reg + reference
in [Design]
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From: Joerg on 27 Mar 2010 11:47 John Larkin wrote: > On Fri, 26 Mar 2010 17:22:36 -0700, Joerg <invalid(a)invalid.invalid> > wrote: > >> John Larkin wrote: >>> On Fri, 26 Mar 2010 15:36:09 -0700, John Larkin >>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>> >>>> On Fri, 26 Mar 2010 14:34:19 -0700, Joerg <invalid(a)invalid.invalid> >>>> wrote: >>>> >>>>> John Larkin wrote: >>>>>> ftp://jjlarkin.lmi.net/V220_reg.JPG >>>>>> >>>>> Just curious, why not replace all this with one of these: >>>>> >>>>> http://www.analog.com/static/imported-files/Data_Sheets/ADR01_02_03_06.pdf >>>>> >>>>> Much less space, cheaper. Unless you need the 3.3V somewhere. >>>> Yes, I need 3.3 to run an ARM processor, 40 mA or so maybe. >>>> >>>> The ADR could run off the +15, so maybe that's a better way to go. It >>>> would cost a little more, but the TC would be a lot better. >>>> >>>> Maybe I'll do that, the LM1117 for power and the ADR for the >>>> reference. >>>> >>>> Wait! LM4120 will work. +3 ref with 120 mV dropout! >>>> >>>> >>>> John >>> And of course, the LM4120 is a stability land-mine of its own. >>> >>> ThestandardapplicationcircuitfortheLM4120isshownin >>> Figure1.Itisdesignedtobestablewithceramicoutput >>> capacitorsintherangeof0.022�Fto0.047�F.Notethat >>> 0.022�Fistheminimumrequiredoutputcapacitor.These >>> capacitorstypicallyhaveanESRofabout0.1to0.5. >>> SmallerESRcanbetolerated,howeverlargerESRcannot. >>> Theoutputcapacitorcanbeincreasedtoimproveloadtran- >>> sientresponse,uptoabout1�F.However,valuesabove >>> 0.047�Fmustbetantalum.Withtantalumcapacitors,inthe >>> 1�Frange,asmallcapacitorbetweentheoutputandthe >>> referencepinisrequired.Thiscapacitorwilltypicallybein >>> the50pFrange.Caremustbetakenwhenusingoutput >>> capacitorsof1�Forlarger.Theseapplicationmustbethor- >>> oughlytestedovertemperature,lineandload. >>> >>> >>> Thanks, National. >>> >> I was just gonna tell you. If there was one I'd have a bumper sticker on >> my car "LDO, no, no, no!" >> >> My advice to clients there is always the same, don't ever use an LDO >> unless there is absolutely no other way (and in my 25 years on the beat >> there always was). No matter what the marketeers sing from the trees. > > Unless I work off the +15, which has its own complications, I need > this LDO voltage reference. After all, they say it's stable over - > wait for it - a 2:1 load capacitor range! Assuming you get the ESR > right. > And nothing seems to be guaranteed. When I read page 9 in the datasheet I am beginning to feel sick. Why does working off of the +15V have complications? >> It's like press-fit where I sat in a meting where a company pitched it >> to a client. Tappa-di-tap went my calculator. "I calculated that 3 out >> of all the contacts will be open. Can you tell us which ones those will >> be?". The meeting and the sales pitch ended that instant ... > > > I sold a lot of CAMAC crates that used pressfit backplanes. They > always worked. Luck'o the Irish I guess. > My client also did, and AFAIR some engineers had Irish blood in them. At least they fancied Guinness like I do. But then one fine day luck ran out, bus errors crept in. Initially they didn't believe me but after a lot of cajoling agreed to adjust the solder machine temp profile and to run dozens of boards through. That took some doing because it was a huge and thick backplane. The number of bus error messages dropped to zero but only on machines with soldered backplanes. _Then_, Mr. sales dude came in touting a new and improved pressfit process ... -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 27 Mar 2010 11:49 Nico Coesel wrote: > Joerg <invalid(a)invalid.invalid> wrote: > >> It's like press-fit where I sat in a meting where a company pitched it >> to a client. Tappa-di-tap went my calculator. "I calculated that 3 out >> of all the contacts will be open. Can you tell us which ones those will >> be?". The meeting and the sales pitch ended that instant ... > > Sorry but that must be some calculating error on your side. All PABXes > I've seen had press-fit backplanes for a very good reason > (reliability). But just like soldering and crimping connections > mounting press-fit is something that needs to be done right. > On the first series it had been done by the pros. Sorry, but my experience with pressfit is not positive. Not as bad as with wire-wrap but close. