Measuring sub picoamp DC by electrical or chemical means

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 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.

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.

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?

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

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.

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

Their Model 6487 Picoammeter is purported to measure to 20 fA accuracy.

Some design rules for subpicoamp circuits

Since this is kind of old, searching the Texas Instruments database should get you more detailed information.

A discussion on subfemtoamp current detection:

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

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

circuits

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.

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.

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.

--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University

Don't you plan to use a current follower?

The input impedance is close to zero, provided you can reference the signal to ground. There are op amps with

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?

follower?

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.

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.

Here is one:

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

Actually that may work. I have a SUBCKT spice model for the LMC6001. Lets see if it works. :-)

Thanks! Paul Lowrance

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