diodes are such rotten diodes
in [Design]
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From: miso on 11 Jun 2010 00:09 On Jun 10, 11:20 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 10 Jun 2010 09:37:57 -0700 (PDT), "m...(a)sushi.com" > > > > <m...(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 Just make sure the 5V supply can't be "lifted" by the diode.
From: Joerg on 11 Jun 2010 00:23 John Larkin wrote: > 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. > A >$1 opamp for clamping a rail? ... <gasp> ... That's like using Hennessy Cognac as rubbing alcohol. Us regular folk will grudgingly spring for a TL431, at the most, and then agonize over the 6-7 cents it costs along with the two resistors. BTW, it wasn't me who suggested the BAV199. I would have but Monsieur Fred beat me by three hours :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 11 Jun 2010 00:45 On Thu, 10 Jun 2010 21:09:17 -0700 (PDT), "miso(a)sushi.com" <miso(a)sushi.com> wrote: >On Jun 10, 11:20�am, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Thu, 10 Jun 2010 09:37:57 -0700 (PDT), "m...(a)sushi.com" >> >> >> >> <m...(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 > >Just make sure the 5V supply can't be "lifted" by the diode. No problem: it's an entire VME crate! John
From: John Larkin on 11 Jun 2010 00:57 On Thu, 10 Jun 2010 21:23:39 -0700, Joerg <invalid(a)invalid.invalid> wrote: >John Larkin wrote: >> 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. >> > >A >$1 opamp for clamping a rail? ... <gasp> ... That's like using >Hennessy Cognac as rubbing alcohol. Us regular folk will grudgingly >spring for a TL431, at the most, and then agonize over the 6-7 cents it >costs along with the two resistors. Hennesey? Martell is better, in my humble opinion. But I recently discovered Ron Zacapa 23, and the Cordon Bleu is gathering dust. There's only one LM8261 on the board now, to make the 2.048 volt Vcm supply for the ADCs. I love these opamps. We just figured out that the parts cost on this board will be about $750. 16 16-bit ADCs, a mess of LT1124s (*three* dollar opamps!), a $50 FPGA, 33 relays, a bazillion Susumu thinfilm resistors... The bare board alone will cost about $42 each at qty 100. > >BTW, it wasn't me who suggested the BAV199. I would have but Monsieur >Fred beat me by three hours :-) Just how many beers am I supposed to buy? John
From: Joerg on 11 Jun 2010 11:20
John Larkin wrote: > On Thu, 10 Jun 2010 21:23:39 -0700, Joerg <invalid(a)invalid.invalid> > wrote: > >> John Larkin wrote: >>> 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. >>> >> A >$1 opamp for clamping a rail? ... <gasp> ... That's like using >> Hennessy Cognac as rubbing alcohol. Us regular folk will grudgingly >> spring for a TL431, at the most, and then agonize over the 6-7 cents it >> costs along with the two resistors. > > Hennesey? Martell is better, in my humble opinion. But I recently > discovered Ron Zacapa 23, and the Cordon Bleu is gathering dust. > > There's only one LM8261 on the board now, to make the 2.048 volt Vcm > supply for the ADCs. I love these opamps. We just figured out that the > parts cost on this board will be about $750. 16 16-bit ADCs, a mess of > LT1124s (*three* dollar opamps!), a $50 FPGA, 33 relays, a bazillion > Susumu thinfilm resistors... The bare board alone will cost about $42 > each at qty 100. > Most of my clients would have me flogged if I ran up a tab like that :-) >> BTW, it wasn't me who suggested the BAV199. I would have but Monsieur >> Fred beat me by three hours :-) > > Just how many beers am I supposed to buy? > I suppose Fred would prefer du vin rouge, I'd go with a Russian Ale at Zeitgeist :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |