On 01/10/2018 09:14 PM, whit3rd wrote:> On Wednesday, January 10, 2018 at 5:21:39 PM UTC-8, George Herold wrote: > > >> The grey haired tech turned him onto an LM134. >> (A new IC for me too.) >> http://www.ti.com/lit/ds/symlink/lm134.pdf >> Apparently they are running it dry down at 1uA >> and either need more voltage or less current.** >> (Supply is a 9V alkaline battery.) > > For low noise at low current, bipolars aren't ideal (shot > noise). How about the matched MOSFETs from > ALD? Cascode current mirror will have good compliance. > > <http://aldinc.com/pdf/ALD1101.pdf> > > There's good references with smallish currents, but mainly > (because they're bandgap based) there's minimum current > 100 uA (TLVH431) and shot noise: you'll want to filter them >Nah, bipolars are the bomb. You just have to use one whose beta holds up at low current (e.g. a BFT25A), and use enough emitter degeneration. It's no problem making quiet current sources well down into the nanoamps that way. I have a laser driver product whose noise is pretty nearly unmeasurable: > 30 dB below shot noise at 100 mA--SPICE says > 50 dB, except at very low frequency where op amp noise is important. To get that quiet, you need a low noise transistor and have to be willing to drop several volts across the degeneration resistor. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net https://hobbs-eo.com
Sub uA battery powered current source.
Started by ●January 10, 2018
Reply by ●January 11, 20182018-01-11
Reply by ●January 11, 20182018-01-11
George Herold wrote...> >On Thursday, January 11, 2018 at 6:09:44 AM UTC-5, Winfield Hill wrote: >> George Herold wrote... >> > >> > Anyway I promised a ~100nA and lower design. >> > >> > My first (and only) thought was a voltage reference >> > feeding a big R ... ascci art. >> > >> >Vin----+ Vref +---RRR--+->I out >> > | /| | >> > +----< |-----+ >> > \| >> >> That's Figure 13.16 in AOE III. I made one of those >> as my first bench project at the Institute in 1988.> I found it in the 'circuit ideas' at the end of some > chapter in AoE1 or 2. I've used it several times.Yes, I'm sorry I couldn't finish the editing to keep those. Have gotten lots of complaints!>> But Figure 13.15 on the next page is more interesting, >> part of our "Designs by the Masters" series. You can >> simplify it since you don't need the DAC programming. >> >> . ,-- zener ----, >> . | A2 -in --+----, >> . +---out +in ---, | >> . | | Rbias >> . | | | >> . | ,------------+ Vminus >> . | '- -in A1 | >> . +--- +in out --+---- Vout >> . | monitor >> . Iout > > Huh, OK I'll have to redraw, I was trying to think > how to use a shunt reference. Aren't you missing > the current setting resistor between out of A2 and > (+) in of A1?Right! . ,-- zener ----, . | A2 -in --+----, . +---out +in ---, | . | | Rbias . Rset | | . | ,------------+ Vminus . | '- -in A1 | . +--- +in out --+---- Vout . | monitor . Iout> Hmm where does Vminus come from? (another battery?)From your opamp's minus power rail.> I'll probably use a three terminal voltage ref... > I've already ordered some LT6654's from DK.It's just that those may be more noisy. -- Thanks, - Win
Reply by ●January 11, 20182018-01-11
On Thursday, January 11, 2018 at 7:18:04 AM UTC-8, George Herold wrote:> On Thursday, January 11, 2018 at 4:02:53 AM UTC-5, whit3rd wrote:> > Thinking something like this: > > > > <https://www.digikey.com/schemeit/project/low-current-control-SH68M9G30290/>> Scratch, scratch... OK I'm not getting that. Is it like a high side > current source. ref-opamp-pfet?Yes, starting with a current sink; a bridge balances an I*R drop on a known resistor against a Zener voltage, both not-near-the-rail. The op amp drives a current mirror; the mirror's output compliance can be nearly the full voltage of the 9V battery. Where I wanted to put a bandgap reference (diode/TL431), it's a one-cell battery symbol, because there's a quirky symbol set available. Would you believe there's a Shockley diode, but not a Zener symbol?
