Maker Pro
Maker Pro

sensitive electrometer

R

Robert Latest

Jan 1, 1970
0
No, they say 2fA on the front page, and the "typical" specs column
says 0.002 with the unit of pA on the far right side of the page.
So that's 2fA typical for the LMC660 and LMC662. Anyway, ignore
all that, we're relying on Bob Pease's remarks about these ICs.

What/where are BP's remarks about these ICs?

Thanks,

robert
 
R

Robert Baer

Jan 1, 1970
0
Winfield said:
Jonathan Kirwan wrote...



The 10^14 number is clearly far too conservative, e.g., having
been thoroughly beaten by NSC. My comment about using NSC's
chips to make measurements below 1fA refers to their low leakage
drift rate, which allows you to correct for the standing level
and then observe changes from that with a sensitivity below 1fA.

As far as a PCB is concerned, no-one would let this measurement
node anywhere near the PCB: I've used small teflon standoffs, as
well as raised-leg soldered-in-air connections, which works well.
...and had *no* triboelectric, accumulated surface charge orother
teflon-related problems?
 
W

Winfield Hill

Jan 1, 1970
0
Robert Latest wrote...
What/where are BP's remarks about these ICs?

In various of his columns. He and his technician developed the
test setups for these opamps. I imagine he was involved in their
design as well. Didn't he have a few columns along the lines of,
What's all this Teflon stuff?
 
J

John Larkin

Jan 1, 1970
0
Jonathan Kirwan wrote...

Specsmanship.
Inability to create a perfect plastic *all the time* - who knows.


In fact there are about 20 problems you may run into in the under-
100fA region, so watch out and don't have your expectations up too
high. I have written about 6 to 10 of the problems, and solutions,
if you search my previous postings here on s.e.d. Add the search
word Keithley to your Google advanced usenet search form.


No, they say 2fA on the front page, and the "typical" specs column
says 0.002 with the unit of pA on the far right side of the page.
So that's 2fA typical for the LMC660 and LMC662. Anyway, ignore
all that, we're relying on Bob Pease's remarks about these ICs.


If I pull a random 2N7000 out of stock (smallish mosfet, TO-92
package), and connect a supply and LED in its source-drain path, I can
tease the floating gate to barely turn on the led, then let it sit
with the drain lead hanging. The led stays part-on for days, and the
current must be in the 1e-17 amp range, ballpark 100 electrons/second.

John
 
W

Winfield Hill

Jan 1, 1970
0
John Larkin wrote...
If I pull a random 2N7000 out of stock (smallish mosfet, TO-92
package), and connect a supply and LED in its source-drain path, I can
tease the floating gate to barely turn on the led, then let it sit
with the drain lead hanging. The led stays part-on for days, and the
current must be in the 1e-17 amp range, ballpark 100 electrons/second.

I've done the same experiment, only with a 5.5-digit meter
measuring the drain current. It's clear the plastic packages
of these parts are essentially pefect when it comes to leakage.
 
R

Robert

Jan 1, 1970
0
Winfield Hill said:
Robert Latest wrote...

In various of his columns. He and his technician developed the
test setups for these opamps. I imagine he was involved in their
design as well. Didn't he have a few columns along the lines of,
What's all this Teflon stuff?

Is that the one where test time was limited by cosmic rays passing through
the air volume by the inputs? He said something about the ionization left by
them created enough current to bugger the low input current test.

IIRC they had to make a little enclosure around the inputs to limit the
volume of air to be ionized by the stray whatzit and after that they could
reliably test the parts.

Robert
 
R

Robert

Jan 1, 1970
0
John Larkin said:
Jonathan Kirwan wrote...
[snip]


If I pull a random 2N7000 out of stock (smallish mosfet, TO-92
package), and connect a supply and LED in its source-drain path, I can
tease the floating gate to barely turn on the led, then let it sit
with the drain lead hanging. The led stays part-on for days, and the
current must be in the 1e-17 amp range, ballpark 100 electrons/second.

John

You disconnect the drain "...drain lead hanging" supply?

Or you disconnect the Gate lead?

Robert
 
R

Robert Baer

Jan 1, 1970
0
John said:
If I pull a random 2N7000 out of stock (smallish mosfet, TO-92
package), and connect a supply and LED in its source-drain path, I can
tease the floating gate to barely turn on the led, then let it sit
with the drain lead hanging. The led stays part-on for days, and the
current must be in the 1e-17 amp range, ballpark 100 electrons/second.

John
Now *that* seems to be a good starting point for a rather sebsitive
electrometer!
 
R

Robert Baer

Jan 1, 1970
0
Robert said:
Is that the one where test time was limited by cosmic rays passing through
the air volume by the inputs? He said something about the ionization left by
them created enough current to bugger the low input current test.

IIRC they had to make a little enclosure around the inputs to limit the
volume of air to be ionized by the stray whatzit and after that they could
reliably test the parts.

Robert
That also is the way i remember what ws said.
 
R

Robert Latest

Jan 1, 1970
0
On 9 Oct 2005 05:35:51 -0700,
in Msg. said:
In various of his columns. He and his technician developed the
test setups for these opamps. I imagine he was involved in their
design as well. Didn't he have a few columns along the lines of,
What's all this Teflon stuff?

Ah that. Yes, I remember. Thanks.

robert
 
J

John Larkin

Jan 1, 1970
0
John Larkin said:
Jonathan Kirwan wrote...

That said, if the package itself is so good (and I read that you used
the word 'perfect' to possibly describe it), then why is it that the
better COTO relays tend to specify leakage resistances in the 10^12
ohms region?
[snip]


If I pull a random 2N7000 out of stock (smallish mosfet, TO-92
package), and connect a supply and LED in its source-drain path, I can
tease the floating gate to barely turn on the led, then let it sit
with the drain lead hanging. The led stays part-on for days, and the
current must be in the 1e-17 amp range, ballpark 100 electrons/second.

John

You disconnect the drain "...drain lead hanging" supply?

Or you disconnect the Gate lead?

Robert

Gate of course.

John
 
R

Robert

Jan 1, 1970
0
John Larkin said:
message
On 8 Oct 2005 09:49:38 -0700, Winfield Hill

Jonathan Kirwan wrote...

That said, if the package itself is so good (and I read that you used
the word 'perfect' to possibly describe it), then why is it that the
better COTO relays tend to specify leakage resistances in the 10^12
ohms region?
[snip]


If I pull a random 2N7000 out of stock (smallish mosfet, TO-92
package), and connect a supply and LED in its source-drain path, I can
tease the floating gate to barely turn on the led, then let it sit
with the drain lead hanging. The led stays part-on for days, and the
current must be in the 1e-17 amp range, ballpark 100 electrons/second.

John

You disconnect the drain "...drain lead hanging" supply?

Or you disconnect the Gate lead?

Robert

Gate of course.

John

Whew!

Thought I was going to have to relearn some electronics.

Robert
 
R

Robert Latest

Jan 1, 1970
0
Hello,

are you shure this is not caused by drain-gate leakage?

Could be, but two leakages cancelling each other out with such
precision would be rather improbable.

robert
 
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