Cooling the electrometer front-end...

[Piotr Wyderski]

Gentlemen,

Since there is a discussion about ovenized oscillators, here\'s something
for restoring the broken symmetry.

I am planning to build an electrometer based on ADA4530-1. Due to the
insanely low leakage currents required, a regular PCB seems to be a
no-go. This instrument will be a one-off, so I can safely ignore the
usual DFM rules. Namely, I am considering a dead-bug arrangement of the
critical components and using air as insulation. For mechanical
stiffness and reasonable thermal conductance to avoid gradients, a
ceramic base plate would be one idea. But the ceramic can be a part of a
TEC module. Question: would it be worth the trouble to cool the
ADA4530-1 (and maybe the ADC and the reference) to ~10 degrees C, in
terms of noise/thermal input offset etc.?

Best regards, Piotr

 

[John Larkin]

On Tue, 24 May 2022 20:37:34 +0200, Piotr Wyderski
<bombald@protonmail.com> wrote:

Gentlemen,

Since there is a discussion about ovenized oscillators, here\'s something
for restoring the broken symmetry.

I am planning to build an electrometer based on ADA4530-1. Due to the
insanely low leakage currents required, a regular PCB seems to be a
no-go. This instrument will be a one-off, so I can safely ignore the
usual DFM rules. Namely, I am considering a dead-bug arrangement of the
critical components and using air as insulation. For mechanical
stiffness and reasonable thermal conductance to avoid gradients, a
ceramic base plate would be one idea. But the ceramic can be a part of a
TEC module. Question: would it be worth the trouble to cool the
ADA4530-1 (and maybe the ADC and the reference) to ~10 degrees C, in
terms of noise/thermal input offset etc.?

Best regards, Piotr

Here\'s my fA parts tester. The critical node is mid-air.

https://www.dropbox.com/sh/5b2fs47b1qukefv/AAAQgOWmjbeYYsxglN23Py2ya?dl=0

The crummy RatShack binding posts were leaky so I had to add the
polycarb.

I was measuring down to about 10 fA. There are better opamps around
now.

--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon

 

[John Larkin]

On Tue, 24 May 2022 20:37:34 +0200, Piotr Wyderski
<bombald@protonmail.com> wrote:

Gentlemen,

Since there is a discussion about ovenized oscillators, here\'s something
for restoring the broken symmetry.

I am planning to build an electrometer based on ADA4530-1. Due to the
insanely low leakage currents required, a regular PCB seems to be a
no-go. This instrument will be a one-off, so I can safely ignore the
usual DFM rules. Namely, I am considering a dead-bug arrangement of the
critical components and using air as insulation. For mechanical
stiffness and reasonable thermal conductance to avoid gradients, a
ceramic base plate would be one idea. But the ceramic can be a part of a
TEC module. Question: would it be worth the trouble to cool the
ADA4530-1 (and maybe the ADC and the reference) to ~10 degrees C, in
terms of noise/thermal input offset etc.?

Best regards, Piotr

Cooling will cause condensation. Bad.

--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon

 

[Piotr Wyderski]

John Larkin wrote:

> Here\'s my fA parts tester. The critical node is mid-air.

Thank you, John. So I am on the right track.

> https://www.dropbox.com/sh/5b2fs47b1qukefv/AAAQgOWmjbeYYsxglN23Py2ya?dl=0

IMHO, if you glued the opamp upside-down, the remaining small components
could be floating in mid-air without that supporting structure. Was
high-g a real concern?

I was measuring down to about 10 fA. There are better opamps around
now.

LMC6001 is not that bad. The ADA4530-1 supports input guarding, though.

Best regards, Piotr

 

[Mike Monett]

Piotr Wyderski <bombald@protonmail.com> wrote:

Gentlemen,

Since there is a discussion about ovenized oscillators, here\'s something
for restoring the broken symmetry.