Reply by ●January 11, 20182018-01-11
On Thursday, January 11, 2018 at 3:00:58 PM UTC-5, whit3rd wrote:> On Thursday, January 11, 2018 at 7:18:04 AM UTC-8, George Herold wrote: > > On Thursday, January 11, 2018 at 4:02:53 AM UTC-5, whit3rd wrote: > > > > Thinking something like this: > > > > > > <https://www.digikey.com/schemeit/project/low-current-control-SH68M9G30290/> > > > Scratch, scratch... OK I'm not getting that. Is it like a high side > > current source. ref-opamp-pfet? > > Yes, starting with a current sink; a bridge balances an I*R drop > on a known resistor against a Zener voltage, both not-near-the-rail. > The op amp drives a current mirror; the mirror's output compliance can > be nearly the full voltage of the 9V battery. > > Where I wanted to put a bandgap reference (diode/TL431), it's a one-cell battery > symbol, because there's a quirky symbol set available. Would you believe > there's a Shockley diode, but not a Zener symbol?OK, that makes more sense. I really like the Vref, R, opamp for it's simplicity. Say I looked up battery noise, there was some Fed. (nasa?) report from 1995 saying NiCd's had the lowest noise. But I could find nothing about LiPo's. Has anyone looked at battery noise? Maybe I could do that some time. They had to use that cool dual amp chain correlation trick to beat down the amp noise, I've always wanted to try that. (Two amps looking at same noise, the amp noise is un-correlated and averages to zero.) Actually I'm more interested in the much bigger microphonic noise that I saw in the same lab years ago. George H.
Reply by ●January 11, 20182018-01-11
On Thursday, January 11, 2018 at 1:32:46 PM UTC-5, Winfield Hill wrote:> George Herold wrote... > > > >On Thursday, January 11, 2018 at 6:09:44 AM UTC-5, Winfield Hill wrote: > >> George Herold wrote... > >> > > >> > Anyway I promised a ~100nA and lower design. > >> > > >> > My first (and only) thought was a voltage reference > >> > feeding a big R ... ascci art. > >> > > >> >Vin----+ Vref +---RRR--+->I out > >> > | /| | > >> > +----< |-----+ > >> > \| > >> > >> That's Figure 13.16 in AOE III. I made one of those > >> as my first bench project at the Institute in 1988. > > > I found it in the 'circuit ideas' at the end of some > > chapter in AoE1 or 2. I've used it several times. > > Yes, I'm sorry I couldn't finish the editing to keep > those. Have gotten lots of complaints!Complaints? Ignore them. They can buy the earlier versions. You know the earlier versions are much better for beginners, are they still in print? And of more importance to me, any guess on the X-chapters?... maybe you could dribble those out in few chapter sections, you might make more money that way. (Maybe not)> > >> But Figure 13.15 on the next page is more interesting, > >> part of our "Designs by the Masters" series. You can > >> simplify it since you don't need the DAC programming. > >> > >> . ,-- zener ----, > >> . | A2 -in --+----, > >> . +---out +in ---, | > >> . | | Rbias > >> . | | | > >> . | ,------------+ Vminus > >> . | '- -in A1 | > >> . +--- +in out --+---- Vout > >> . | monitor > >> . Iout > > > > Huh, OK I'll have to redraw, I was trying to think > > how to use a shunt reference. Aren't you missing > > the current setting resistor between out of A2 and > > (+) in of A1? > > Right! > > . ,-- zener ----, > . | A2 -in --+----, > . +---out +in ---, | > . | | Rbias > . Rset | | > . | ,------------+ Vminus > . | '- -in A1 | > . +--- +in out --+---- Vout > . | monitor > . Iout > > > Hmm where does Vminus come from? (another battery?) > > From your opamp's minus power rail. > > > I'll probably use a three terminal voltage ref... > > I've already ordered some LT6654's from DK. > > It's just that those may be more noisy.OK, but if the noise is less than that of R_set you hardly care. (For the physics lab measurement we'd run with a battery, and series R cooled down to the same temp as the detector. ~4K.) George H.> > > -- > Thanks, > - Win
Reply by ●January 11, 20182018-01-11
