diodes are such rotten diodes
in [Design]
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From: Jan Panteltje on 10 Jun 2010 14:17 On a sunny day (Thu, 10 Jun 2010 08:43:02 -0700) it happened John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in <470216hcjq48g2habirjboe31hskafo9sk(a)4ax.com>: >The input to the filter comes from an INA154 powered from +-17 volts, >and it can potentially rail. So I can't clamp that. The input of the >first opamp in the SK filter is conveniently about 7K ohms downstream, >an ideal place to clamp. But clamping there changes the filter characteristics no? The caps will get pushed to strange values, how does it come out of a clamped situation? Would it not be better to add a separate opamp stage before this to do the clamping?
From: John Larkin on 10 Jun 2010 14:20 On Thu, 10 Jun 2010 09:37:57 -0700 (PDT), "miso(a)sushi.com" <miso(a)sushi.com> wrote: >On Jun 10, 8:43�am, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Wed, 9 Jun 2010 23:08:09 -0700 (PDT), "m...(a)sushi.com" >> >> >> >> <m...(a)sushi.com> wrote: >> >On Jun 9, 2:58�pm, John Larkin >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> I have a signal that can potentially swing +-15 volts maybe, and I'm >> >> going into a 4-pole Sallen-Key filter, then an ADC with swing range 0 >> >> to 4.096. So I added a dual diode, BAV99, at the input of the first >> >> opamp, connecting to clamp rails of 0 and 4.1. The filter input >> >> resistors add up to about 10K. >> >> >> This works, but it's not safe over temperature. Turns out a BAV99 >> >> leaks around 5 nA at room temp alone. >> >> >> The collector-base junction of a cheap transistor, like a BCX70, leaks >> >> about 150 fA at room temp, -5 volts, kinda hard to measure. >> >> >> Transistors are so much better diodes than diodes. Do they still make >> >> diodes by dicing up featureless wafers, exposing the damaged edges? >> >> Barbaric. Or are they just big junctions? >> >> >> Maybe I'll test some high-voltage dual diodes; they might leak less. I >> >> could use the BCX70 or BFT25 junctions (we created a PADS schematic >> >> symbol for a transistor used as a diode) but it will take two parts. >> >> >> Central makes a "low-leakage" SOT-23 dual diode, samples coming. >> >> >> John >> >> >Is there some reason you wouldn't clamp at the input of the filter >> >rather than at the op amp? I'm assuming leakage at the input wouldn't >> >be a problem since if it was, the Salen Key filter wouldn't work so >> >well. That is, the filter assumes it is driven by a low impedance. >> >> The input to the filter comes from an INA154 powered from +-17 volts, >> and it can potentially rail. So I can't clamp that. The input of the >> first opamp in the SK filter is conveniently about 7K ohms downstream, >> an ideal place to clamp. It's tricky, because the input range of the >> AD7699 ADC is 0 to 4.096, so I have to clamp just below 0 volts, to >> avoid jamming the ADC ESD diodes too hard, but I don't want to add >> drift or nonlinearity. >> >> ftp://jjlarkin.lmi.net/22S490B_sh22.pdf >> >> This is basically an anti-aliasing/noise filter. We have lots of >> digital filtering downstream of the ADC, and customers will usually be >> down-filtering considerably, so the AC response of this filter isn't >> critical. >> >> >> >> >A lot of engineers get stuck in the Salen Key mode, but if you learn >> >leap frog design, you can come up with filters that use a "stockable" >> >cap, that is always use a 0.1uF for example in every filter stage, >> >then use that cap for all your products. Cost you more op amps though. >> >There are other solutions besides leap front that give this >> >flexibility. Generally the one op amp per pole designs do this. You >> >stock less parts and can get a price break on the caps. I examined a >> >lot of modems back in the day that used this kind of scheme. >> >> One nice thing about S-K is that the DC gain is 1.000 and doesn't >> depend on component values. I use the TI FilterPro software, which >> comes up with standard value caps. If my transient or frequency >> response is a little off, my customers won't notice. If my DC gain >> drifts with temperature, they sure will. So I want the filter gain to >> not depend on resistor TCs. >> >> I sometimes design software/FPGA versions of S-K and state-variable >> filters. They have the same advantage as the analog SK, namely unity >> gain per section. The coefficients and gains don't get insane like a >> butterfly tends to