# Sub uA battery powered current source.

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So I was out at UB, (picking up some stuff) I was talking with
a resistive FIR detector with a current source.
The grey haired tech turned him onto an LM134.
(A new IC for me too.)
Apparently they are running it dry down at 1uA
and either need more voltage or less current.**
(Supply is a 9V alkaline battery.)
I don't know what happens when you try to run at
less than 1 uA, the spec sheet stops at 10.

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
|     /|     |
+----< |-----+
\|

But I'm open to other ideas.
It needs to be low noise.  The LM134 is about 4 times
shot noise, so that's a target.

First question; How about some other battery.  Those 9V
transistor batters are noisy.  My son uses 2 and 3 cell
Lipo's (rechargeable), how are they noise wise?

For the opmap I'll need a R-R input I've got opa2192
that should work.  Then what voltage reference?
I've got REF02's 5V, but they are kinda noisy.
Checking AoE3 the LT6654 looks nice, (and not all that spendy)
http://cds.linear.com/docs/en/datasheet/6654fh.pdf

Is there any reason not to use the 1.25V one?

1.25 V /10Meg is ~ 100nA, and the 100nV/rtHz of noise is
10 fA/rtHz... DC (0.1 - 10 Hz) is ~1uV/10M so 100fA(p-p)

Oh the current noise of the R is
(1k=4nV/rtHz, 100k = 40, 10M= 400nV = 40 fA/rtHz, the resistor
the biggest source.  I guess I might gain a little with a 2.5V ref.

George H.

*Bruce was my post-doc PI (he got the grants)
also a good friend.

**(They need to run the magnet colder, and that
mean the detector is colder, with higher resistance.)

Re: Sub uA battery powered current source.
On Wed, 10 Jan 2018 17:21:32 -0800 (PST), George Herold

Can you apply a voltage across the detector and measure current?

I wonder if a transistor with an emitter resistor would work. The more
voltage across the resistor, the lower the Johnson current noise. How
much voltage drop will the detector have?

--

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement

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Re: Sub uA battery powered current source.
On Wednesday, January 10, 2018 at 8:37:32 PM UTC-5, John Larkin wrote:

Sure, the pre-amp would have to be different... At the moment it's
a three wire connection to a photoresistor down the bottom of a
~4' probe. liquid He.

I think it's about 1-2 Meg ohm at 4.2 K.
They need to cool to ~2K, which raises the resistance.

(Seems like 1 uA should be low enough.. the grad student might be
confused...)

Re: Sub uA battery powered current source.
On Wed, 10 Jan 2018 18:05:10 -0800 (PST), George Herold

Why not a power supply and a resistor?

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

Re: Sub uA battery powered current source.
On Thursday, January 11, 2018 at 12:00:53 AM UTC-5, John Larkin wrote:

Yeah that's what they were using.  Battery is noisy, so maybe just
some filter.  Voltage reference and filter?
Anyway I promised a current source.

George H.

Re: Sub uA battery powered current source.
On Wednesday, January 10, 2018 at 5:21:39 PM UTC-8, George Herold wrote:

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

Re: Sub uA battery powered current source.
On 01/10/2018 09:14 PM, whit3rd wrote:

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
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Re: Sub uA battery powered current source.
On Wednesday, January 10, 2018 at 5:21:39 PM UTC-8, George Herold wrote:

Thinking something like this:

<https://www.digikey.com/schemeit/project/low-current-control-SH68M9G30290/>

except the single cell battery spozed to be a programmable zener (TLVH431A), and the
current mirror shown isn't the cascode that I envisioned, and a capacitor or two
would help... and every time I try to edit it, the wires snarl differently.

ALD's matched MOS device determines what R2 value biases the '431,
and R3 ~= R1 for bias current compensation,
and the op amp can be slow but low-noise and low power.

Re: Sub uA battery powered current source.
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?

George H.

Re: Sub uA battery powered current source.
On Thursday, January 11, 2018 at 7:18:04 AM UTC-8, George Herold wrote:

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?

Re: Sub uA battery powered current source.
On Thursday, January 11, 2018 at 3:00:58 PM UTC-5, whit3rd wrote:

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.

Re: Sub uA battery powered current source.
On Thursday, January 11, 2018 at 3:08:06 PM UTC-8, George Herold wrote:

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.

Re: Sub uA battery powered current source.
On Thursday, January 11, 2018 at 8:35:01 PM UTC-5, whit3rd wrote:

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.

Re: Sub uA battery powered current source.
On Friday, January 12, 2018 at 12:01:32 PM UTC-8, George Herold wrote:

My concern was more with output isolation.   There's a microsecond or five
while an op amp resettles, that could disturb pulse-shapes.

Re: Sub uA battery powered current source.
On Friday, January 12, 2018 at 4:01:49 PM UTC-5, whit3rd wrote:

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

Re: Sub uA battery powered current source.
George Herold wrote...

That's Figure 13.16 in AOE III.  I made one of those
as my first bench project at the Institute in 1988.
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

This circuit has the advantage that it can work
up toward the positive rail, without dramatically
lowering the zener bias current.  The current
might instead only double.  Only A1 needs to be
a good JFET or MOSFET opamp.

As for the zener, rather than use a two-terminal
IC Vbe reference, etc., to get lower e_n, use a
5.6 or 6.2-volt reference zener, a real zener
diode, operating in optimum combo field-effect /
zener region, see AoE III page 674.

I remember Dave Jones looking for the precision
voltage reference in his teardown of a Keithley
5-digit instrument, repeatedly passing over an
innocent looking DO-41 glass zener diode, and
not realizing that it was the vaunted beast!

--
Thanks,
- Win

Re: Sub uA battery powered current source.
On Thursday, January 11, 2018 at 6:09:44 AM UTC-5, Winfield Hill wrote:

I found it in the 'circuit ideas' at the end of some chapter
in AoE1 or 2.  I've used it several times.

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?

Hmm where does Vminus come from?  (another battery?)
I'll probably use a three terminal voltage ref...
I've already ordered some LT6654's from DK.

Thanks,
George H.

Re: Sub uA battery powered current source.
George Herold wrote...

Yes, I'm sorry I couldn't finish the editing to keep
those.  Have gotten lots of complaints!

Right!

.   ,-- zener ----,
.   | A2    -in --+----,
.   +---out +in ---,   |
.   |              |  Rbias
.  Rset            |   |
.   | ,------------+  Vminus
.   | '- -in   A1  |
.   +--- +in out --+---- Vout
.   |                   monitor
.  Iout

From your opamp's minus power rail.

It's just that those may be more noisy.

--
Thanks,
- Win

Re: Sub uA battery powered current source.
On Thursday, January 11, 2018 at 1:32:46 PM UTC-5, Winfield Hill wrote:

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)

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.

Re: Sub uA battery powered current source.
On Thursday, January 11, 2018 at 6:25:44 PM UTC-5, George Herold wrote:

Hmm, is that wrong?  Does the active voltage reference
cancel out the Rset Johnson noise?  (at least up to
some frequency.)

GH