On Friday, January 12, 2018 at 4:01:49 PM UTC-5, whit3rd wrote:
> 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.
Oh, you mean the time response? If the load changes or set point
voltage is modulated. I've never really looked at that.
For this case there's a ~1meg resistor (changing ~1-10% with the light,
IIRC) with cable capacitance down a probe... ~10-100pF? The time
response is audio.
Oh I needed to give myself a head slap. Two circuits ago in the
current project (at work), The 10 uA current source* is as above,
with lnd150's on the the output line... in series with the output and
not just the feedback to the opamp. (at 10 uA you don't really care about
a kilo ohm.)
George H.
*diode temp sensor
Reply by whit3rd●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.
Reply by George Herold●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 whit3rd●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 Winfield Hill●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 George Herold●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 George Herold●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 George Herold●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 whit3rd●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:
> 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 Winfield Hill●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?