On Tue, 11 Sep 2012 14:35:23 -0700 (PDT), Bill Sloman <bill.sloman@ieee.org> wrote:>On Sep 11, 8:30�pm, John Larkin <jlar...@highlandtechnology.com> >wrote: >> On Tue, 11 Sep 2012 10:55:24 -0700 (PDT),BillSloman >> >> <bill.slo...@ieee.org> wrote: >> >On Sep 11, 4:19�pm, John Larkin >> ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> >> On Tue, 11 Sep 2012 00:32:46 -0700 (PDT), whit3rd <whit...@gmail.com> >> >> wrote: >> >> >> >On Monday, September 10, 2012 7:53:37 PM UTC-7, John Larkin wrote: >> >> >> >[about light sensitivity of a LED used as voltage reference] >> >> >> >> Well, I measured it this afternoon. There is no discernable >> >> >> >> photoelectric effect, even with intense illumination. I didn't expect >> >> >> >> any, and Phil didn't expect any. >> >> >> >When I put a light chopper on a laser and detect a weak >> >> >response signal with a lock-in amplifier, the sensitivity will be >> >> >much more clear. �Pay the extra nickel, use a REAL >> >> >reference. �LM4140adj �is suitable, just add two >> >> >resistors (the op amp is built in). >> >> >> I don't want a voltage reference, I want a low-capacitance current >> >> source. >> >> >You are being disingenuous. >> >> >But you want a more or less stable current source, and the >> >compensating drifts of the LED and the transistor gives the stable >> >voltage that controls the current output. The transistor makes for a >> >simpler, cheaper and more compact output than anything fast enough/low >> >capacitance enough for you that you could wrap around an LM4140. >> >> >If you needed something more accurate, you'd need a better reference >> >voltage, but it seems that here you can get away with being a >> >cheapskate. >> >> The board is 2" x 2" and it's brickwalled with parts. This current >> source is small, cute, glows a nice warm orange color, and is plenty >> good enough for my needs here. >> >> Is there some virtue to wasting more board area and more money, for no >> advantage? > >Absolutely none. One definition of an engineer is somebody who can do >for $1 what any fool can do for $2. Cheapskate is a term of >approbation in this context, though Phil Hobbs - as a physicist - >doesn't seem to realise this. > >> Actually, I've known some engineers who would usually >> select a more expensive part, because it apparently made them feel >> more important. > >The world is full of lunatics, but lunacy tends to take different >forms in different people. > >> I measured a TC of 90 PPM/degC, which is better than the spec on the >> metal-film emitter resistor on the breadboard. > >I tended to use 15ppm/C metal film resistors where it mattered - they >aren't that expensive and they are easy to get hold of. > >> And as Phil points out, >> this circuit will have much lower noise than something based on a >> bandgap reference and an op amp. > >Perhaps not. You've probably got a lot more bandwidth than you'd end >up with if you were engineering something around a bandgap reference. >Phil's being a physicist rather than thinking about the circuit.Wow, you have this overwhelming desire to be disliked. And to look like an idiot. Enjoy. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
LED reference current source
Started by ●September 9, 2012
Reply by ●September 11, 20122012-09-11
Reply by ●September 11, 20122012-09-11
On Sep 12, 12:23=A0am, John Larkin <jlar...@highlandtechnology.com> wrote:> On Tue, 11 Sep 2012 14:35:23 -0700 (PDT),BillSloman > > <bill.slo...@ieee.org> wrote: > >On Sep 11, 8:30=A0pm, John Larkin <jlar...@highlandtechnology.com> > >wrote: > >> On Tue, 11 Sep 2012 10:55:24 -0700 (PDT),BillSloman > > >> <bill.slo...@ieee.org> wrote: > >> >On Sep 11, 4:19=A0pm, John Larkin > >> ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > >> >> On Tue, 11 Sep 2012 00:32:46 -0700 (PDT), whit3rd <whit...@gmail.co=m>> >> >> wrote: > > >> >> >On Monday, September 10, 2012 7:53:37 PM UTC-7, John Larkin wrote: > > >> >> >[about light sensitivity of a LED used as voltage reference] > > >> >> >> Well, I measured it this afternoon. There is no discernable > > >> >> >> photoelectric effect, even with intense illumination. I didn't e=xpect> > >> >> >> any, and Phil didn't expect any. > > >> >> >When I put a light chopper on a laser and detect a weak > >> >> >response signal with a lock-in amplifier, the sensitivity will be > >> >> >much more clear. =A0Pay the extra nickel, use a REAL > >> >> >reference. =A0LM4140adj =A0is suitable, just add two > >> >> >resistors (the op amp is built in). > > >> >> I don't want a voltage reference, I want a