I am planning to build an electrometer based on ADA4530-1. Due to the
insanely low leakage currents required, a regular PCB seems to be a
no-go. This instrument will be a one-off, so I can safely ignore the
usual DFM rules. Namely, I am considering a dead-bug arrangement of the
critical components and using air as insulation. For mechanical
stiffness and reasonable thermal conductance to avoid gradients, a
ceramic base plate would be one idea. But the ceramic can be a part of a
TEC module. Question: would it be worth the trouble to cool the
ADA4530-1 (and maybe the ADC and the reference) to ~10 degrees C, in
terms of noise/thermal input offset etc.?

Best regards, Piotr

ADA4530 Ouch. $29.370, Single Amp

How about the LMC662 - $1.800 at Rochester, qty 1, Dual, Input Bias Current
2 fA

https://octopart.com/search?q=lmc662&currency=USD&specs=0

As far as cooling, I don\'t think you see much until you get down to liquid
nitrogen temperatures. Then you need ventilation to avoid death.


--
MRM

 

[Jeroen Belleman]

On 2022-05-24 21:38, Mike Monett wrote:

Piotr Wyderski <bombald@protonmail.com> wrote:

Gentlemen,

Since there is a discussion about ovenized oscillators, here\'s something
for restoring the broken symmetry.

I am planning to build an electrometer based on ADA4530-1. Due to the
insanely low leakage currents required, a regular PCB seems to be a
no-go. This instrument will be a one-off, so I can safely ignore the
usual DFM rules. Namely, I am considering a dead-bug arrangement of the
critical components and using air as insulation. For mechanical
stiffness and reasonable thermal conductance to avoid gradients, a
ceramic base plate would be one idea. But the ceramic can be a part of a
TEC module. Question: would it be worth the trouble to cool the
ADA4530-1 (and maybe the ADC and the reference) to ~10 degrees C, in
terms of noise/thermal input offset etc.?

Best regards, Piotr

ADA4530 Ouch. $29.370, Single Amp

How about the LMC662 - $1.800 at Rochester, qty 1, Dual, Input Bias Current
2 fA

https://octopart.com/search?q=lmc662&currency=USD&specs=0

As far as cooling, I don\'t think you see much until you get down to liquid
nitrogen temperatures. Then you need ventilation to avoid death.

I had good fun with LPC661 amplifiers. They\'re specified as having a 2fA
bias current, but the ones I measured only had about 150aA, more than
fifteen times better. I had an tiny ionization chamber attached, so the
real bias current may well be lower still.

This was at room temperature, with the critical node wired in the air.
Cost not even 1$/pc for a reel-full.

Jeroen Belleman

 

[server]

On Tue, 24 May 2022 21:26:04 +0200, Piotr Wyderski
<bombald@protonmail.com> wrote:

John Larkin wrote:

Here\'s my fA parts tester. The critical node is mid-air.

Thank you, John. So I am on the right track.

https://www.dropbox.com/sh/5b2fs47b1qukefv/AAAQgOWmjbeYYsxglN23Py2ya?dl=0

IMHO, if you glued the opamp upside-down, the remaining small components
could be floating in mid-air without that supporting structure. Was
high-g a real concern?

I was measuring down to about 10 fA. There are better opamps around
now.

LMC6001 is not that bad. The ADA4530-1 supports input guarding, though.

Best regards, Piotr

You could surface-mount it and bend the +IN pin out into free space.

The driven guard is very nice. That should block leakage across the
SO8 package.

Ib is about 600 electrons per second.

Don\'t expect anything to happen fast.



--

Anybody can count to one.

- Robert Widlar

 

[whit3rd]

On Tuesday, May 24, 2022 at 11:37:55 AM UTC-7, Piotr Wyderski wrote:

I am planning to build an electrometer based on ADA4530-1. Due to the
insanely low leakage currents required, a regular PCB seems to be a
no-go. This instrument will be a one-off, so I can safely ignore the
usual DFM rules. Namely, I am considering a dead-bug arrangement of the
critical components and using air as insulation.