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 27 Mar 2010 11:50 Nico Coesel wrote: > John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Fri, 26 Mar 2010 14:34:19 -0700, Joerg <invalid(a)invalid.invalid> >> wrote: >> >>> John Larkin wrote: >>>> ftp://jjlarkin.lmi.net/V220_reg.JPG >>>> >>> Just curious, why not replace all this with one of these: >>> >>> http://www.analog.com/static/imported-files/Data_Sheets/ADR01_02_03_06.pdf >>> >>> Much less space, cheaper. Unless you need the 3.3V somewhere. >> Yes, I need 3.3 to run an ARM processor, 40 mA or so maybe. > > AFAIK the LPC1768 can run on lower voltages as well like down to 2.6V. > Not at full speed ofcourse. 40mA may be too optimistic though. > >> The ADR could run off the +15, so maybe that's a better way to go. It >> would cost a little more, but the TC would be a lot better. >> >> Maybe I'll do that, the LM1117 for power and the ADR for the >> reference. >> >> Wait! LM4120 will work. +3 ref with 120 mV dropout! > > Be sure to decouple the reference properly. The ADC sinks quite some > current from the reference pin when sampling. > Yup, and then the LM4120 datasheet says, quote "Care must be taken when using output capacitors of 1?F or larger. These application must be thoroughly tested over temperature, line and load." That would give me the creeps. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 27 Mar 2010 12:46 On Sat, 27 Mar 2010 07:08:36 -0700 (PDT), "langwadt(a)fonz.dk" <langwadt(a)fonz.dk> wrote: >On 27 Mar., 02:40, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Fri, 26 Mar 2010 17:01:38 -0700 (PDT), "langw...(a)fonz.dk" >> >> >> >> <langw...(a)fonz.dk> wrote: >> >On 26 Mar., 22:24, John Larkin >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> On Fri, 26 Mar 2010 19:21:00 GMT, Jan Panteltje >snip >> >> Or maybe there exists a 3 volt 3-wire reference that would work from >> >> 3.3 volts. >> >> >use the ~3.3V as reference, measure a 1.2V reference voltage and >> >correct for it? >> >> I could use a 1.2 volt shunt reference, with a reasonable resistor >> from 3.3, and use an opamp to scale the 1.2 up to 3. More parts! The >> National low-dropout reference is ideal, if it doesn't oscillate. >> >snip > >I meant connect the 1.2V reference to a spare ADC channel, no scaling >needed. > > >-Lasse Oh. Then we'd use 3.3 as the ADC reference, measure 1.2, and normalize measured values against the 1.2. That would work, with a bit more math... unfortunately a divide, but not too often. I could do that at 2.5 volts, too, and still have a reasonable voltage drop across a resistor to 3.3. Nice idea. John
From: John Larkin on 27 Mar 2010 12:51
On Sat, 27 Mar 2010 12:09:34 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >On a sunny day (Fri, 26 Mar 2010 18:27:35 -0700) it happened John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in ><imnqq51aro61mta0ehao2copasiga90627(a)4ax.com>: > >>>>The current into the reference will be more constant than if I just >>>>used a resistor with a nominal 0.3 volt drop across it. The BCX71K has >>>>a very tight beta spec. It would probably be better than using the >>>>current mirror discussed in another thread recently. And we have >>>>BCX71Ks in stock for under 4 cents each. >>> >>>It is senile, >>>the base will pick up all the noise it can get from the universe. >> >>I suppose all transistors do that. Low-noise preamps become >>impossible. > >This circuit of yours is basically a rectifier (be junction). As any common-emitter amplifier is. >In good opamps there is a current source in the emitter. And this is a current source into the base! Since average base current is nearly constant, RF should not affect average collector current. >Indeed very few opamps are free from RF interference, >Dave had some chip on his video blog that actually could stand GSM signals close up, most cannot. > >If you do not believe that, get a GSM phone, and put it next to your circuit. >Low pass the reference your create, and use your nnnn+ digit volt meter to see any changes, > > >As for creating the reference, you have plenty of volts on the input, >why not use something like a MCP1525? >Or simply a LM317, I use LM317 for everything, very stable output at low loads. The TC would be terrible, espacially as it self-heats. I want overall channel accuracy of at least 0.1%, and there are other errors to deal with, so I can't use a rotten reference. John |