low-capacitance current > >> >> source. > > >> >You are being disingenuous. > > >> >But you want a more or less stable current source, and the > >> >compensating drifts of the LED and the transistor gives the stable > >> >voltage that controls the current output. The transistor makes for a > >> >simpler, cheaper and more compact output than anything fast enough/lo=w> >> >capacitance enough for you that you could wrap around an LM4140. > > >> >If you needed something more accurate, you'd need a better reference > >> >voltage, but it seems that here you can get away with being a > >> >cheapskate. > > >> The board is 2" x 2" and it's brickwalled with parts. This current > >> source is small, cute, glows a nice warm orange color, and is plenty > >> good enough for my needs here. > > >> Is there some virtue to wasting more board area and more money, for no > >> advantage? > > >Absolutely none. One definition of an engineer is somebody who can do > >for $1 what any fool can do for $2. Cheapskate is a term of > >approbation in this context, though Phil Hobbs - as a physicist - > >doesn't seem to realise this. > > >> Actually, I've known some engineers who would usually > >> select a more expensive part, because it apparently made them feel > >> more important. > > >The world is full of lunatics, but lunacy tends to take different > >forms in different people. > > >> I measured a TC of 90 PPM/degC, which is better than the spec on the > >> metal-film emitter resistor on the breadboard. > > >I tended to use 15ppm/C metal film resistors where it mattered - they > >aren't that expensive and they are easy to get hold of. > > >> And as Phil points out, > >> this circuit will have much lower noise than something based on a > >> bandgap reference and an op amp. > > >Perhaps not. You've probably got a lot more bandwidth than you'd end > >up with if you were engineering something around a bandgap reference. > >Phil's being a physicist rather than thinking about the circuit. > > Wow, you have this overwhelming desire to be disliked. And to look > like an idiot. Enjoy.I may have such an overwhelming desire - it would come as a surprise to me and my acquaintances, but perhaps we are a more robust lot than American pantywaists - but anything I may achieve in that direction pales into insignificance when compared with your extravagant pratfalls and malicious irrelevancies. Find a specific idiocy in the post you responded to, and identify it. I haven't had a good laugh for several days, and the only way you are going to deliver on that claim is going to be with a John Fields/Jamie level of reading incomprehension. -- Bill Sloman, Nijmegen
Reply by ●September 11, 20122012-09-11
On 9/10/2012 11:32 PM, Jon Kirwan wrote:> On Sun, 09 Sep 2012 19:01:02 -0700, Jon Kirwan > <jonk@infinitefactors.org> wrote: > >> On Sun, 09 Sep 2012 18:50:25 -0700, John Larkin >> <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >> >>> Has anybody done this? >>> >>> https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG >> >> Must have been, since I just saw some discussion about it on >> LTspice group less than a month ago. >> >> Jon > > Just doing a few searches found these: > > http://www.ciphersbyritter.com/RADELECT/MEASNOIS/NOISMEA1.HTM > > Also, AN-211 from Analog Devices discussing the use of a pair > of back to back 2N3904 BE junctions zenered to supply a 15V > reference with very little noise (bypassed with 10uF tants.) > See Figure 3 on it, Q14 through Q17. > > And check out DIYAudio, from 2002: > > http://www.diyaudio.com/forums/parts/35821-some-noise-measurements-leds-zener-diodes.html > > Jon >It seems to me you want the lowest voltage LED that has the magic tempco to produce a constant delta-V. The lowest voltage implies the smallest resistor going from the voltage supply to the BJT emitter. It follows that the smallest resistor has the least thermal noise. Thus the IR led looks like something worthy of examination. I'm not so sure I'd run one at 20ma unless someone has lifetime stability data on this.
Reply by ●September 11, 20122012-09-11
> A bandgap takes a Vbe and adds 10x the difference of two others. That > makes it about 20-25 dB noisier than a Vbe at the same bias current. > Then factor in the very low currents used in most IC references, and > they get noisier still. Not all frequencies are easy to filter. > > Cheers > > Phil Hobbs >For sure the low current requirement is an issue. I've done references under 10uA. The problem is there are products that have to be in "shutdown", but the bandgap is still on.