Air is good, as is a PTFE-insulated standoff or ceramic; some PCB materials with guard-rings
are good, too, I suspect, but this is a TSSOP package. It might be hard to fit in a guard ring,
but you can still bore a hole under the high-Z pin, and carefully hand-wire it to a
thin coax with shield driven appropriately. It doesn\'t matter if the PCB leaks, if the sensitive
lead isn\'t in contact.

It might be possible too, to plunge the coax center wire THROUGH the PCB and butt
to the lead wire ; a snug fit will keep the wire steady during soldering.

Most of the PTFE insulators I see in my junk box are BNC jacks. Solder type ends for
those are circa 2mm, and TSSOP gives you about 1mm to work with. You could
mount such a jack next to the chip, and just flying-wire the last centimeter or two.

 

[Piotr Wyderski]

whit3rd wrote:

Air is good, as is a PTFE-insulated standoff or ceramic; some PCB materials with guard-rings
are good, too, I suspect, but this is a TSSOP package.

It\'s actually a regular SO8, so it makes things simpler. But, given the
QTY 1 and the inadequacy of FR4, I am not considering making a PCB for
this front-end. The current idea is to enclose it in a metal box and
machine a 0.6mm copper plate with a cavity in the middle of the bottom,
edge, just big enough to embrace the SO package tangentially. This
U-shaped plate will then be soldered to both GRD pins, splitting the SO8
into two halves and providing reasonable stiffness. This plate, in turn,
will be then soldered to the enclosure using a small number of those
nifty SMD AlN thermal bridges due to their \"infinite\" resistance and
solderable ends. This levitated and GRD-screened op-amp should take full
advantage of the superior resistance and low soakage of air.

In addition to the 1x, 10x, 100x and 1000x selectable gains I would like
to be able to connect one of several shunts too. Heck, what is the
off-state resistance of a reed relay? The specs say 10G+, but I
believe they were too lazy to measure it.

And what triax connector is actually good?

It might be possible too, to plunge the coax center wire THROUGH the PCB and butt
to the lead wire ; a snug fit will keep the wire steady during soldering.

Most of the PTFE insulators I see in my junk box are BNC jacks. Solder type ends for
those are circa 2mm, and TSSOP gives you about 1mm to work with. You could
mount such a jack next to the chip, and just flying-wire the last centimeter or two.

No problem with flying parts and connections, this is a lab unit not
subject to automated assembly anyway.

Best regard, Piotr

 

[whit3rd]

On Wednesday, May 25, 2022 at 12:14:14 AM UTC-7, Piotr Wyderski wrote:

whit3rd wrote:

Air is good, as is a PTFE-insulated standoff or ceramic; some PCB materials with guard-rings
are good, too...

...U-shaped plate will then be soldered to both GRD pins, splitting the SO8
into two halves and providing reasonable stiffness. This plate, in turn,
will be then soldered to the enclosure using a small number of those
nifty SMD AlN thermal bridges due to their \"infinite\" resistance and
solderable ends. This levitated and GRD-screened op-amp should take full
advantage of the superior resistance and low soakage of air.

The thermal bridges sound like a nifty repurposing; those are usually
kinda short, though, so cleaning is important. I\'ve heard of using small-value
ceramic capacitors, and a burnt-out ceramic fuse should have some standoff virtues:
that\'s something a good electrometer can test!

In addition to the 1x, 10x, 100x and 1000x selectable gains I would like
to be able to connect one of several shunts too. Heck, what is the
off-state resistance of a reed relay? The specs say 10G+, but I
believe they were too lazy to measure it.

Pease had a jig for low-current testing, using plain old springy
pushbutton switch contacts and off-the-shelf relays

<https://www.electronicdesign.com/technologies/test-measurement/article/21766108/whats-all-this-femtoampere-stuff-anyhow>

For quick-connecting a shunt, you might consider
the magnetic-power-connector gizmos like here

<https://www.aliexpress.com/item/3256803873053430.html>

which eliminates the spring tension uncertainty.

