Science > Physics > Measuring sub picoamp DC by electrical or chemical means
| Topic: |
Science > Physics |
| User: |
"Paul" |
| Date: |
15 Oct 2007 12:48:29 PM |
| Object: |
Measuring sub picoamp DC by electrical or chemical means |
I'm trying to think of methods that may detect ~ 1/2 picoamp DC
without applying any appreciable *bias* current.
I have a passive analog amp meter that with the aid of a microscope
can accurately detect down to 100 pA. Close, but no cigar.
One possible method is to place a thin wire on a float (perhaps
Styrofoam). The Styrofoam would float on liquid. One end of the wire
would dip in a liquid bath that will conduct an appreciable amount of
electricity. The other end of the wire would dip into another liquid
bath consisting of the same type of liquid. Both liquid baths would be
the input. The total resistance could be as high as a few megaohms.
Over the wire on Styrofoam is a magnet. Current flowing through the
wire will produce a rotational force on the wire, thereby causing the
floating wire to slowly rotate.
Perhaps another method is to direct the 1/2 picoamp DC current through
some type of material that may cause a slow chemical change. Would
flowing the current through some type of photographic film work?
Any ideas is greatly appreciated.
Regards,
Paul Lowrance
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| User: "Paul" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
15 Oct 2007 01:16:15 PM |
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On Oct 15, 10:48 am, Paul <energymo...@gmail.com> wrote:
I'm trying to think of methods that may detect ~ 1/2 picoamp DC
without applying any appreciable *bias* current.
I have a passive analog amp meter that with the aid of a microscope
can accurately detect down to 100 pA. Close, but no cigar.
One possible method is to place a thin wire on a float (perhaps
Styrofoam). The Styrofoam would float on liquid. One end of the wire
would dip in a liquid bath that will conduct an appreciable amount of
electricity. The other end of the wire would dip into another liquid
bath consisting of the same type of liquid. Both liquid baths would be
the input. The total resistance could be as high as a few megaohms.
Over the wire on Styrofoam is a magnet. Current flowing through the
wire will produce a rotational force on the wire, thereby causing the
floating wire to slowly rotate.
I just thought of a simpler method. What about flowing current through
just one liquid bath where a piece of Styrofoam is floating below a
permanent magnet. The vertical magnetic field would cause a horizontal
force on the current, which would cause an eddy flow in the liquid,
thereby moving the Styrofoam. One concern would be surface tension
between the liquid and Styrofoam. Therefore, the Styrofoam would have
to repel the liquid, otherwise the Styrofoam would eventually drift to
the edge of the liquid bath.
Perhaps another method is to direct the 1/2 picoamp DC current through
some type of material that may cause a slow chemical change. Would
flowing the current through some type of photographic film work?
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| User: "Stanislaw Flatto" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
15 Oct 2007 02:50:02 PM |
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Paul wrote:
On Oct 15, 10:48 am, Paul <energymo...@gmail.com> wrote:
[snip]
Read this thread from one end to the other.
a) during the postings you change conditions so no definitive answer can
be given. You mention microscope, what influence does it have on
magnetic enviroment of your measurement. Does the presence of the
observer affect conditions?
On such sensitive measurements you should consider "Trying to perform a
measurement you change the measured quantity!"
Good luck
Stanislaw
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| User: "Paul" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
15 Oct 2007 03:40:05 PM |
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On Oct 15, 12:50 pm, Stanislaw Flatto <comp...@brownbear.com.au>
wrote:
Paul wrote:
On Oct 15, 10:48 am, Paul <energymo...@gmail.com> wrote:
[snip]
Read this thread from one end to the other.
a) during the postings you change conditions so no definitive answer can be given.
What did I change?
You mention microscope, what influence does it have on
magnetic enviroment of your measurement.
I never said it have any affect on the magnetic environment. I said
using a microscope on an analog amp meter can view changes down to
100pA.
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| User: "Paul" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
15 Oct 2007 03:45:59 PM |
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On Oct 15, 12:50 pm, Stanislaw Flatto <comp...@brownbear.com.au>
wrote:
Paul wrote:
On Oct 15, 10:48 am, Paul <energymo...@gmail.com> wrote:
[snip]
Read this thread from one end to the other.
a) during the postings you change conditions so no definitive answer can be given.
