• Cooling the electrometer front-end

    From Piotr Wyderski@21:1/5 to All on Tue May 24 20:37:34 2022
    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

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  • From John Larkin@21:1/5 to bombald@protonmail.com on Tue May 24 12:06:05 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Larkin@21:1/5 to bombald@protonmail.com on Tue May 24 12:05:00 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Mike Monett@21:1/5 to Piotr Wyderski on Tue May 24 19:38:44 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Piotr Wyderski@21:1/5 to John Larkin on Tue May 24 21:26:04 2022
    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

    --- SoupGate-Win32 v1.05
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  • From Jeroen Belleman@21:1/5 to Mike Monett on Tue May 24 22:17:24 2022
    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

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  • From jlarkin@highlandsniptechnology.com@21:1/5 to bombald@protonmail.com on Tue May 24 16:11:20 2022
    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

    --- SoupGate-Win32 v1.05
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  • From whit3rd@21:1/5 to Piotr Wyderski on Tue May 24 23:15:09 2022
    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.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Piotr Wyderski@21:1/5 to All on Wed May 25 09:13:56 2022
    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

    --- SoupGate-Win32 v1.05
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  • From whit3rd@21:1/5 to Piotr Wyderski on Wed May 25 12:36:53 2022
    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.

    --- SoupGate-Win32 v1.05
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  • From Phil Hobbs@21:1/5 to All on Wed May 25 15:58:38 2022
    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

    --- SoupGate-Win32 v1.05
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  • From Piotr Wyderski@21:1/5 to All on Wed May 25 22:44:16 2022
    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

    --- SoupGate-Win32 v1.05
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  • From Bill Beaty@21:1/5 to All on Wed May 25 16:27:58 2022
    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.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Mike Monett@21:1/5 to Bill Beaty on Thu May 26 01:51:34 2022
    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

    --- SoupGate-Win32 v1.05
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  • From Jan Panteltje@21:1/5 to billb@eskimo.com on Thu May 26 07:11:47 2022
    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..

    --- SoupGate-Win32 v1.05
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  • From DecadentLinuxUserNumeroUno@decadenc@21:1/5 to Jan Panteltje on Thu May 26 09:54:10 2022
    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>

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Bill Beaty@21:1/5 to Mike Monett on Thu May 26 21:45:01 2022
    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.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Martin Brown@21:1/5 to Piotr Wyderski on Fri May 27 13:22:00 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to Martin Brown on Fri May 27 11:27:18 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Martin Brown@21:1/5 to Phil Hobbs on Tue May 31 14:31:34 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to Martin Brown on Tue May 31 12:50:01 2022
    Martin Brown wrote:
    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.


    The highest-impedance thing I ever did for a production application was
    a 1 G / 50 G TIA for a scanning surface voltage tool used in
    semiconductor manufacturing.

    Here's a post about it from 2013. (Ignore the stuff about
    reverse-polarity diodes--there was a package outline mixup, long fixed.)

    Cheers

    Phil Hobbs

    =========
    On Mon, 09 Sep 2013 17:15:01 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote in <msmdnROcIY7Lp7PPnZ2dnUVZ_vWdnZ2d@supernews.com>:

    On 09/09/2013 04:27 PM, Jan Panteltje wrote:
    On a sunny day (Mon, 09 Sep 2013 16:02:25 -0400) it happened Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote in <522E2951.4070404@electrooptical.net>:

    Hi all,

    Because of a miscommunication with a client's layout person, I find
    myself in need of a series-connected dual Schottky diode in SC70 whose
    pinout is opposite to the BAV54SW, i.e. I need something like this:


    *------*
    K ==| |
    | |== COM
    A ==| |
    *------*

    It'll be used at 20 mA, 12V, no inductive kick. I can use the BAV99RW,
    but that'll cost me a volt's worth of drop, which I'd rather avoid. Any
    suggestions?

    Mount upside down?


    Might be possible, but the package is a lot taller above the leads than
    below.

    There are reverse-polarity Schottky pairs, but the ones I've found are
    all 4 volt RF mixer/detector things.

    The application is a super high-Z front end, where I need to bootstrap
    the contact-to-coil capacitance of a relay. The circuit is like this.
    K2 is single-coil latching, and switches the actual feedback resistors,
    while K1 engages disengages K2 for about 30 ms after a transition of
    either polarity. (The coils are both about 1.3k, so 650 ohms * 47 uF ~=
    30 ms.) K1 is the wrong way up, so I need to be able to invert the
    polarity of D1 and D2 without a board spin.

    *------------*------------*
    | | | 0 *---*
    D1A --- | --- | |
    A | D2A A -12 --* *---
    | . C + |
    *-----* . O K1 *------*---------0 From gate
    | | . ..I | | driver IC
    | --- . L - --- |
    | D1B A . | . D2B A |
    | | . | . | |
    | *----.-------*----.-------* |
    | . . |
    | . . |
    | . K1A .K1B |
    | /0 . 0\ |
    *----0/ + K2 - . \0----*
    | 0---*---COIL-------*---0
    | | . |
    | | . |
    CCC *--.-RRRR--||--*
    CCC 47uF | . |
    | CCC . CCC
    GND CCC . CCC
    | . |
    *--.-----------*---< Bootstrap
    .
    .
    . 50 G total
    *------RRRRRRR-.--RRRRRRR------*
    | . |
    | . K2A |
    *--------------0\ 1G |
    | \0----RRRRRR--*
    | 0 |
    | |\ | |
    | | \ *---------------*----0 Vout
    0----*-------| -\ |
    I_in | \ |
    | >-----------------*

    ... etc....

    It's interesting that the contact capacitance of K2 is large enough (0.2
    pF or so) that I have to short out the 1G resistor. If I'd let it
    float, its Johnson noise current would have dominated at high frequency,
    i.e. above a few hundred hertz. The capacitance itself is noiseless, so shorting out the 1G resistor allows the frequency response to be fixed
    in the second stage without hurting the noise.

    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

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    * Origin: fsxNet Usenet Gateway (21:1/5)