diodes are such rotten diodes
in [Design]
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From: miso on 12 Jun 2010 01:05 On Jun 10, 9:57 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 10 Jun 2010 21:23:39 -0700, Joerg <inva...(a)invalid.invalid> > wrote: > > > > >John Larkin 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. > > >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 Has anyone done an extensive study on chips to make common mode voltages? TI still makes a rail splitter.
From: Joerg on 12 Jun 2010 10:50 miso(a)sushi.com wrote: [...] > Has anyone done an extensive study on chips to make common mode > voltages? TI still makes a rail splitter. You mean some sort of artifical ground? Everyone would be using cheap opamps there :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 12 Jun 2010 11:01 On Sat, 12 Jun 2010 07:50:03 -0700, Joerg <invalid(a)invalid.invalid> wrote: >miso(a)sushi.com wrote: > >[...] > >> Has anyone done an extensive study on chips to make common mode >> voltages? TI still makes a rail splitter. > > >You mean some sort of artifical ground? Everyone would be using cheap >opamps there :-) Intermediate rails are usually bypassed, and lots of opamps don't like that. It takes a few more parts to have a regular opamp drive a big capacitive load. The LM8261 doesn't care. Are there other c-load opamps? The 8261 is a bit of overkill sometimes. John
From: Joerg on 12 Jun 2010 11:48 John Larkin wrote: > On Sat, 12 Jun 2010 07:50:03 -0700, Joerg <invalid(a)invalid.invalid> > wrote: > >> miso(a)sushi.com wrote: >> >> [...] >> >>> Has anyone done an extensive study on chips to make common mode >>> voltages? TI still makes a rail splitter. >> >> You mean some sort of artifical ground? Everyone would be using cheap >> opamps there :-) > > Intermediate rails are usually bypassed, and lots of opamps don't like > that. It takes a few more parts to have a regular opamp drive a big > capacitive load. The LM8261 doesn't care. > Yes, needs to be isolated. If you don't want to spring for a transistor pair that means your virtual ground becomes a bit soft. > Are there other c-load opamps? The 8261 is a bit of overkill > sometimes. > They are all expensive but audio amps can be nice here. Many live happily with 4ohms or less resistive. They are really cheap because of the masses that want to be blasted with tchk .. tchk .. *BOOM*, the louder the better. Although, nowadays I'd consider a sync buck or class-D amp. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: MooseFET on 12 Jun 2010 12:42
On Jun 12, 11:01 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Sat, 12 Jun 2010 07:50:03 -0700, Joerg <inva...(a)invalid.invalid> > wrote: > > >m...(a)sushi.com wrote: > > >[...] > > >> Has anyone done an extensive study on chips to make common mode > >> voltages? TI still makes a rail splitter. > > >You mean some sort of artifical ground? Everyone would be using cheap > >opamps there :-) > > Intermediate rails are usually bypassed, and lots of opamps don't like > that. It takes a few more parts to have a regular opamp drive a big > capacitive load. The LM8261 doesn't care. > > Are there other c-load opamps? The 8261 is a bit of overkill > sometimes. > > John One section of an LM339 will make a nice sink-only rail splitter that is good for a few mA. The outputs of rail-rail opamps are really current sources that are servoed to make the voltage. It is too bad that they don't make the internal node that sets the output current available on the singles in the SO-8. It would be a very nice way to over compensate to allow large capacitive loads to be driven. |