reg + reference
in [Design]
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From: langwadt on 26 Mar 2010 20:01 On 26 Mar., 22:24, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Fri, 26 Mar 2010 19:21:00 GMT, Jan Panteltje > > <pNaonStpealm...(a)yahoo.com> wrote: > >On a sunny day (Fri, 26 Mar 2010 10:19:55 -0700) it happened John Larkin > ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote in > ><13rpq5lqd1lj1k9flb9orfr2vcl2tuu...(a)4ax.com>: > > >>ftp://jjlarkin.lmi.net/V220_reg.JPG > > >>John > > >No money for a real current source or current mirror? > >Does that 1M depending on beta circuit change a lot over temperature? > >Its Vbe will change, so will the Ib. > > 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. > > The +15 is not very well regulated, and I'm tight for space. > > I guess I could use a depletion mode fet from +15. That's just one > part. The current regulation would be fair, but I wouldn't have any > flexibility on setting current. An LND150 is 33 cents, no problem > there, but Idss is 1 to 3 mA and it may change a bunch with > temperature. > > 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? > > This will be the ADC voltage reference for an NXP ARM processor. I'm > doing a 12-channel 4-20 mA thing, electrically isolated per channel, > with an ARM per channel. The ARM and other stuff might need 45 mA or > so, less if we can sleep the code a lot. why not pick an ARM with a build in reference? something like stm32... ? 12 bits too, think the nxp is only 10bits I'm sure analog has some too > snip -Lasse
From: Joerg on 26 Mar 2010 20:22 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. 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 ... -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 26 Mar 2010 21:26 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. > >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. John
From: John Larkin on 26 Mar 2010 21:27 On Fri, 26 Mar 2010 23:28:41 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >On a sunny day (Fri, 26 Mar 2010 14:24:39 -0700) it happened John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in ><ds6qq5pog4jacgoi7fdnunckt584e7emkr(a)4ax.com>: > >>On Fri, 26 Mar 2010 19:21:00 GMT, Jan Panteltje >><pNaonStpealmtje(a)yahoo.com> wrote: >> >>>On a sunny day (Fri, 26 Mar 2010 10:19:55 -0700) it happened John Larkin >>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>><13rpq5lqd1lj1k9flb9orfr2vcl2tuucpi(a)4ax.com>: >>> >>>> >>>>ftp://jjlarkin.lmi.net/V220_reg.JPG >>>> >>>> >>>>John >>>> >>> >>>No money for a real current source or current mirror? >>>Does that 1M depending on beta circuit change a lot over temperature? >>>Its Vbe will change, so will the Ib. >> >>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. That's awfully good to know. John
From: John Larkin on 26 Mar 2010 21:40
On Fri, 26 Mar 2010 17:01:38 -0700 (PDT), "langwadt(a)fonz.dk" <langwadt(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 >> >> <pNaonStpealm...(a)yahoo.com> wrote: >> >On a sunny day (Fri, 26 Mar 2010 10:19:55 -0700) it happened John Larkin >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote in >> ><13rpq5lqd1lj1k9flb9orfr2vcl2tuu...(a)4ax.com>: >> >> >>ftp://jjlarkin.lmi.net/V220_reg.JPG >> >> >>John >> >> >No money for a real current source or current mirror? >> >Does that 1M depending on beta circuit change a lot over temperature? >> >Its Vbe will change, so will the Ib. >> >> 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. >> >> The +15 is not very well regulated, and I'm tight for space. >> >> I guess I could use a depletion mode fet from +15. That's just one >> part. The current regulation would be fair, but I wouldn't have any >> flexibility on setting current. An LND150 is 33 cents, no problem >> there, but Idss is 1 to 3 mA and it may change a bunch with >> temperature. >> >> 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. > >> >> This will be the ADC voltage reference for an NXP ARM processor. I'm >> doing a 12-channel 4-20 mA thing, electrically isolated per channel, >> with an ARM per channel. The ARM and other stuff might need 45 mA or >> so, less if we can sleep the code a lot. > >why not pick an ARM with a build in reference? something like >stm32... ? >12 bits too, think the nxp is only 10bits The NXP LPC1768 we're going to use has a 12 bit mux'd ADC and a 10 bit DAC, and all the other stuff like ram, flash, SPI we need. It's a 100 MHz CPU with single-cycle multiply, pretty impressive for around $7. We're going to be running a couple of PID loops as fast as the ADC can feed us data, 100K hits a second maybe. Some of the ST parts look nice, but we already have the compiler/jtag infrastructure in place for the NXP, and we're blinking LEDs, so we'll stick with that. > >I'm sure analog has some too ARM seems to have won the embedded game. Freescale/Coldfire is rumored to be seriously ill. John |