diodes are such rotten diodes
in [Design]
|
From: John Larkin on 12 Jun 2010 13:03 On Sat, 12 Jun 2010 09:42:10 -0700 (PDT), MooseFET <kensmith(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
From: miso on 12 Jun 2010 13:23 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. The TI rail splitter (2426) has the advantage of lower Iq.
From: Joerg on 12 Jun 2010 14:58 John Larkin wrote: > On Sat, 12 Jun 2010 09:42:10 -0700 (PDT), MooseFET > <kensmith(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. > On regular opamps you can try how small of an output series resistor you can get away with. Plus a small capacitor from VIN- directly to OUT, of course. With cheap boom box amps that resistor can be surprisingly small. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Phil Hobbs on 12 Jun 2010 21:44 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. > > Are there other c-load opamps? The 8261 is a bit of overkill > sometimes. > > John > I used to like the lM6361--RIP. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: Nemo on 14 Jun 2010 18:27
In article <76452696-3efc-4ab9-a5ba-28fce6f8be58(a)a30g2000yqn.googlegroups.com>, "miso(a)sushi.com" <miso(a)sushi.com> writes >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. > >The TI rail splitter (2426) has the advantage of lower Iq. I tried the TI rail splitters last year. I had +5V and -5V rails and I needed to use one IC which could only take max 5V supply total, and needed its input near mid rail. (Actually this was a poor solution which I abandoned, but that's not my point.) I created +/-2.5V rails with two rail splitters. This caused problems, firstly because you could get lockup if one powered up before the other - the other's output was pulled beyond 0V through the IC I was trying to power and latched off; and there seemed to be an oscillation as if they were fighting each other (though that may have been another problem with another IC!) So I decided to avoid rail splitters after that, perhaps others have found similar problems and that is why they are not widely discussed. I eventually found another approach, which did not need +/-2.5V. -- Nemo |