18:11:47 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Thu, 5 Aug 2010 04:39:59 -0500, "Tim Williams" ><tmoranwms(a)charter.net> wrote: > >><miso(a)sushi.com> wrote in message >>news:37c28984-3697-42e3-b1e9-30edbfe39f3b(a)j8g2000yqd.googlegroups.com... >>> But you have to provide two new power rails? This isn't a passive >>> solution. At some point you are just better off buying the protection >>> diodes, probably from both a cost and reliability standpoint. >> >>What's hard about this? I've been doing this for years, I hope it's not >>novel? >>http://myweb.msoe.edu/williamstm/Images/Input_Protection.png >>Clamp diodes unmarked, substitute as desired. > >What's hard is that I'd need 256 low-leakage diodes. That's a lot of >parts. How do you intend to do the signal conditioning ? The anti-aliasing filtering needs to be done individually for each channel before the mux (it is no use of doing between the mux and the ADC :-). Adding two protection diodes for each channel should not be a big issue.
From: linnix on 6 Aug 2010 13:30
On Aug 6, 3:57 am, Tim Shoppa <sho...(a)trailing-edge.com> wrote: > On Aug 3, 6:09 pm, 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? > > I'm going to state the problem a little more generally. Signal > processing power per square inch of PCB space is simply ginormous > these days. A chip the size of a fingernail can (like John's analog > mux) marshall a hundred analog inputs. > > The issue is, (correct me if I'm wrong John), is that the input signal > conditioning to protect against even mild overloads often takes many > more parts and (this is the kick in the teeth for me) PCB space. Sometimes, there is just no space on the PCB. In our case, we have a 26 pins connectors with signals coming from all sides (top, bottom PCB, left and right routings). The ESD diodes (0201 SMD) have to sit in the middle. But the assembler is cursing us (behind our back, of course). We are seriously thinking about building the ESD diodes inside the connector (26 pins 2.54mm zig zag). The rest of the board does not need such high density anyway. see: http://linnix.com/esd.jpg |