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sensitive electrometer

Hello,

do you know the operating principle of the keithly 640 electrometer?


I know there is a very sensitive electrometer using a membrane with a
capacitor plate on one side. To this plate the DC voltage to be
measured is applied + a voltage at the resonance frequency of the
membrane (around 200 V).
The resulting force is proportional to (U_0+U_1 sin(wt))^2.

One the other side of the membrane another capacitor changes its
capacitance when the membrane oscillates. An AC coupled amplifier then
amplifies the result.

The description is from the following interesting paper:

Vibrating Membrane Electrometer with High Conversion Gain
John Dimeff and James W. Lane Rev. Sci. Instr. 35 p666 1964

http://scitation.aip.org/getpdf/ser...35000006000666000001&idtype=cvips&prog=normal

The Keithly manual

http://www.keithly.com/com/data?asset=5685

shows the same principle, but is says the membrane is excited at 400
kHz but resonates at 6 kHz.


Many thanks


Daniel
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that [email protected] wrote (in
The Keithly manual

http://www.keithly.com/com/data?asset=5685

shows the same principle, but is says the membrane is excited at 400
kHz but resonates at 6 kHz.

The 400 kHz acts as a polarizing voltage, and variations in its
amplitude are amplified by the AC amplifier much more accurately (and/or
easily) than could be done using a DC polarizing voltage (or none at
all) and a DC amplifier. As the manual implies, it needs to be a
frequency much higher than the resonance frequency of the membrane.

You could make a toy one from an electret microphone capsule with the
polarising electrode attached over the sound entrance but insulated from
the capsule body.
 
W

Winfield Hill

Jan 1, 1970
0
[email protected] wrote...
do you know the operating principle of the keithly 640 electrometer?

BTW, Daniel, you do know the 640 vibrating membrane approach
was superseded by low-gate-current MOSFETs in the 642, which
was Keithley's next high-performance remote-head model? Now
similar fA capability can be had with a few NSC MOSFET opamps.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Winfield Hill
Now
similar fA capability can be had with a few NSC MOSFET opamps.

You can get fA capability much easier than that. In fact, there's a glut
of it.
 
Hello

I know of the 642 but the paper by Dimeff talks about 10^-19 A current
sensitivity and the NSC OPs only have bias current of around 2 fA.
(in fact I was not going to build one but the mechanical principle is
fascinating)
I am just wondering about the principle of the 642 because the Dimeff
thing needs an excitation frequency of exactly the resonance frequency.
Without DC bias the membrane will not resonate, because force is
proportional to squared voltage and as a result there is only an
excitation at DC and at the double resonance frequency, where the
membrane will not follow the signal.

Does anybody know about a paper explaining how the 640 works?

Many thanks

Daniel
 
R

Robert Baer

Jan 1, 1970
0
Hello,

do you know the operating principle of the keithly 640 electrometer?


I know there is a very sensitive electrometer using a membrane with a
capacitor plate on one side. To this plate the DC voltage to be
measured is applied + a voltage at the resonance frequency of the
membrane (around 200 V).
The resulting force is proportional to (U_0+U_1 sin(wt))^2.

One the other side of the membrane another capacitor changes its
capacitance when the membrane oscillates. An AC coupled amplifier then
amplifies the result.

The description is from the following interesting paper:

Vibrating Membrane Electrometer with High Conversion Gain
John Dimeff and James W. Lane Rev. Sci. Instr. 35 p666 1964

http://scitation.aip.org/getpdf/ser...35000006000666000001&idtype=cvips&prog=normal

The Keithly manual

http://www.keithly.com/com/data?asset=5685

shows the same principle, but is says the membrane is excited at 400
kHz but resonates at 6 kHz.


Many thanks


Daniel
That "scitation" reference is not too useful; Ya gotsta reggiesterr
eN *pay* !!
 
Robert,

I am sorry about this, but publishers are criminal in my opinion.
If I download the paper my IP and my institution are included in the
pdf and I am not allowed to give this to anyone.

Daniel
 
W

Winfield Hill

Jan 1, 1970
0
[email protected] wrote...
I know of the 642 but the paper by Dimeff talks about 10^-19 A
current sensitivity and the NSC OPs only have bias current of
around 2 fA.

These are not mutually exclusive. All sensitive electrometers
have input leakage, which they deal with by means of an input-
current offset control, on the front panel, or invoked under uP
control during an autozero cycle, etc. This means the ultimate
sensitivity is in effect determined by the noise and drift rate
away from the auto-zero value. Moreover, if the open-circuit
drift rate can be modeled, the claimed measurement sensitivity
can be further improved. NSC's opamps are much better than 2fA,
assuming you can create an appropriate environment for them.
I am just wondering about the principle of the 642 ...

Differential pair of selected MOSFETs. Sapphire insulators, etc.
 
B

Boris Mohar

Jan 1, 1970
0
[email protected] wrote...