Reply by ●September 11, 20122012-09-11
On 9/11/2012 10:54 AM, Dave Platt wrote:> In article <87wr01yw1u.fsf@devereux.me.uk>, > John Devereux <john@devereux.me.uk> wrote: > >>> Because there isn't one? The LED is a voltage reference and a pilot >>> light. >> >> Hey, I used to be able to crash a quartz-windowed 8749 microcontroller >> by taking a picture of the machine it was in with a flash camera! :) > > A guy I worked with some years ago, told a sad tale of the development > of one of the first very-high-performance "clones" of the Sun > Microsystems SPARC processor. > > The development team got the first sample back from the fab (in a > quartz-lidded ceramic package), carefully powered it up, and it began > running diagnostics just fine. One of the engineers pulled out a > camera and took a photograph of the system board... with flash. There > was immediately a second flash from within the CPU package, as a whole > bunch of the circuitry on the chip latched up and crowbarred the power > supply to ground. Poof. That particular CPU never ran again. > > One of the more expensive "victory" photographs ever taken, I believe. > >If the process was old, i.e. no SOI or even EPI, then only the peripheral pins are designed to handle injection. [Guard rings, etc.] But I'm not accustomed to getting parts back from the fab with a window. Was their UV prom on the chip?
Reply by ●September 12, 20122012-09-12
On Tue, 11 Sep 2012 11:31:58 -0700, John Larkin <jlarkin@highlandtechnology.com> wrote:> >>>>> When I put a light chopper on a laser and detect a weak >>>>> response signal with a lock-in amplifier, the sensitivity will be >>>>> much more clear. Pay the extra nickel, use a REAL >>>>> reference. LM4140adj is suitable, just add two >>>>> resistors (the op amp is built in). >>>> >>>> I don't want a voltage reference, I want a low-capacitance current >>>> source. >>> >>> You are being disingenuous. >>> >>> But you want a more or less stable current source, and the >>> compensating drifts of the LED and the transistor gives the stable >>> voltage that controls the current output. The transistor makes for a >>> simpler, cheaper and more compact output than anything fast =enough/low>>> capacitance enough for you that you could wrap around an LM4140. >>> >>> If you needed something more accurate, you'd need a better reference >>> voltage, but it seems that here you can get away with being a >>> cheapskate. >>> >> >>Congratulations! You've just won this months SED Snarkiest Way Of=20 >>Agreeing With Somebody Award. This prestigious honour is hotly=20 >>contested--it's almost as sought after as the Stupidest Flame War =Award.>> >>Cheers >> >>Phil Hobbs > >Hey, it's only September 11. Give some other guys a chance. >You obviously think you can do better then. Go for it of you want. ?-)
Reply by ●September 12, 20122012-09-12
On Tue, 11 Sep 2012 06:55:21 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>miso wrote: >>=20 >> On 9/10/2012 6:25 AM, Phil Hobbs wrote: >> > Mr Stonebeach wrote: >> >> >> >> On Sep 10, 11:17 am, miso <m...@sushi.com> wrote: >> >>> On 9/9/2012 6:50 PM, John Larkin wrote: >> >>>> Has anybody done this? >> >>>> https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG >> >>> >> >>> Analog Device SSM-2220 data sheet uses that scheme. Nelson Pass =amps>> >>> have used it in production. LED biasing was a wave or sorts in =high end>> >>> audio. >> >> >> >> Also in SQUID readout amplifiers, like Pasquarelli & Rossi in =the>> >> ISEC'97 conference. >> >> >> >>> The RF transistor I presume is for low capacitance. The low beta =could>> >>> be an issue. >> >> >> >> What about NESG4030, hFE=3D400 typ, Cre=3D0.12pF typ? I know, =costs way>> >> too much... >> >> >> >> Regards, >> >> Mikko >> > >> > The LED trick is about 20 dB quieter than a bandgap, and reasonably >> > competitive with buried zeners for noise, but of course not for >> > stability. I haven't done this recently enough to have tempco data =on>> > modern LEDs. (I've only used the LED + emitter follower voltage >> > reference form.) >> > >> > Cheers >> > >> > Phil Hobbs >> > >> Can you elaborate on the noise aspect? After all, most bandgaps are >> filtered. Or are you referring to an op amp circuit using the bandgap =to>> create the current reference? >>=20 >> It seems questionable to make such a blanket statement about the LED >> reference being 20dB quieter without describing the competing circuit. > >A bandgap takes a Vbe and adds 10x the difference of two others. That >makes it about 20-25 dB noisier than a Vbe at the same bias current.=20 >Then factor in the very low currents used in most IC references, and >they get noisier still. Not all frequencies are easy to filter. > >Cheers > >Phil HobbsMmmm. Flicker and popcorn noises. Even the shot noise at those = currents, not pretty. Now i will remember even better. Thanx. ?-)
