diodes are such rotten diodes
in [Design]
|
From: John Larkin on 14 Jun 2010 19:05 On Sat, 12 Jun 2010 10:23:11 -0700 (PDT), "miso(a)sushi.com" <miso(a)sushi.com> wrote: >On Jun 12, 10:03�am, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Sat, 12 Jun 2010 09:42:10 -0700 (PDT), MooseFET >> >> >> >> <kensm...(a)rahul.net> wrote: >> >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. >> >> The LM8261 compensation is effectively Miller caps on the two output >> transistors. That makes a dominant pole right at the output. Adding >> external load caps just slows down that pole. If you hang a cap on >> most opamps, ones where the compensation is buried further inside, it >> creates a 2nd order loop that is unstable. >> >> Lots of opamps and LDOs would benefit from this trick. >> >> John > >The LM8261 looks like a good part for the fake ground. What I like is >the loop gain isn't very high. In many applications, you are better >off with less gain since that often (but not necessarily) means better >stability. Those designers that insist on rolling their own regulators >should look at this op amp. > >I wonder with the LM8261 if there is a, for lack of a better phrase, >"sour spot" where you have killed the internal Miller multiplication, >but having rolled off the high frequency gain enough with the external >cap. I think it's stable for any C. And probably for any C+ESR. John
From: miso on 14 Jun 2010 23:39 On Jun 14, 4:05 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Sat, 12 Jun 2010 10:23:11 -0700 (PDT), "m...(a)sushi.com" > > > > <m...(a)sushi.com> wrote: > >On Jun 12, 10:03 am, John Larkin > ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Sat, 12 Jun 2010 09:42:10 -0700 (PDT), MooseFET > > >> <kensm...(a)rahul.net> wrote: > >> >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. > > >> The LM8261 compensation is effectively Miller caps on the two output > >> transistors. That makes a dominant pole right at the output. Adding > >> external load caps just slows down that pole. If you hang a cap on > >> most opamps, ones where the compensation is buried further inside, it > >> creates a 2nd order loop that is unstable. > > >> Lots of opamps and LDOs would benefit from this trick. > > >> John > > >The LM8261 looks like a good part for the fake ground. What I like is > >the loop gain isn't very high. In many applications, you are better > >off with less gain since that often (but not necessarily) means better > >stability. Those designers that insist on rolling their own regulators > >should look at this op amp. > > >I wonder with the LM8261 if there is a, for lack of a better phrase, > >"sour spot" where you have killed the internal Miller multiplication, > >but having rolled off the high frequency gain enough with the external > >cap. > > I think it's stable for any C. And probably for any C+ESR. > > John Then LM8261 is probably the way to go then unless you need low power, The Natl design certainly looks good, especially that it is push/pull.
From: John Larkin on 15 Jun 2010 14:25 On Wed, 09 Jun 2010 14:58:42 -0700, John Larkin <jjlarkin(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 I got some samples of the Central CMPD6001S dual SOT-23 diodes. Through a local rep, since their registration/password thing is so absurd. I measured about 50 fA leakage at -5 volts. I say "about" because this is sorta hard to measure... 5 mV across a 100G resistor. That's a lot better than most diodes at around 5 na. Pricing is 0.099 by the reel, as compared to 0.085 for the BAV199, which I haven't measured yet. Looks like 1.8 pF at zero bias, pretty nice part. John
From: Grant on 15 Jun 2010 17:15 On Tue, 15 Jun 2010 11:25:17 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Wed, 09 Jun 2010 14:58:42 -0700, John Larkin ><jjlarkin(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 > > >I got some samples of the Central CMPD6001S dual SOT-23 diodes. >Through a local rep, since their registration/password thing is so >absurd. > >I measured about 50 fA leakage at -5 volts. I say "about" because this >is sorta hard to measure... 5 mV across a 100G resistor. That's a lot >better than most diodes at around 5 na. Pricing is 0.099 by the reel, >as compared to 0.085 for the BAV199, which I haven't measured yet. And just how does one go about measuring 5mV across 100G? Seems like an open circuit that might not collect a static charge, to me ;) > >Looks like 1.8 pF at zero bias, pretty nice part. Grant. -- http://bugs.id.au/
From: John Larkin on 15 Jun 2010 17:33
On Wed, 16 Jun 2010 07:15:17 +1000, Grant <omg(a)grrr.id.au> wrote: >On Tue, 15 Jun 2010 11:25:17 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Wed, 09 Jun 2010 14:58:42 -0700, John Larkin >><jjlarkin(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 >> >> >>I got some samples of the Central CMPD6001S dual SOT-23 diodes. >>Through a local rep, since their registration/password thing is so >>absurd. >> >>I measured about 50 fA leakage at -5 volts. I say "about" because this >>is sorta hard to measure... 5 mV across a 100G resistor. That's a lot >>better than most diodes at around 5 na. Pricing is 0.099 by the reel, >>as compared to 0.085 for the BAV199, which I haven't measured yet. > >And just how does one go about measuring 5mV across 100G? Seems like >an open circuit that might not collect a static charge, to me ;) >> >>Looks like 1.8 pF at zero bias, pretty nice part. > >Grant. Here's my pA parts tester: ftp://jjlarkin.lmi.net/99A260A1.JPG ftp://jjlarkin.lmi.net/99A260A3.JPG ftp://jjlarkin.lmi.net/99S260A.JPG I plugged a 100G resistor into the Z2 slot, and put the diode in as Z1. With a 100M or 1G resistor as Z2, nanoamps or tens of pA are easy to measure. When you get to fA, everything has to be very well shielded, and you have to wait a long time for the inherent time constants to settle down. Without a solid shield, just moving around in the vicinity will induce huge offsets. Those damned RatShack banana terminals are conductive as hell, so I had to machine the hole and add the lexan *after* it was all built. Grrrrr. John |