> And what triax connector is actually good?

None, probably; to active-shield a long wire, I\'d use coax for signal (center) and shield
(outer), with a braid sleeve overall for a ground. The logic is, capacitance signal-to-shield
is constant (and has null effect due to driven shield) while capacitance shield-to-ground
is high impedance compared to shield drive, so it can vary without harm.

My solution, of course, is a quick and dirty one-off.

The old electrometers used sapphire insulators, and instructed on
careful cleaning; I\'d think quartz or white ceramic is good,
too. The epoxy of an IC package will set a leakage minimum.

 

[Phil Hobbs]

whit3rd wrote:

On Wednesday, May 25, 2022 at 12:14:14 AM UTC-7, Piotr Wyderski wrote:
whit3rd wrote:

Air is good, as is a PTFE-insulated standoff or ceramic; some PCB materials with guard-rings
are good, too...

...U-shaped plate will then be soldered to both GRD pins, splitting the SO8
into two halves and providing reasonable stiffness. This plate, in turn,
will be then soldered to the enclosure using a small number of those
nifty SMD AlN thermal bridges due to their \"infinite\" resistance and
solderable ends. This levitated and GRD-screened op-amp should take full
advantage of the superior resistance and low soakage of air.

The thermal bridges sound like a nifty repurposing; those are usually
kinda short, though, so cleaning is important. I\'ve heard of using small-value
ceramic capacitors, and a burnt-out ceramic fuse should have some standoff virtues:
that\'s something a good electrometer can test!

Ceramics and glass are generally disappointing insulators in the
femtoamp range. PTFE is very good, and a clean plastic DIP package is
also very very good. Anything hydrophilic such as polyamide (nylon) is
a disaster.

Glass can have low leakage, but the soakage (dielectric absorption) of a
glass capacitor has to be seen to be believed.

I do electrometer stuff using small through-hole parts assembled
dead-bug style. Good Medicine.

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
http://hobbs-eo.com

 

[Piotr Wyderski]

whit3rd wrote:

The thermal bridges sound like a nifty repurposing; those are usually
kinda short, though, so cleaning is important.

They come in various sizes, but there is another option:

https://www.mouser.pl/new/ohmite/ohmite-aln-chip-resistors/

These are pretty big. One can solder them to a copper base plate and
then apply precision abrasive Dremel trimming to move their resistance a
bit closer to infinity. Same can be done to any SMD resistor, so I don\'t
know if in this application there are any inherent advantages of AlN as
compared to Al2O3. I don\'t know if grinding would not contaminate the
substrate, though. Not an issue for generic structural support, but
might be for sensitive wire holders.

The old electrometers used sapphire insulators, and instructed on
careful cleaning; I\'d think quartz or white ceramic is good,
too. The epoxy of an IC package will set a leakage minimum.

They discuss 1TOhm resistors and 100TOhm leakage in the datasheet, so
the IC package is not a primary concern IMHO.

Thank you for your hits, much appreciated!

Best regards, Piotr

 

[Bill Beaty]

Question: would it be worth the trouble to cool the ADA4530-1 (and maybe the
ADC and the reference) to ~10 degrees C, in terms of noise/thermal input offset etc.?

Axon Instruments cools their \"Axopatch\" front end ...inside a tiny gold-plated
DIP14 box, epoxy sealed, N2 charged, with a huge piece of dessicant pellet
sealed inside. Their TEC is internal, few mm on a side, with a heavy copper
finger against the bottom of the little box. -30C operating temp. w/thermistor,
for shutting down power until leakage gets low enough.

See photo http://staff.washington.edu/wbeaty/chem_axopatch.html

Gold-plated box is a driven shield. The input is via a gold pin through a little
ceramic disk, then a large ?hdpe? threaded part. They used JFET dies on
ceramic, deadbug leadbond, also no feedback RC, instead it\'s operated as
an integrator, with a slow periodic FET reset, then a differentiator to get back
the DC signal. Supposedly this thing can watch the state of single ion channels
in neurons. $5000 ea, iirc.