Reply by ●September 12, 20122012-09-12
On 9/11/2012 7:25 PM, miso wrote:> On 9/10/2012 11:32 PM, Jon Kirwan wrote: >> On Sun, 09 Sep 2012 19:01:02 -0700, Jon Kirwan >> <jonk@infinitefactors.org> wrote: >> >>> On Sun, 09 Sep 2012 18:50:25 -0700, John Larkin >>> <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >>> >>>> Has anybody done this? >>>> >>>> https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG >>> >>> Must have been, since I just saw some discussion about it on >>> LTspice group less than a month ago. >>> >>> Jon >> >> Just doing a few searches found these: >> >> http://www.ciphersbyritter.com/RADELECT/MEASNOIS/NOISMEA1.HTM >> >> Also, AN-211 from Analog Devices discussing the use of a pair >> of back to back 2N3904 BE junctions zenered to supply a 15V >> reference with very little noise (bypassed with 10uF tants.) >> See Figure 3 on it, Q14 through Q17. >> >> And check out DIYAudio, from 2002: >> >> http://www.diyaudio.com/forums/parts/35821-some-noise-measurements-leds-zener-diodes.html >> >> >> Jon >> > It seems to me you want the lowest voltage LED that has the magic tempco > to produce a constant delta-V. The lowest voltage implies the smallest > resistor going from the voltage supply to the BJT emitter. It follows > that the smallest resistor has the least thermal noise. Thus the IR led > looks like something worthy of examination. > > I'm not so sure I'd run one at 20ma unless someone has lifetime > stability data on this. > >The lowest LED voltage implies the higher LED biasing resistor, which in turn means it will have the least current variation. It also seems to me that you could bootstrap the LED biasing to make it "stiffer" via negative feedback. Add another BJT and resistor to make another current source. Feed that current source into the LED biasing resitor. Now it would have to be a small fraction of the current that would otherwise flow in the LED biasing resistor else it would perhaps have start up issues. The negative feedback goes as follows. For increased LED voltage, the extra current source BJT shunts some current away from the LED, which in turn reduces the current in the LED and thus the voltage.
Reply by ●September 12, 20122012-09-12
On Tue, 11 Sep 2012 20:30:47 -0700, josephkk <joseph_barrett@sbcglobal.net> wrote:>On Tue, 11 Sep 2012 11:31:58 -0700, John Larkin ><jlarkin@highlandtechnology.com> wrote: > >> >>>>>> When I put a light chopper on a laser and detect a weak >>>>>> response signal with a lock-in amplifier, the sensitivity will be >>>>>> much more clear. Pay the extra nickel, use a REAL >>>>>> reference. LM4140adj is suitable, just add two >>>>>> resistors (the op amp is built in). >>>>> >>>>> I don't want a voltage reference, I want a low-capacitance current >>>>> source. >>>> >>>> You are being disingenuous. >>>> >>>> But you want a more or less stable current source, and the >>>> compensating drifts of the LED and the transistor gives the stable >>>> voltage that controls the current output. The transistor makes for a >>>> simpler, cheaper and more compact output than anything fast enough/low >>>> capacitance enough for you that you could wrap around an LM4140. >>>> >>>> If you needed something more accurate, you'd need a better reference >>>> voltage, but it seems that here you can get away with being a >>>> cheapskate. >>>> >>> >>>Congratulations! You've just won this months SED Snarkiest Way Of >>>Agreeing With Somebody Award. This prestigious honour is hotly >>>contested--it's almost as sought after as the Stupidest Flame War Award. >>> >>>Cheers >>> >>>Phil Hobbs >> >>Hey, it's only September 11. Give some other guys a chance. >> >You obviously think you can do better then. Go for it of you want. > >?-)That wasn't very good; Sloman remains in the lead. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Reply by ●September 12, 20122012-09-12
Am 11.09.2012 20:30, schrieb John Larkin:> The board is 2" x 2" and it's brickwalled with parts.If it is not too rf-ish and there is not much space you might consider this one <http://www.digikey.de/product-detail/de/BCR%20400W%20E6327/BCR400WE6327INTR-ND/1281377> (infineon BCR400W) regards, Gerhard
