Measuring sub picoamp DC by electrical or chemical means

[Paul]

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

 

[Bill Penrose]

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

 

[Paul]

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.

 

[Paul]

You've re-invented the Galvanometer. You'll find the mechanical
constraints are extreme.

Would this be similar to Keithley electrometers? I wasn't sure if the
Keithley electrometers were completely passive in terms of the input
or are they transistor based input. Also what I found was the Keithley
electrometers are used for extremely low current, but high voltage
relative to my 0.5 uV. I should have posted the voltage range.

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 own a Keithley picoamp meter. The only issue is the 6 pA bias
current. Other Keithley meters have fA bias, but they expect input
voltages far higher than my 0.5 uV, or at least the used Keithley
meters I can afford. What about the INA116PA op-amp that has under 3
fA bias? The only possible issue is its relatively high Vos of 1mV.
I'm expecting to measure voltages near 0.5 uV. I'm just concerned the
INA116PA op-amps 1mV offset will interfere with the measured results.
IOW, is the INA116PA intended to measure sub microvolts?

 

[Bob M]

The classic instrument is the vibrating reed electrometer. The
detectable charge limit on the old Cary unit (late 1960s) corresponds
to approximately 2000 electrons. To measure current all you use is an
appropriate valued shunt resistor. Whem measuring very small current
you often feed the meter output to a chart recorder. Everytime a
cosmic ray hits the sensor you get a blip on the recorder.
I have one of these toys working it came secondhand with a pile of lab
equipment and cost little. I had been used for measuring for measuring
pH inside living animal cells using capillary glass electrode
underneath a microscope.
I suspect the machine would have cost a lot of money when new.

Bob M

 

[Paul]

The classic instrument is the vibrating reed electrometer. The
detectable charge limit on the old Cary unit (late 1960s) corresponds
to approximately 2000 electrons. To measure current all you use is an
appropriate valued shunt resistor. Whem measuring very small current
you often feed the meter output to a chart recorder. Everytime a
cosmic ray hits the sensor you get a blip on the recorder.
I have one of these toys working it came secondhand with a pile of lab
equipment and cost little. I had been used for measuring for measuring
pH inside living animal cells using capillary glass electrode
underneath a microscope.
I suspect the machine would have cost a lot of money when new.

I would like to try a Keithley electrometer. Although one issue is the
input impedance. For example the Keithley 616 electrometer has "INPUT
IMPEDANCE: Greater than 2 x 10^14 ohms." Therefore, my 0.5 uV would
generate 0.5uV / 2E+14 = 2.5E-21 amps. So unfortunately there's no
chance of the Keithley electrometer detecting my 0.5 uV, which would
generate 2.5E-21 amps in the Keithley 616.

 

[Paul]

You've re-invented the Galvanometer. You'll find the mechanical
constraints are extreme.


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

 

[Bill Penrose]

I own a Keithley picoamp meter. The only issue is the 6 pA bias
current.

Their Model 6487 Picoammeter is purported to measure to 20 fA
accuracy.
http://news.thomasnet.com/fullstory/16027

Some design rules for subpicoamp circuits
http://focus.ti.com/lit/an/sboa061/sboa061.pdf
Since this is kind of old, searching the Texas Instruments database
should get you more detailed information.

A discussion on subfemtoamp current detection:
http://tinyurl.com/255c4l

Remember, the big issues with this type of measurement are
capacitances in the current source and everywhere in the circuit, plus
tracking down all the leakages. Use of guard rings and other noise-
control tricks is essential.

DB

 

[Paul]

You've re-invented the Galvanometer. You'll find the mechanical
constraints are extreme.


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

 

[Paul]

On Oct 15, 10:48 am, Paul <[email protected]> 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.

 

[Paul]

Their Model 6487 Picoammeter is purported to measure to 20 fA
accuracy.http://news.thomasnet.com/fullstory/16027

Some design rules for subpicoamp circuitshttp://focus.ti.com/lit/an/sboa061/sboa061.pdf
Since this is kind of old, searching the Texas Instruments database
should get you more detailed information.

A discussion on subfemtoamp current detection:http://tinyurl.com/255c4l

Remember, the big issues with this type of measurement are
capacitances in the current source and everywhere in the circuit, plus
tracking down all the leakages. Use of guard rings and other noise-
control tricks is essential.

DB

Thanks for the links. The reason I'm pursuing other methods other than
those used by Keithley is because these electrometers meters have
extremely high input impedance, or at least all of the meters I've
seen, including the Keithley 6487. The Keithley 6487 claims it can
measure down to 200uV, which is substantially higher than my 0.5uV. I
am trying to measure current, but I mention the voltage and output
resistance as well, which is perhaps what confused Stanislaw Flatto.

 

[Paul]

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.

 

[Andy Resnick]

I'm trying to think of methods that may detect ~ 1/2 picoamp DC
without applying any appreciable *bias* current.

<snip>

We do patch clamping and can "measure" extremely small currents- the
details of voltage vs. current clamping are not germane here, but the
point is that we need a very high resistance ('gigaseal') in order to
perform these measurements.

 

[Bill Penrose]

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

 

[Mark Thorson]

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.

 

[Paul]

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?

 

[Paul]

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.

 

[Paul]

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

 

[Paul]

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

 

[N:dlzc D:aol T:com \(dlzc\)]

Dear Paul:

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