What did I change?
You mention microscope, what influence does it have on
magnetic enviroment of your measurement.
I never said it have any affect on the magnetic environment. I said
using a microscope on an analog amp meter can view changes down to
100pA.
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| User: "Paul" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
15 Oct 2007 05:15:55 PM |
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On Oct 15, 10:48 am, Paul <energymo...@gmail.com> wrote:
I'm trying to think of methods that may detect ~ 1/2 picoamp DC
without applying any appreciable *bias* current.
I have a passive analog amp meter that with the aid of a microscope
can accurately detect down to 100 pA. Close, but no cigar.
One obvious version of the above method is to replace the microscope
with a sensitive motion detector. A laser beam and phototransistor
would work. The amp meters needle would block the laser beam. Any
movement of the amp meter needle would cause more light to shine on
the phototransistor. Connected to phototransistor would be an op-amp
and low pass filter. This method should be able to detect small amp
meter needle movements than a high power microscope, perhaps achieving
the goal of measuring down to 1/2 picoamp.
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| User: "dlzc" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
18 Oct 2007 02:36:25 PM |
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On Oct 15, 10:48 am, Paul <energymo...@gmail.com> wrote:
I'm trying to think of methods that may detect ~ 1/2 picoamp DC
without applying any appreciable *bias* current.
I wonder if, since you are talking about only 10^7 electrons, that you
could not apply this to a heated vacuum tube plate and *count* the
durned things?
David A. Smith
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| User: "Fred Kasner" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
18 Oct 2007 06:21:33 PM |
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dlzc wrote:
On Oct 15, 10:48 am, Paul <energymo...@gmail.com> wrote:
I'm trying to think of methods that may detect ~ 1/2 picoamp DC
without applying any appreciable *bias* current.
I wonder if, since you are talking about only 10^7 electrons, that you
could not apply this to a heated vacuum tube plate and *count* the
durned things?
David A. Smith
First refer to the "Johnson Noise Limit".
I worked with DC circuits (for measuring very small amounts of thermal
changes) about a factor of 10 above that level. It took a nastily long
time for the system to get to a relatively quiet equilibrium before the
small emf produced in the DC circuit could be detected above the noise
level.
FK
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| User: "Bill Penrose" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
15 Oct 2007 01:12:15 PM |
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On Oct 15, 10:48 am, Paul <energymo...@gmail.com> wrote:
Over the wire on Styrofoam is a magnet. Current flowing through the
wire will produce a rotational force on the wire, thereby causing the
floating wire to slowly rotate.
You've re-invented the Galvanometer. You'll find the mechanical
constraints are extreme.
Perhaps another method is to direct the 1/2 picoamp DC current through
some type of material that may cause a slow chemical change.
It works, but you can calculate the length of time that would be
needed to achieve a measurable change in mass, and it will be very
long. A charge of 57000 coulombs is needed to change 1 mole to
something into something else. And that's if only one electron is
involved.
Conventional electronics has come a long way since the 100 pA meter
was built. Femtoamp detection is next to routine. Check it out.
Dangerous Bill
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| User: "Paul" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
15 Oct 2007 03:30:13 PM |
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On Oct 15, 11:12 am, Bill Penrose <penr...@iit.edu> wrote:
On Oct 15, 10:48 am, Paul <energymo...@gmail.com> wrote:
Over the wire on Styrofoam is a magnet. Current flowing through the
wire will produce a rotational force on the wire, thereby causing the
floating wire to slowly rotate.
You've re-invented the Galvanometer. You'll find the mechanical
constraints are extreme.
Perhaps another method is to direct the 1/2 picoamp DC current through
some type of material that may cause a slow chemical change.
It works, but you can calculate the length of time that would be
needed to achieve a measurable change in mass, and it will be very
long. A charge of 57000 coulombs is needed to change 1 mole to
something into something else. And that's if only one electron is
involved.
Conventional electronics has come a long way since the 100 pA meter
was built. Femtoamp detection is next to routine. Check it out.
Dangerous Bill
I did the calculation. All of these fA bias op-amps that I'm aware of
have exceptionally high input resistance. For example, the INA116PA op-
amp has 1000T ohms. My 0.5 uV DC voltage source would generate 5E-22
amps. The INA116PA could not detect no where near 5E-22 amps input.
Does anyone have Any ideas?
Thanks,
Paul Lowrance
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| User: "Paul" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
15 Oct 2007 03:32:53 PM |
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On Oct 15, 11:12 am, Bill Penrose <penr...@iit.edu> wrote:
On Oct 15, 10:48 am, Paul <energymo...@gmail.com> wrote:
Over the wire on Styrofoam is a magnet. Current flowing through the
wire will produce a rotational force on the wire, thereby causing the
floating wire to slowly rotate.
You've re-invented the Galvanometer. You'll find the mechanical
constraints are extreme.
Perhaps another method is to direct the 1/2 picoamp DC current through
some type of material that may cause a slow chemical change.
It works, but you can calculate the length of time that would be
needed to achieve a measurable change in mass, and it will be very
long. A charge of 57000 coulombs is needed to change 1 mole to
something into something else. And that's if only one electron is
involved.
Conventional electronics has come a long way since the 100 pA meter
was built. Femtoamp detection is next to routine. Check it out.
Dangerous Bill
I just did the calculation. All of these fA bias op-amps that I'm
aware of have exceptionally high input resistance. For example, the
INA116PA op-amp has 1000T ohms. My 0.5 uV DC voltage source would
generate 5E-22 amps. The INA116PA could not detect no where near
5E-22 amps input.
Does anyone have Any ideas?
Thanks,
Paul Lowrance
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| User: "Bill Penrose" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
16 Oct 2007 02:53:10 PM |
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On Oct 15, 1:32 pm, Paul <energymo...@gmail.com> wrote:
I just did the calculation. All of these fA bias op-amps that I'm
aware of have exceptionally high input resistance. For example, the
INA116PA op-amp has 1000T ohms. My 0.5 uV DC voltage source would
generate 5E-22 amps. The INA116PA could not detect no where near
5E-22 amps input.
Don't you plan to use a current follower?
http://en.wikipedia.org/wiki/Current-to-voltage_converter
The input impedance is close to zero, provided you can reference the
signal to ground. There are op amps with <15 fA input bias current.
The output voltage is equal to the product of the input current x the
feedback resistor.
The hard part is getting the very high value resistors. The even
harder part is defeating all the leakage currents and stray noise
sources.
DB
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| User: "Mark Thorson" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
16 Oct 2007 03:54:55 PM |
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Bill Penrose wrote:
The hard part is getting the very high value resistors. The even
harder part is defeating all the leakage currents and stray noise
sources.
Old CD radiation detectors based on electrometer tubes
often have resistors in the gigaohm range. You can also
make one by cutting the coils off an old RF inductor, and
drawing a line with India ink between the two terminals.
Everything must be cleaned with alcohol or acetone
after handling, because your fingerprints conduct
electricity.
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| User: "Paul" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
16 Oct 2007 05:46:12 PM |
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On Oct 16, 1:54 pm, Mark Thorson <nos...@sonic.net> wrote:
Bill Penrose wrote:
The hard part is getting the very high value resistors. The even
harder part is defeating all the leakage currents and stray noise
sources.
Old CD radiation detectors based on electrometer tubes
often have resistors in the gigaohm range. You can also
make one by cutting the coils off an old RF inductor, and
drawing a line with India ink between the two terminals.
Everything must be cleaned with alcohol or acetone
after handling, because your fingerprints conduct
electricity.
What I think would work is an op-amp with less than 100 fA bias and ~
a few megohm input resistance. Does anyone know such an op-amp?
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| User: "John Popelish" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
16 Oct 2007 08:02:58 PM |
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Paul wrote:
What I think would work is an op-amp with less than 100 fA bias and ~
a few megohm input resistance. Does anyone know such an op-amp?
Here is one:
http://cache.national.com/ds/LM/LMC6001.pdf
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| User: "Paul" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
16 Oct 2007 08:55:59 PM |
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On Oct 16, 6:02 pm, John Popelish <jpopel...@rica.net> wrote:
Paul wrote:
What I think would work is an op-amp with less than 100 fA bias and ~
a few megohm input resistance. Does anyone know such an op-amp?
Here is one:http://cache.national.com/ds/LM/LMC6001.pdf
Actually that may work. I have a SUBCKT spice model for the LMC6001.
Lets see if it works. :-)
Thanks!
Paul Lowrance
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| User: "N:dlzc D:aol T:com \dlzc" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
16 Oct 2007 09:03:46 PM |
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Dear Paul:
"Paul" <energymover@gmail.com> wrote in message
news:1192586159.090831.312120@q5g2000prf.googlegroups.com...
On Oct 16, 6:02 pm, John Popelish <jpopel...@rica.net> wrote:
Paul wrote:
What I think would work is an op-amp with less than
100 fA bias and ~ a few megohm input resistance.
Does anyone know such an op-amp?
Here is one:http://cache.national.com/ds/LM/LMC6001.pdf
Actually that may work. I have a SUBCKT spice model for
the LMC6001. Lets see if it works. :-)
Just wondering, could you put the current into a small capacitor
at the input of one opamp (forming a differential), and take that
output and put it into the input of an opamp rigged to integrate?
That way, the input impedance would not be an issue. You'd just
need to short the input cap every so often (while isolating the
input to the integrator).
David A. Smith
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| User: "John Popelish" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
16 Oct 2007 09:55:44 PM |
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N:dlzc D:aol T:com (dlzc) wrote:
Dear Paul:
"Paul" <energymover@gmail.com> wrote in message
news:1192586159.090831.312120@q5g2000prf.googlegroups.com...
On Oct 16, 6:02 pm, John Popelish <jpopel...@rica.net> wrote:
Paul wrote:
What I think would work is an op-amp with less than
100 fA bias and ~ a few megohm input resistance.
Does anyone know such an op-amp?
Here is one:http://cache.national.com/ds/LM/LMC6001.pdf
Actually that may work. I have a SUBCKT spice model for
the LMC6001. Lets see if it works. :-)
Just wondering, could you put the current into a small capacitor
at the input of one opamp (forming a differential), and take that
output and put it into the input of an opamp rigged to integrate?
That way, the input impedance would not be an issue. You'd just
need to short the input cap every so often (while isolating the
input to the integrator).
Integration is a common way to amplify small unidirectional
currents. The process adds much less noise than a high
value feedback resistor in a current to voltage converter.
The trick is producing a low leakage current reset
mechanism. A glass encased reed relay is one of those ways.
.
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| User: "Bill Penrose" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
16 Oct 2007 11:00:01 PM |
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On Oct 16, 6:55 pm, John Popelish <jpopel...@rica.net> wrote:
Integration is a common way to amplify small unidirectional
currents. The process adds much less noise than a high
value feedback resistor in a current to voltage converter.
The trick is producing a low leakage current reset
mechanism. A glass encased reed relay is one of those ways.
Do you have a link (or a paper reference)? I'm interested.
DB
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| User: "John Popelish" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
17 Oct 2007 12:04:32 AM |
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Bill Penrose wrote:
On Oct 16, 6:55 pm, John Popelish <jpopel...@rica.net> wrote:
Integration is a common way to amplify small unidirectional
currents. The process adds much less noise than a high
value feedback resistor in a current to voltage converter.
The trick is producing a low leakage current reset
mechanism. A glass encased reed relay is one of those ways.
Do you have a link (or a paper reference)? I'm interested.
The general concept of an integrate and hold/reset is shown
on page 4 of this application note:
http://www.national.com/an/AN/AN-20.pdf#page=4
Note that with a current input instead of a voltage input
you don't need R1. And with such a low bias current opamp
as the LMC6001, you also don't need R2, so it is a pretty
simple circuit, except for the logic controls for the the
input and capacitor dump -- neglecting the supply and
bypass, just an opamp, a capacitor and one or two reed switches.
.
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| User: "Bill Penrose" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
17 Oct 2007 12:07:48 PM |
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On Oct 16, 10:04 pm, John Popelish <jpopel...@rica.net> wrote:
Note that with a current input instead of a voltage input
you don't need R1.
Thanks, David and John. I'm familiar with the concept, but never
thought of it in terms of measuring charge.
DB
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| User: "N:dlzc D:aol T:com \dlzc" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
16 Oct 2007 11:46:52 PM |
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Dear Bill Penrose:
"Bill Penrose" <penrose@iit.edu> wrote in message
news:1192593601.466486.142890@y27g2000pre.googlegroups.com...
On Oct 16, 6:55 pm, John Popelish <jpopel...@rica.net> wrote:
Integration is a common way to amplify small
unidirectional currents. The process adds much
less noise than a high value feedback resistor in
a current to voltage converter. The trick is
producing a low leakage current reset mechanism.
A glass encased reed relay is one of those ways.
Do you have a link (or a paper reference)? I'm interested.
http://www.allaboutcircuits.com/vol_3/chpt_8/11.html
http://www.ee.psu.edu/grimes/publications/BHLooperPaper.pdf
David A. Smith
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| User: "Paul" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
17 Oct 2007 12:01:20 AM |
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On Oct 16, 7:03 pm, "N:dlzc D:aol T:com \(dlzc\)" <dl...@cox.net>
wrote:
Dear Paul:
"Paul" <energymo...@gmail.com> wrote in message
news:1192586159.090831.312120@q5g2000prf.googlegroups.com...
On Oct 16, 6:02 pm, John Popelish <jpopel...@rica.net> wrote:
Paul wrote:
What I think would work is an op-amp with less than
100 fA bias and ~ a few megohm input resistance.
Does anyone know such an op-amp?
Here is one:http://cache.national.com/ds/LM/LMC6001.pdf
Actually that may work. I have a SUBCKT spice model for
the LMC6001. Lets see if it works. :-)
Just wondering, could you put the current into a small capacitor
at the input of one opamp (forming a differential), and take that
output and put it into the input of an opamp rigged to integrate?
That way, the input impedance would not be an issue. You'd just
need to short the input cap every so often (while isolating the
input to the integrator).
David A. Smith
On Oct 16, 7:55 pm, John Popelish <jpopel...@rica.net> wrote:
N:dlzc D:aol T:com (dlzc) wrote:
Dear Paul:
"Paul" <energymo...@gmail.com> wrote in message
news:1192586159.090831.312120@q5g2000prf.googlegroups.com...
On Oct 16, 6:02 pm, John Popelish <jpopel...@rica.net> wrote:
Paul wrote:
What I think would work is an op-amp with less than
100 fA bias and ~ a few megohm input resistance.
Does anyone know such an op-amp?
Here is one:http://cache.national.com/ds/LM/LMC6001.pdf
Actually that may work. I have a SUBCKT spice model for
the LMC6001. Lets see if it works. :-)
Just wondering, could you put the current into a small capacitor
at the input of one opamp (forming a differential), and take that
output and put it into the input of an opamp rigged to integrate?
That way, the input impedance would not be an issue. You'd just
need to short the input cap every so often (while isolating the
input to the integrator).
Integration is a common way to amplify small unidirectional
currents. The process adds much less noise than a high
value feedback resistor in a current to voltage converter.
The trick is producing a low leakage current reset
mechanism. A glass encased reed relay is one of those ways.- Hide quoted text -
- Show quoted text -
Thanks David and John. That's actually a great idea, and applicable to
this design. I've never tried that method. Google patents has an
interesting patent describing such a method-- Figure 2. They use a FET
to reset the cap, but I'd imagine John's idea of using a glass encased
reed relay would help reduce noise.
http://www.google.com/patents?id=9fAAAAAAEBAJ&dq=6011252
Regards,
Paul Lowrance
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| User: "John Popelish" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
17 Oct 2007 12:21:11 AM |
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Paul wrote:
Thanks David and John. That's actually a great idea, and applicable to
this design. I've never tried that method. Google patents has an
interesting patent describing such a method-- Figure 2. They use a FET
to reset the cap, but I'd imagine John's idea of using a glass encased
reed relay would help reduce noise.
http://www.google.com/patents?id=9fAAAAAAEBAJ&dq=6011252
Be sure to get one that has a magnetic shield over the coil,
so you don't induce voltage spikes in the input circuit when
the coil switches.