These are not mutually exclusive. All sensitive electrometers
have input leakage, which they deal with by means of an input-
current offset control, on the front panel, or invoked under uP
control during an autozero cycle, etc. This means the ultimate
sensitivity is in effect determined by the noise and drift rate
away from the auto-zero value. Moreover, if the open-circuit
drift rate can be modeled, the claimed measurement sensitivity
can be further improved. NSC's opamps are much better than 2fA,
assuming you can create an appropriate environment for them.


Differential pair of selected MOSFETs. Sapphire insulators, etc.

Who makes 3N155A these days?



Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca

void _-void-_ in the obvious place
 
J

Jonathan Kirwan

Jan 1, 1970
0
[email protected] wrote...

These are not mutually exclusive. All sensitive electrometers
have input leakage, which they deal with by means of an input-
current offset control, on the front panel, or invoked under uP
control during an autozero cycle, etc. This means the ultimate
sensitivity is in effect determined by the noise and drift rate
away from the auto-zero value. Moreover, if the open-circuit
drift rate can be modeled, the claimed measurement sensitivity
can be further improved. NSC's opamps are much better than 2fA,
assuming you can create an appropriate environment for them.
<snip>

I'm curious about this comment, Win. If I recall correctly, I
remember reading something about the IC packaging having a bulk
impedance figure around the vicinity of 10^12-10^14 ohms. If so, this
would seem to be a barrier and would suggest to me the asking for
unpackaged dice. But is it truly possible to achieve "much better"
than 2fA in packaged components automatically placed on common
(perhaps with cutouts in them) PCB materials, even assuming a sealed
metal can bonded to the PCB?

Jon
 
W

Winfield Hill

Jan 1, 1970
0
Jonathan Kirwan wrote...
I'm curious about this comment, Win. If I recall correctly, I
remember reading something about the IC packaging having a bulk
impedance figure around the vicinity of 10^12-10^14 ohms. If so,
this would seem to be a barrier and would suggest to me the asking
for unpackaged dice. But is it truly possible to achieve "much
better" than 2fA in packaged components automatically placed on
common (perhaps with cutouts in them) PCB materials, even
assuming a sealed metal can bonded to the PCB?

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.
 
C

Chris Jones

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.

Hi Win,
Do you know if NSC bootstrap the ESD proteciton diodes on their op-amps?
I'd be pretty sure that the leakage of the ESD protection would be the
dominant leakage mechanism on the chip (but perhaps not as bad as the
package though). If they were to drive the other end of the ESD diodes
with a guard signal, that guard signal being in turn ESD protected with
diodes to the supplies, then I would expect that lower leakage could be
obtained than with plain diodes from the input pins to the supplies.
Perhaps they already do this - do you think so? Although I have never seen
it done, they could also drive the pins surrounding the input pins with
this guard signal if low leakage through the packace were the problem. I
wonder if anyone makes such a thing.

Chris
 
W

Winfield Hill

Jan 1, 1970
0
Chris Jones wrote...
Hi Win,
Do you know if NSC bootstrap the ESD proteciton diodes on their op-amps?

They do not, to my knowledge.
I'd be pretty sure that the leakage of the ESD protection would be the
dominant leakage mechanism on the chip (but perhaps not as bad as the
package though).

Right, creating low-leakage protection diodes is the trick.
There's reason to believe the plastic package may be "perfect."
If they were to drive the other end of the ESD diodes with a guard
signal, that guard signal being in turn ESD protected with diodes
to the supplies...

This would be a useful idea, but only for specific configurations
for the opamp. Bob Pease isn't telling how they accomplished the
low leakage level, but it seems clear they haven't used a guard,
at least in the conventional sense (i.e. from the "+" pin). It's
possible each gate pin guards its own ESD diode, ultimately from
the same MOSFET's source, but IIRC, there's reason to believe they
don't do that either.
 
F

Fred Bloggs

Jan 1, 1970
0
Winfield said:
Chris Jones wrote...



They do not, to my knowledge.




Right, creating low-leakage protection diodes is the trick.
There's reason to believe the plastic package may be "perfect."




This would be a useful idea, but only for specific configurations
for the opamp. Bob Pease isn't telling how they accomplished the
low leakage level, but it seems clear they haven't used a guard,
at least in the conventional sense (i.e. from the "+" pin). It's
possible each gate pin guards its own ESD diode, ultimately from
the same MOSFET's source, but IIRC, there's reason to believe they
don't do that either.

Isn't it just a matter of balancing the reverse leakage currents- there
is no telling what can be achieved with a 4-layer integrated structure
looking like two diodes between the supplies and a weakly enhanced
MOSFET transversely.
 
J

Jonathan Kirwan

Jan 1, 1970
0
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.

Okay. Thanks. Although I'm very inexperienced in electronics, I
understand at least somewhat about the difference between a bias
current that can be mostly accounted for (leaving some residual error)
and also time drift away from that accounting.

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? (I'm thinking here that one of the reasons that someone
would bother using a COTO relay to switch gains on the 1st stage of a
transimpedance amp would be for its OFF leakage, and that COTO would
be driven to use some of the better packaging available, if possible
-- at least, that is the inspiration for my further question.) Or is
it that NSC truly has done something very special that others cannot
achieve in their packaging? I guess I'm still just having a hard time
with the idea that bulk impedances in packaging can really achieve
that kind of level of insulation, which must be in the 10^16 region or
better (yes?) in very tiny packages.
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.

