diodes are such rotten diodes
in [Design]
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From: MooseFET on 9 Jun 2010 22:21 On Jun 10, 5:58 am, 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 How about BAS116 or in the extreme case, SSTDPAD1 from Linear Integrated Systems.
From: John Larkin on 9 Jun 2010 23:30 On Wed, 9 Jun 2010 19:21:00 -0700 (PDT), MooseFET <kensmith(a)rahul.net> wrote: >On Jun 10, 5:58 am, 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 > >How about BAS116 or in the extreme case, SSTDPAD1 from Linear >Integrated Systems. Thanks. The 116 is nice, just about like the BAV199 that Joerg suggested. The 199 is a dual, which will drop into my BAV99 footprint. The SSTPADs are actually jfets. They have a lot of series channel resistance, ballpark 2K, so aren't good as clamps. And they really leak around a pA. The c-b junction of an NPN transistor, BAT25 or BCX70 maybe, leaks less, costs less, and has way lower series R. But I need a dual SOT23, and can tolerate a few nA over temperature, so the BAV199 is ideal. John
From: miso on 10 Jun 2010 02:08 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. 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.
From: John Larkin on 10 Jun 2010 11:43 On Wed, 9 Jun 2010 23:08:09 -0700 (PDT), "miso(a)sushi.com" <miso(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
From: miso on 10 Jun 2010 12:37
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. 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. |