do. >> >> John > >It could be clamped after R413, though 17V to ground would be about >0.4W through the resistor. And it's an 0603! The additional current would just ride the voltage further up the diode curve and make the clamp that much worse. I want an ideal diode and I want it now. > >I'm stating the obvious here, but the power supply going to the >clamping diodes needs to be able to sink current. Many supplies >regulate well when sourcing current, but are just fine being yanked >high when pulled by a diode. This is a common problem in latchup >testing components. That is, the person doing the test forgets to load >the power supply with a resistor so that it can sink the latchup test >current. I'm currently using an LM8261 opamp to make the clamp rails. If I add a small resistor, 50 ohms maybe, from the last opamp into the ADC, I can clamp to ground and +5, same place as now, and not stuff too much current into the ADC esd diodes. Using BAV199s, that should work. John
From: John Larkin on 10 Jun 2010 15:25 On Thu, 10 Jun 2010 18:17:36 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >On a sunny day (Thu, 10 Jun 2010 08:43:02 -0700) it happened John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in ><470216hcjq48g2habirjboe31hskafo9sk(a)4ax.com>: > >>The input to the filter comes from an INA154 powered from +-17 volts, >>and it can potentially rail. So I can't clamp that. The input of the >>first opamp in the SK filter is conveniently about 7K ohms downstream, >>an ideal place to clamp. > >But clamping there changes the filter characteristics no? Only if the customer has railed the signal, past my 5% headroom. At that point, my obligations as regards signal quality cease. >The caps will get pushed to strange values, how does it come out of a clamped situation? >Would it not be better to add a separate opamp stage before this to do the clamping? The board is packed with parts as is! Any clamping is going to do weird things to a signal; you just get to decide what sort of weird. Actually, clamping at this point in the filter is good, because it passively attenuates spikes before things get nonlinear. John
From: Tim Williams on 10 Jun 2010 16:50 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:tpa216pm0rk5s5idibcccm8i3251gf1qqa(a)4ax.com... > And it's an 0603! The additional current would just ride the voltage > further up the diode curve and make the clamp that much worse. I want > an ideal diode and I want it now. What about a current limiter? Signal goes into the AC side of a FWB. Bridge is biased with CCS's from + to -. Signal goes out the other AC pin, into a load resistor. When current through the load resistor exceeds CCS, the output clips. Downsides include lots of diodes in the path (potentially requiring matched offsets and tempcos) and doing nothing about capacitive coupling (though for a filter, that's probably not a problem). Do something with op-amps. Op-amps and diodes make great ideal diodes. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: John Larkin on 10 Jun 2010 17:47
On Thu, 10 Jun 2010 15:50:21 -0500, "Tim Williams" <tmoranwms(a)charter.net> wrote: >"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >news:tpa216pm0rk5s5idibcccm8i3251gf1qqa(a)4ax.com... >> And it's an 0603! The additional current would just ride the voltage >> further up the diode curve and make the clamp that much worse. I want >> an ideal diode and I want it now. > >What about a current limiter? Signal goes into the AC side of a FWB. >Bridge is biased with CCS's from + to -. Signal goes out the other AC pin, >into a load resistor. When current through the load resistor exceeds CCS, >the output clips. > >Downsides include lots of diodes in the path (potentially requiring matched >offsets and tempcos) and doing nothing about capacitive coupling (though for >a filter, that's probably not a problem). Yeah, lots of parts and potentially lots of DC error. One LSB of the ADC is 78 microvolts, which is why the diode leakage was so annoying. 10 nA makes 1 LSB error. > >Do something with op-amps. Op-amps and diodes make great ideal diodes. Lots of parts again. The board is currently rev B, about to be C, and it's very dense, so I can't add much stuff. Rev A had fatal pinout errors on the ADCs, impossible to kluge, so I didn't learn much from it. Rev B works, and we have customers playing with them and writing drivers and such, but the clamps are scary as regards offset and linearity over temperature. So, rev C. John |