Measuring nanoamps and below like a Ninja

[VintageNut]

This thread is a place to post your techniques, results, tips and tricks for forcing and measuring low current.

In my opinion the definition of low current starts somewhere between 1nA and 10nA. That is about the noise floor of unshielded wires in open air. You can test this waving your hand near some unshielded wire that is connected to an instrument that has a noise floor well below 1nA.

 

[VintageNut]

I will begin with an example of how to characterize the noise floor of a measurement instrument.

The attached plot has current on the Y axis and "time" on the X axis for a KE6430.

Since this plot is pictorial only with no spreadsheet, I will use pk-pk amplitude to define and quantify the noise of the current measurement.

The technique is to place a metal triax cap over the trax jack of the instrument, force 0V and measure current.

In this case, the pk-pk noise is 3.78 x 10^-16.

There is an offset which is defined as the midpoint of the Y axis on the graph. Removing this offset leaves the user with just the p-p noise as the uncertainty of the measurement.

[tggzzz]

Ah. An easy question to answer.

To measure current like a ninja, you just do nothing whatsoever.

OTOH, to measure current like a cowboy, you stick the two electrodes on your tongue.

[HighVoltage]

I am using my trusted Keithley 410 PicoAmMeter

[3roomlab]

The technique is to place a metal triax cap over the trax jack of the instrument, force 0V and measure current.

i am curious about the measurement cables. i got some PTFE 2 core shielded, i applied 1000v 2 any 2 of the conductors, and measured the leakage (0.04uA/m or 26Gohm/m). so im not sure, how does this compare to a good PTFE insulated cable suitable for "low low loss" ?

[Alex Nikitin]

Here is my Keithley 617 current baseline measurements for 5min, measured every 2sec (the red line is averaging for 10 readings), the bias current (about 1.5fA) is suppressed. I will try to do some wire resistance measurements in a near future, the K617 can output +/- 100V and so would be able to measure resistance up to about 10^17 ohm.

Cheers

Alex

[Gyro]

I'll throw my cheap little Picoammeter project into the mix. You could easilly de-sensititize it if you only want to go as low as nA's 

https://www.eevblog.com/forum/projects/picoammeter-design/msg790045/#msg790045

P.S: Just reading through the thread again. In terms of 'tips', there are plenty among the replies, including a number from Alex.

[e61_phil]

Sorry it is in german, but here are some quick&dirty measurements of mine with a breadboard setup

https://lowcurrent.wordpress.com/2015/09/07/messung-von-leckstroemen/

The resulting standard deviation was about 5fA and peak to peak value was about 23fA

Proper shielding will decrease the noise a lot, of course.

[VintageNut]

The technique is to place a metal triax cap over the trax jack of the instrument, force 0V and measure current.

i am curious about the measurement cables. i got some PTFE 2 core shielded, i applied 1000v 2 any 2 of the conductors, and measured the leakage (0.04uA/m or 26Gohm/m). so im not sure, how does this compare to a good PTFE insulated cable suitable for "low low loss" ?

If you are using the cables to measure mA, then a 40nA leakage is not important. If you are measuring pA, then 40 nA of leakage is much larger than your intended measurement. I think that "low" is a relative term.

One question I have is why measure leakage at 1000V? Will you be using this cable to carry 1000V?

[VintageNut]

You can test this waving your hand near some unshielded wire that is connected to an instrument that has a noise floor well below 1nA.

I once tried to measure insulation resistance of a piece of ceramic substrate. I set the bias voltage to 1kV, and my meter reads as low as a couple tens of nA. Forget about moving hands or any conductive metal parts around, even only blowing air can cause displacement current of air dielectric, which considerably alters readings by up to hundreds of nA, so one word or suggestion: shield your DUT, not only electrically, but also mechanically. Don't let air to pass by it, or any thing that can carry charges.

Exactly. Air can carry charge from HI to LO. That is why having a hermetically sealed enclosure around the DUT and energizing that enclosure to GUARD potential is an important technique. All of the air inside of the sealed enclosure is charged to GUARD potential creating a fluid that insulates against leakage from HI to LOW.

I set one of these up and measured some wire. During one of the tests I encountered a very large current which was traced to a thumbprint that spanned the wire insulation and the copper core. Cleaning the sample restored the low current. I then purposely added a thumbprint to verify the cause and the measurement was ruined as I expected.

[VintageNut]

I'll throw my cheap little Picoammeter project into the mix. You could easilly de-sensititize it if you only want to go as low as nA's 

https://www.eevblog.com/forum/projects/picoammeter-design/msg790045/#msg790045

P.S: Just reading through the thread again. In terms of 'tips', there are plenty among the replies, including a number from Alex.

DIY pA meter is very cool. Someone had to build the first one!

[Gyro]

DIY pA meter is very cool. Someone had to build the first one!

Actually credit for the first (hobby) one probably [edit: possibly] goes to Alan Yates for this one:  http://www.vk2zay.net/article/251   It inspired mine anyway, although his didn't have Vos trimming. Come to think, i've seen several based on the CA3140 too.

[plesa]

For every beginner Ninja in this thread:
http://www.tek.com/sites/tek.com/files/media/document/resources/LowLevelHandbook_7Ed.pdf
Low current measurement or electron counting is like ppm chasing in voltage standard section, sometimes more difficult I must say

VintageNut
What is your application for K6430?

[David Hess]

I sometimes hack together a transimpedance amplifier using an LMC6081 selected for low input current to make measurements below 1 picoamp but often there is an easier way.

Common good digital voltmeters have a 10 megohm input shunt resistance and negligible input current so if DC volts mode is used to make a current measurement, the sensitivity is 100nA/V or 100pA/mV.  My 3.5 and 4.5 digit bench meters have a resolution of 100uV on their most sensitive range yielding 10pA resolution.  I have some higher count meters which do better yielding 1pA resolution.

So how good are the results in real life?  Well, my ancient 4.5 digit Tektronix DM501 with 100uV resolution uses a discrete differential JFET input buffer (no autozero means no charge pumping) with a specified input current below 50 picoamps which in practice it is less than 10 picoamps (I measured it) which is only 1 count.  The noise when making this measurement with a direct connection to the input is 1 count or less which is as good as it gets.  The integration time nulls out ambient 60 Hz noise and its harmonics.

My 3.5 digit Tektronix DM502 seems to do just as well even though its autozero MOSFET front end (Siliconix LD111) probably adds some noise from charge pumping.

[VintageNut]

For every beginner Ninja in this thread:
http://www.tek.com/sites/tek.com/files/media/document/resources/LowLevelHandbook_7Ed.pdf
Low current measurement or electron counting is like ppm chasing in voltage standard section, sometimes more difficult I must say

VintageNut
What is your application for K6430?

I have participated is several; photon counting, direct current measurement of circuit boards in sleep mode, etc. Measuring a 1T resistor with less than 1V was the first project.

[VintageNut]

This is a K6430 measuring a 1T resistor using 0.5VDC. The measurement yields 6 significant digits.

The metal pans are at GUARD potential.

[HighVoltage]

What shape is this 1 TOhm resistor?
That is kind of impressive, measuring such a resistor at less than 1V
 

[Alex Nikitin]

This is a K6430 measuring a 1T resistor using 0.5VDC. The measurement yields 6 significant digits.

The metal pans are at GUARD potential.

  What do you mean by "6 significant digits" ?! The last digit in this case equals about 3 electrons a second . The Keithley is a nice unit, but it can manage only about 0.4fA p-p noise, or ~2500 electrons/sec . That means at least the last two (and more likely three) digits are noise.

Cheers

Alex

[VintageNut]

What shape is this 1 TOhm resistor?
That is kind of impressive, measuring such a resistor at less than 1V

The goal was sub-pA stable current. The resistor is pictured below. It is a Russian glass enclosed device bearing Cyrillic lettering. I bought a small assortment from eBay. The seller was from Bulgaria. You can see the black GUARD connection to the enclosure. The measurement requires a very long time to stabilize. All of the capacitance in the cables etc are being charged with less than 1 pA. All of the air in the enclosure has to charge to 0.5V as well. This is a long slow process.

[VintageNut]

This is a K6430 measuring a 1T resistor using 0.5VDC. The measurement yields 6 significant digits.

The metal pans are at GUARD potential.

  What do you mean "6 significant digits" ?! The last digit in this case equals about 3 electrons a second . The Keithley is a nice unit, but it can manage only about 0.4fA p-p noise, or ~2500 electrons/sec .

Cheers

Alex

Understood and agreed as presented. The most significant three digits are "above" the noise.

If you look at the noise plot, it is not random. If someone were so inclined, that noise looks suspiciously periodic and may likely be characterized and largely removed.

[plesa]

You are using same resistors like me I purchased few of them on eBay about 5 years ago for experiments.
I like your barbecue style shielding, when I saw it in some video/appnote made by Keithley or Agilent I was laughing, but after test I must confirm it works

[VintageNut]

The aluminum pans are qty 3 for USD $1.00. Almost for free. The edges can be folded over to make the air current almost zero.

[plesa]

The aluminum pans are qty 3 for USD $1.00. Almost for free. The edges can be folded over to make the air current almost zero.

Yep it is pretty smart, but I cannot remember when I saw it for first time
If it was invented by Keithley or by Agilent or someone else...
For normal measurement I prefer light-tight enclosure.
It can be interesting to see what is inside preamp box, can expect teardown of it? 

BTW why you are measuring resistor with such a low voltage, these resistors should withstand 200V.

[VintageNut]

The aluminum pans are qty 3 for USD $1.00. Almost for free. The edges can be folded over to make the air current almost zero.

Yep it is pretty smart, but I cannot remember when I saw it for first time
If it was invented by Keithley or by Agilent or someone else...
For normal measurement I prefer light-tight enclosure.
It can be interesting to see what is inside preamp box, can expect teardown of it? 

BTW why you are measuring resistor with such a low voltage, these resistors should withstand 200V.

The 6430 did not belong to me. It was on loan to me.

I used 0.5V because the project was to explore measuring below 1pA. The resistor was a convenient device to help accomplish the goal at a safe voltage.

Those aluminum pans will not allow any light inside if you fold over the edges carefully.

My preferred enclosure now is that KE 8101-PIV. It closes securely and will no allow air or light inside. I use the 7078-TRX-BNC adapter that makes the enclosure at GUARD potential for the HI SMU cable. The LO smu cable is converted to isolated BNC/banana. It works well for small devices. It is not safe for high voltage because the enclosure is at GUARD. I usually do not mix high voltage and low current this way. For voltages above 40V, I use an interlocked enclosure for safety.

[HighVoltage]

I also have some of these military glass resistors with high GOhm values but they never showed any stable readings on voltages below 10V. So, I have usually tested them with 100V on the SMU.

Your way of shielding is pretty clever.
Thanks for the explanation, I will give this a try.