 

[Mike Monett]

Bill Beaty <billb@eskimo.com> wrote:

Question: would it be worth the trouble to cool the ADA4530-1 (and
maybe the ADC and the reference) to ~10 degrees C, in terms of
noise/thermal input offset etc.?


Axon Instruments cools their \"Axopatch\" front end ...inside a tiny
gold-plated DIP14 box, epoxy sealed, N2 charged, with a huge piece of
dessicant pellet sealed inside. Their TEC is internal, few mm on a
side, with a heavy copper finger against the bottom of the little box.
-30C operating temp. w/thermistor, for shutting down power until leakage
gets low enough.

See photo http://staff.washington.edu/wbeaty/chem_axopatch.html

Gold-plated box is a driven shield. The input is via a gold pin through
a little ceramic disk, then a large ?hdpe? threaded part. They used
JFET dies on ceramic, deadbug leadbond, also no feedback RC, instead
it\'s operated as an integrator, with a slow periodic FET reset, then a
differentiator to get back the DC signal. Supposedly this thing can
watch the state of single ion channels in neurons. $5000 ea, iirc.

Neurons would freeze solid at -30C. Not much ion channel activity at that
temperature.

https://en.wikipedia.org/wiki/Neuron



--
MRM

 

[Jan Panteltje]

On a sunny day (Wed, 25 May 2022 16:27:58 -0700 (PDT)) it happened Bill Beaty
<billb@eskimo.com> wrote in
<4b46d4b6-de8f-49d5-9349-76483da8558fn@googlegroups.com>:

Question: would it be worth the trouble to cool the ADA4530-1 (and maybe the
ADC and the reference) to ~10 degrees C, in terms of noise/thermal input offset etc.?


Axon Instruments cools their \"Axopatch\" front end ...inside a tiny gold-plated
DIP14 box, epoxy sealed, N2 charged, with a huge piece of dessicant pellet
sealed inside. Their TEC is internal, few mm on a side, with a heavy copper
finger against the bottom of the little box. -30C operating temp. w/thermistor,
for shutting down power until leakage gets low enough.

See photo http://staff.washington.edu/wbeaty/chem_axopatch.html

Gold-plated box is a driven shield. The input is via a gold pin through a little
ceramic disk, then a large ?hdpe? threaded part. They used JFET dies on
ceramic, deadbug leadbond, also no feedback RC, instead it\'s operated as
an integrator, with a slow periodic FET reset, then a differentiator to get back
the DC signal. Supposedly this thing can watch the state of single ion channels
in neurons. $5000 ea, iirc.

For cellphone towers there exist superconducting filters, so cooled to a few degrees above absolute zero,
I happen to have one:
http://panteltje.com/pub/super_filter/super_filter_front_plate_img_2576.jpg
http://panteltje.com/pub/super_filter/super_filter_cryo_cooler_img_2545.jpg
dewar with filter on the left side
Use it for all sort of experiments..

 

[server]

Jan Panteltje <pNaonStpealmtje@yahoo.com> wrote in
news:t6n998$6ak$1@dont-email.me:

On a sunny day (Wed, 25 May 2022 16:27:58 -0700 (PDT)) it happened
Bill Beaty <billb@eskimo.com> wrote in
4b46d4b6-de8f-49d5-9349-76483da8558fn@googlegroups.com>:

Question: would it be worth the trouble to cool the ADA4530-1
(and maybe the ADC and the reference) to ~10 degrees C, in terms
of noise/thermal input offset etc.?


Axon Instruments cools their \"Axopatch\" front end ...inside a tiny
gold-plated DIP14 box, epoxy sealed, N2 charged, with a huge piece
of dessicant pellet sealed inside. Their TEC is internal, few mm
on a side, with a heavy copper finger against the bottom of the
little box. -30C operating temp. w/thermistor, for shutting down
power until leakage gets low enough.