Relays with the ends of the glass capsule sticking out of
the coil forms used to be common, but I don't see many, any
more. You may have to buy the switches and wind your own
coils around them. Got any 40 AWG wire?
http://www.hamlin.com/specsheets/MDCG-4.pdf
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| User: "mike" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
21 Oct 2007 01:23:06 PM |
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"John Popelish" <jpopelish@rica.net> wrote in message
news:SMmdnUsQWdszBojanZ2dnUVZ_s6mnZ2d@comcast.com...
Paul wrote:
Thanks David and John. That's actually a great idea, and applicable to
this design. I've never tried that method. Google patents has an
interesting patent describing such a method-- Figure 2. They use a FET
to reset the cap, but I'd imagine John's idea of using a glass encased
reed relay would help reduce noise.
http://www.google.com/patents?id=9fAAAAAAEBAJ&dq=6011252
Be sure to get one that has a magnetic shield over the coil, so you don't
induce voltage spikes in the input circuit when the coil switches.
Relays with the ends of the glass capsule sticking out of the coil forms
used to be common, but I don't see many, any more. You may have to buy
the switches and wind your own coils around them. Got any 40 AWG wire?
http://www.hamlin.com/specsheets/MDCG-4.pdf
Do you know of a distributor for these reed switches?
Mike
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| User: "Jamie" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
21 Oct 2007 02:53:42 PM |
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mike wrote:
"John Popelish" <jpopelish@rica.net> wrote in message
news:SMmdnUsQWdszBojanZ2dnUVZ_s6mnZ2d@comcast.com...
Paul wrote:
Thanks David and John. That's actually a great idea, and applicable to
this design. I've never tried that method. Google patents has an
interesting patent describing such a method-- Figure 2. They use a FET
to reset the cap, but I'd imagine John's idea of using a glass encased
reed relay would help reduce noise.
http://www.google.com/patents?id=9fAAAAAAEBAJ&dq=6011252
Be sure to get one that has a magnetic shield over the coil, so you don't
induce voltage spikes in the input circuit when the coil switches.
Relays with the ends of the glass capsule sticking out of the coil forms
used to be common, but I don't see many, any more. You may have to buy
the switches and wind your own coils around them. Got any 40 AWG wire?
http://www.hamlin.com/specsheets/MDCG-4.pdf
Do you know of a distributor for these reed switches?
Mike
http://www.cotorelay.com/html/reed_relay___dry_reed_switch_p.htm
--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
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| User: "John Popelish" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
21 Oct 2007 01:42:03 PM |
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mike wrote:
"John Popelish" <jpopelish@rica.net> wrote in message
news:SMmdnUsQWdszBojanZ2dnUVZ_s6mnZ2d@comcast.com...
Paul wrote:
Thanks David and John. That's actually a great idea, and applicable to
this design. I've never tried that method. Google patents has an
interesting patent describing such a method-- Figure 2. They use a FET
to reset the cap, but I'd imagine John's idea of using a glass encased
reed relay would help reduce noise.
http://www.google.com/patents?id=9fAAAAAAEBAJ&dq=6011252
Be sure to get one that has a magnetic shield over the coil, so you don't
induce voltage spikes in the input circuit when the coil switches.
Relays with the ends of the glass capsule sticking out of the coil forms
used to be common, but I don't see many, any more. You may have to buy
the switches and wind your own coils around them. Got any 40 AWG wire?
http://www.hamlin.com/specsheets/MDCG-4.pdf
Do you know of a distributor for these reed switches?
http://www.digikey.com/
They list 409 hits for the search parameter [reed switch],
including some of the ones shown in that data sheet
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| User: "mike" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
21 Oct 2007 04:13:50 PM |
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"John Popelish" <jpopelish@rica.net> wrote in message
news:g5SdnTGl46XBAIbanZ2dnUVZ_obinZ2d@comcast.com...
mike wrote:
"John Popelish" <jpopelish@rica.net> wrote in message
news:SMmdnUsQWdszBojanZ2dnUVZ_s6mnZ2d@comcast.com...