Gotcha. I assume this part is packaged in such a fashion as to
encourage these configurations.

Just imagining other problems, it seems like the whole thing with some
of the circuit would need to be placed in at least an electrostatic
shielding can and sealed dry (hmm, is the packaging hydrophobic?)

....

Finally, I'm at a loss for the part number to look at. I went to
National's web site (I'm assuming this is "NSC") and did find dies
with an input bias of 2fA, namely the MNLMC662AM-X REV 0B1 data sheet,
but this is unpackaged. The packaged LMC662 appears to spec 2pA (they
say .002nA which I read as 2pA.) The LMC6001 is also 2pA, not 2fA. So
I'm still looking for a packaged 2fA part (rubbishemail had posted
this comment "NSC OPs only have bias current of around 2 fA") and
wouldn't mind being given a clue where I could read about such a
thing. So far, I'm only finding that spec in a national _die_ and not
a _package_.

Jon
 
J

Jonathan Kirwan

Jan 1, 1970
0
So far, I'm only finding that spec in a national _die_ and not
a _package_.

I should add that the die form specifies 3mV offset voltage with a
drift of 1.3uV/C, so it seems to me that a diode detector would either
need to be "very, very cold" to get its own impedance very much higher
or else the 3mV would need to be tweaked away, perhaps with a DAC to a
resistor divider biasing the other, non-summing input node.

Jon
 
W

Winfield Hill

Jan 1, 1970
0
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?

Specsmanship.
Inability to create a perfect plastic *all the time* - who knows.
Just imagining other problems, it seems like the whole thing with some
of the circuit would need to be placed in at least an electrostatic
shielding can and sealed dry (hmm, is the packaging hydrophobic?)

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.
Finally, I'm at a loss for the part number to look at. I went to
National's web site (I'm assuming this is "NSC") and did find dies
with an input bias of 2fA, namely the MNLMC662AM-X REV 0B1 data sheet,
but this is unpackaged. The packaged LMC662 appears to spec 2pA
(they say .002nA which I read as 2pA.)

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.
 
J

Jonathan Kirwan

Jan 1, 1970
0
Jonathan Kirwan wrote...

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

Okay. Trust nothing at these levels, then.
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.

Will do.
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.

I was looking only so far as the web page:

http://www.national.com/pf/LM/LMC662.html

and also the parametric search page (which says the same thing.)

As you can see, that page says: "Max Input Bias Current (nA) .0040,
..0020"

So I downloaded the data sheet PDF and see exactly what you say about
it. At least that explains my confusion on this point. Thanks.

Jon
 
C

Chris Jones

Jan 1, 1970
0
Winfield said:
Chris Jones wrote...

They do not, to my knowledge.


Right, creating low-leakage protection diodes is the trick.
There's reason to believe the plastic package may be "perfect."


This would be a useful idea, but only for specific configurations
for the opamp. Bob Pease isn't telling how they accomplished the
low leakage level, but it seems clear they haven't used a guard,
at least in the conventional sense (i.e. from the "+" pin). It's
possible each gate pin guards its own ESD diode, ultimately from
the same MOSFET's source, but IIRC, there's reason to believe they
don't do that either.
Well for all sensible amplifier configurations, the two inputs have the same
voltage, though admittedly it may not be equal to the output voltage. They
could make the guard voltage using a diode connected device matched to and
running at the same current density as the input pair, with its source
connected to the source terminals of the two input devices, assuming the
input is a differential pair.

Thanks for the info, especially about Bob Pease's comments. It sounds from
what you say that they actually tried to get low input current, whereas I
had previously assumed (from the fact that 2fA is always a 'typical' spec,
and the max is much higher) that they didn't really care about the low
input current and that it was an accident. If it was an accident, then I
thought that there might be the potential for it to be even better if they
were trying.

With these parts, I have found it impossible to clean the package myself to
the same standard as a new part. After I have handled one, therefore, it
must be demoted to a lesser duty and a new part put into the low current
circuit. Thus for me it seems important that the distributor that I buy
from must not handle the parts at all when sending them to me. Maybe I
will have to expect a certain percentage of parts will be unusable, and
just rely on the statistical chance that there are some parts where the
important part of the package hasn't been touched. I might have better
luck if I buy a full tube.

Chris
 
W

Winfield Hill

Jan 1, 1970
0
Chris Jones wrote...
you say that they actually tried to get low input current, whereas I
had previously assumed (from the fact that 2fA is always a 'typical'
spec, and the max is much higher) that they didn't really care about
the low input current and that it was an accident. ...

No, apparently the tough issue for those parts isn't their performance,
it's the difficulty making production measurements at low currents to
support a lower "maximum" spec. Such very-low current measurements
would require specialized home-made circuitry, rather than standard
production test equipment, and would take much more time for each part
in the test fixture. The solution was to use a high tested-max spec,
and list the much lower typical value and then write and talk about it.
 
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