See photo http://staff.washington.edu/wbeaty/chem_axopatch.html

Gold-plated box is a driven shield. The input is via a gold pin
through a little ceramic disk, then a large ?hdpe? threaded part.
They used JFET dies on ceramic, deadbug leadbond, also no feedback
RC, instead it\'s operated as an integrator, with a slow periodic
FET reset, then a differentiator to get back the DC signal.
Supposedly this thing can watch the state of single ion channels
in neurons. $5000 ea, iirc.

For cellphone towers there exist superconducting filters, so
cooled to a few degrees above absolute zero, I happen to have one:

snip

The coolest thing in the universe...

<https://youtu.be/djBleF7-tDE>

 

[Bill Beaty]

On Wednesday, May 25, 2022 at 6:51:41 PM UTC-7, Mike Monett wrote:

Bill Beaty <bi...@eskimo.com> wrote:

Question: would it be worth the trouble to cool the ADA4530-1 (and
maybe the ADC and the reference) to ~10 degrees C, in terms of
noise/thermal input offset etc.?


Axon Instruments cools their \"Axopatch\" front end ...inside a tiny
gold-plated DIP14 box, epoxy sealed, N2 charged, with a huge piece of
dessicant pellet sealed inside. Their TEC is internal, few mm on a
side, with a heavy copper finger against the bottom of the little box.
-30C operating temp. w/thermistor, for shutting down power until leakage
gets low enough.

See photo http://staff.washington.edu/wbeaty/chem_axopatch.html

Gold-plated box is a driven shield. The input is via a gold pin through
a little ceramic disk, then a large ?hdpe? threaded part. They used
JFET dies on ceramic, deadbug leadbond, also no feedback RC, instead
it\'s operated as an integrator, with a slow periodic FET reset, then a
differentiator to get back the DC signal. Supposedly this thing can
watch the state of single ion channels in neurons. $5000 ea, iirc.

Neurons would freeze solid at -30C. Not much ion channel activity at that
temperature.

Heh, better check photos before commenting, and for info, search \"patch clamp,\"
capillary microprobes of in vitro cell surfaces. The -30C cooled JFET front-end
is inside Axon Instruments \"Axopatch\" electrometer.

And no, you don\'t put your microscope slides INSIDE the tiny electrometer!

Only the dual JFET itself is cooled, inside the sealed DIP14 package,
inside the \"headstage\" housing, which is typically bolted to a microscope stage.

 

[Martin Brown]

On 24/05/2022 19:37, Piotr Wyderski wrote:

Gentlemen,

Since there is a discussion about ovenized oscillators, here\'s something
for restoring the broken symmetry.

I am planning to build an electrometer based on ADA4530-1. Due to the
insanely low leakage currents required, a regular PCB seems to be a
no-go. This instrument will be a one-off, so I can safely ignore the
usual DFM rules. Namely, I am considering a dead-bug arrangement of the
critical components and using air as insulation. For mechanical
stiffness and reasonable thermal conductance to avoid gradients, a
ceramic base plate would be one idea. But the ceramic can be a part of a
TEC module. Question: would it be worth the trouble to cool the
ADA4530-1 (and maybe the ADC and the reference) to ~10 degrees C, in
terms of noise/thermal input offset etc.?

I don\'t know about that particular part but some do improve a lot with
active cooling well below ambient. Same is true for CCD operation.

This patent exploited the effects of cooling certain instrumentation
opamps for mass spectrometry Faraday collectors back in the 1980\'s.

https://patents.google.com/patent/EP0286365A3/en

The improvement in LF noise was a bit of a surprise. Later versions of
the kit cooled things a bit more aggressively. It doesn\'t give much away ;-)

I can\'t recall if it was granted or not.

Things were made a bit more interesting by it all having to work in a
high vacuum and also withstand periodic baking to 150C.

