Cooling the electrometer front-end...

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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