Paul wrote:
Thanks David and John. That's actually a great idea, and applicable to
this design. I've never tried that method. Google patents has an
interesting patent describing such a method-- Figure 2. They use a FET
to reset the cap, but I'd imagine John's idea of using a glass encased
reed relay would help reduce noise.
http://www.google.com/patents?id=9fAAAAAAEBAJ&dq=6011252
Be sure to get one that has a magnetic shield over the coil, so you
don't induce voltage spikes in the input circuit when the coil switches.
Relays with the ends of the glass capsule sticking out of the coil forms
used to be common, but I don't see many, any more. You may have to buy
the switches and wind your own coils around them. Got any 40 AWG wire?
http://www.hamlin.com/specsheets/MDCG-4.pdf
Do you know of a distributor for these reed switches?
http://www.digikey.com/
They list 409 hits for the search parameter [reed switch], including some
of the ones shown in that data sheet
I'll take a look, I liked the .2pf capacitance, I'm assuming that's across
the contacts open circuit?
Mike
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| User: "John Popelish" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
21 Oct 2007 08:08:35 PM |
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mike wrote:
"John Popelish" <jpopelish@rica.net> wrote in message
news:g5SdnTGl46XBAIbanZ2dnUVZ_obinZ2d@comcast.com...
mike wrote:
"John Popelish" <jpopelish@rica.net> wrote in message
news:SMmdnUsQWdszBojanZ2dnUVZ_s6mnZ2d@comcast.com...
(snip)
Relays with the ends of the glass capsule sticking out of the coil forms
used to be common, but I don't see many, any more. You may have to buy
the switches and wind your own coils around them. Got any 40 AWG wire?
http://www.hamlin.com/specsheets/MDCG-4.pdf
Do you know of a distributor for these reed switches?
http://www.digikey.com/
They list 409 hits for the search parameter [reed switch], including some
of the ones shown in that data sheet
I'll take a look, I liked the .2pf capacitance, I'm assuming that's across
the contacts open circuit?
I don't know what else it could be.
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| User: "Paul" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
16 Oct 2007 08:15:54 PM |
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On Oct 16, 6:02 pm, John Popelish <jpopel...@rica.net> wrote:
Paul wrote:
What I think would work is an op-amp with less than 100 fA bias and ~
a few megohm input resistance. Does anyone know such an op-amp?
Here is one:http://cache.national.com/ds/LM/LMC6001.pdf
Thanks, that is the best one I've seen so far. Rin is > 1T ohm. Are
there any op-amps in the megohm or Gohm region where Ib is less than
100 fA?
Paul Lowrance
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| User: "Paul" |
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| Title: Re: Measuring sub picoamp DC by electrical or chemical means |
16 Oct 2007 06:24:25 PM |
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On Oct 16, 12:53 pm, Bill Penrose <penr...@iit.edu> wrote:
On Oct 15, 1:32 pm, Paul <energymo...@gmail.com> wrote:
I just did the calculation. All of these fA bias op-amps that I'm
aware of have exceptionally high input resistance. For example, the
INA116PA op-amp has 1000T ohms. My 0.5 uV DC voltage source would
generate 5E-22 amps. The INA116PA could not detect no where near
5E-22 amps input.
Don't you plan to use a current follower?http://en.wikipedia.org/wiki/Current-to-voltage_converter
The input impedance is close to zero, provided you can reference the
signal to ground. There are op amps with <15 fA input bias current.
The output voltage is equal to the product of the input current x the
feedback resistor.
You are talking about the preamp, correct? The preamp cannot have
appreciable voltage offset or bias current relative to the voltage or
current levels one is trying to measure. So what I was saying is that
my 0.5 uV fed into the INA116PA op-amp would cause 5E-22 amps of
current due to the op-amps extremely high resistance. So the highly
sensitive INA116PA op-amp could not detect 0.5 uV. I think Keithley
did an amazing job, as the Keithely electrometer you pointed out can
measure down to 200uV.
The hard part is getting the very high value resistors. The even
harder part is defeating all the leakage currents and stray noise
sources.
A MOSFET would do trick with their ~ 10T ohms and higher resistance
from base to source. I believe an IRF540 has close to 9T ohms from
base to source.
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