--
Regards,
Martin Brown

 

[Phil Hobbs]

Martin Brown wrote:

On 24/05/2022 19:37, Piotr Wyderski wrote:
Gentlemen,

Since there is a discussion about ovenized oscillators, here\'s
something for restoring the broken symmetry.

I am planning to build an electrometer based on ADA4530-1. Due to the
insanely low leakage currents required, a regular PCB seems to be a
no-go. This instrument will be a one-off, so I can safely ignore the
usual DFM rules. Namely, I am considering a dead-bug arrangement of
the critical components and using air as insulation. For mechanical
stiffness and reasonable thermal conductance to avoid gradients, a
ceramic base plate would be one idea. But the ceramic can be a part of
a TEC module. Question: would it be worth the trouble to cool the
ADA4530-1 (and maybe the ADC and the reference) to ~10 degrees C, in
terms of noise/thermal input offset etc.?

I don\'t know about that particular part but some do improve a lot with
active cooling well below ambient. Same is true for CCD operation.

This patent exploited the effects of cooling certain instrumentation
opamps for mass spectrometry Faraday collectors back in the 1980\'s.

https://patents.google.com/patent/EP0286365A3/en

The improvement in LF noise was a bit of a surprise. Later versions of
the kit cooled things a bit more aggressively. It doesn\'t give much away
;-)

I can\'t recall if it was granted or not.

Things were made a bit more interesting by it all having to work in a
high vacuum and also withstand periodic baking to 150C.

1/f noise is caused by conductance fluctuations, which basically require
mass motion. (At very low temperature there are long-lived traps that
contribute as well, iirc.) So for old-timey parts that were made on
less-clean processes, I can easily believe that cooling helps the 1/f
noise a lot.

If you go too cold on a JFET, the transconductance goes into the tank,
so that the voltage noise gets worse.

The noise voltage is ideally sqrt(8 kT/( 3 * g_M)) in 1 Hz, i.e.
sqrt(2/3) of the Johnson noise for the same conductance. A BJT is
slightly better, at sqrt(1/2)--a BJT emitter effectively has a noise
temperature of T_amb / 2.

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
http://hobbs-eo.com

 

[Martin Brown]

On 25/05/2022 20:58, Phil Hobbs wrote:

whit3rd wrote:
On Wednesday, May 25, 2022 at 12:14:14 AM UTC-7, Piotr Wyderski wrote:
whit3rd wrote:

Air is good, as is a PTFE-insulated standoff or ceramic; some PCB
materials with guard-rings
are good, too...

...U-shaped plate will then be soldered to both GRD pins, splitting
the SO8
into two halves and providing reasonable stiffness. This plate, in turn,
will be then soldered to the enclosure using a small number of those
nifty SMD AlN thermal bridges due to their \"infinite\" resistance and
solderable ends. This levitated and GRD-screened op-amp should take full
advantage of the superior resistance and low soakage of air.

The thermal bridges sound like a nifty repurposing; those are usually
kinda short, though, so cleaning is important.  I\'ve heard of using
small-value
ceramic capacitors, and a burnt-out ceramic fuse should have some
standoff  virtues:
that\'s something a good electrometer can test!

Ceramics and glass are generally disappointing insulators in the
femtoamp range.  PTFE is very good, and a clean plastic DIP package is
also very very good.  Anything hydrophilic such as polyamide (nylon) is
a disaster.

ISTR we used PEEK which is a more nearly engineering grade plastic than
cheaper PTFE which tends to deform and creep under stress. Although it
is a thermoplastic it could survive the normal baking for standard mass
specs. It doesn\'t outgas much at all after the first heating session.

https://en.wikipedia.org/wiki/Polyether_ether_ketone

Glass can have low leakage, but the soakage (dielectric absorption) of a
glass capacitor has to be seen to be believed.

I do electrometer stuff using small through-hole parts assembled
dead-bug style.  Good Medicine.

I recall several tricks of that sort relying on the package starting out
clean and staying clean (not always a safe bet in routine
manufacturing). Easy enough for a one off being made carefully though.

--
Regards,
Martin Brown