On Thursday, January 11, 2018 at 6:25:44 PM UTC-5, George Herold wrote:> On Thursday, January 11, 2018 at 1:32:46 PM UTC-5, Winfield Hill wrote: > > George Herold wrote... > > > > > >On Thursday, January 11, 2018 at 6:09:44 AM UTC-5, Winfield Hill wrote: > > >> George Herold wrote... > > >> > > > >> > Anyway I promised a ~100nA and lower design. > > >> > > > >> > My first (and only) thought was a voltage reference > > >> > feeding a big R ... ascci art. > > >> > > > >> >Vin----+ Vref +---RRR--+->I out > > >> > | /| | > > >> > +----< |-----+ > > >> > \| > > >> > > >> That's Figure 13.16 in AOE III. I made one of those > > >> as my first bench project at the Institute in 1988. > > > > > I found it in the 'circuit ideas' at the end of some > > > chapter in AoE1 or 2. I've used it several times. > > > > Yes, I'm sorry I couldn't finish the editing to keep > > those. Have gotten lots of complaints! > Complaints? Ignore them. They can buy the earlier versions. > You know the earlier versions are much better for beginners, > are they still in print? > And of more importance to me, any guess on the X-chapters?... > maybe you could dribble those out in few chapter sections, > you might make more money that way. (Maybe not) > > > > > >> But Figure 13.15 on the next page is more interesting, > > >> part of our "Designs by the Masters" series. You can > > >> simplify it since you don't need the DAC programming. > > >> > > >> . ,-- zener ----, > > >> . | A2 -in --+----, > > >> . +---out +in ---, | > > >> . | | Rbias > > >> . | | | > > >> . | ,------------+ Vminus > > >> . | '- -in A1 | > > >> . +--- +in out --+---- Vout > > >> . | monitor > > >> . Iout > > > > > > Huh, OK I'll have to redraw, I was trying to think > > > how to use a shunt reference. Aren't you missing > > > the current setting resistor between out of A2 and > > > (+) in of A1? > > > > Right! > > > > . ,-- zener ----, > > . | A2 -in --+----, > > . +---out +in ---, | > > . | | Rbias > > . Rset | | > > . | ,------------+ Vminus > > . | '- -in A1 | > > . +--- +in out --+---- Vout > > . | monitor > > . Iout > > > > > Hmm where does Vminus come from? (another battery?) > > > > From your opamp's minus power rail. > > > > > I'll probably use a three terminal voltage ref... > > > I've already ordered some LT6654's from DK. > > > > It's just that those may be more noisy. > OK, but if the noise is less than that of R_set > you hardly care.Hmm, is that wrong? Does the active voltage reference cancel out the Rset Johnson noise? (at least up to some frequency.) GH> (For the physics lab measurement we'd run > with a battery, and series R cooled down to the same > temp as the detector. ~4K.) > > George H. > > > > > > -- > > Thanks, > > - Win
Reply by ●January 11, 20182018-01-11
George Herold wrote...> > Hmm, is that wrong? Does the active voltage reference > cancel out the Rset Johnson noise? (at least up to > some frequency.)The current noise density is (4kT/R)^0.5 + e_n(ref)/R. So you want e_n(ref) < (4kTR)^0.5, so if I = 1uA and R=6.2M for 6.2V zener, then e_n << 314nV/rt-Hz at room temp, say allow an e_n of 100nV/rt-Hz. The 1uA current noise will be 50fA/rt-Hz, which is 11x lower than the 565fA ordinary 1uA shot noise would have been. All are white noise, with RMS = i_n * sqrt BW, etc. -- Thanks, - Win
Reply by ●January 11, 20182018-01-11
On Thursday, January 11, 2018 at 3:08:06 PM UTC-8, George Herold wrote:> ... I really like the Vref, R, opamp > for it's simplicity.It's certainly likeable, but the unknown load connected directly to an op amp input makes me itch. I like my control knobs away from the playful kittens. Thus, a current mirror.
Reply by ●January 12, 20182018-01-12
On Thursday, January 11, 2018 at 8:35:01 PM UTC-5, whit3rd wrote:> On Thursday, January 11, 2018 at 3:08:06 PM UTC-8, George Herold wrote: > > > ... I really like the Vref, R, opamp > > for it's simplicity. > > It's certainly likeable, but the unknown load connected directly to > an op amp input makes me itch. I like my control knobs away from > the playful kittens. Thus, a current mirror.Hmm I never worried about the load connection. I guess you could put ~1k ohm in series with the (+) input. Or if more worried a pair of LND150's (my new favorite input protection, 'hammer'.) George H.
Reply by ●January 12, 20182018-01-12
On Friday, January 12, 2018 at 12:01:32 PM UTC-8, George Herold wrote:> On Thursday, January 11, 2018 at 8:35:01 PM UTC-5, whit3rd wrote: > > On Thursday, January 11, 2018 at 3:08:06 PM UTC-8, George Herold wrote: > > > > > ... I really like the Vref, R, opamp > > > for it's simplicity. > > > > It's certainly likeable, but the unknown load connected directly to > > an op amp input makes me itch. I like my control knobs away from > > the playful kittens. Thus, a current mirror. > > Hmm I never worried about the load connection. I guess you could put > ~1k ohm in series with the (+) input. Or if more worried a pair of > LND150's (my new favorite input protection, 'hammer'.)My concern was more with output isolation. There's a microsecond or five while an op amp resettles, that could disturb pulse-shapes.
