Measuring nanoamps and below like a Ninja

[guenthert]

Youtube thought I might be interested in the "squre root electrometer" https://www.youtube.com/watch?v=xbKiAl2ioBg.  Looks nice to me, but my Keithley 617 just arrived (seemed to be too good a deal to pass on, but I just noticed the busted GPIB connector 

[Alex Nikitin]

Here is a direct input current measurement of the HP3458A, 10V DC range, AZERO OFF, NPLC100 (AZERO ON adds a noticeable current noise on the input in this setup - up to +/-5pA jumps on the Keithley 617, making accurate measurements quite difficult). Without auto-zero the value stabilises to less than +/- 0.2pA variations and I've recorded the mean value at 1V intervals for the input voltage from -11 to +11V. A 1Gohm resistor was placed in series with the input of the meter as otherwise the current fluctuations from the meter's input did confuse the electrometer. A voltage drop on the resistor was used to double check the measured current value with a good agreement (i.e for 10.000V input the measured voltage on the meter was about 9.986V) . This measurement was performed with TEMP?=35.6C and the ambient temperature 23C. Later I've tried a 100G resistor in series with the input and for the measured current 14.5pA the measured voltage was 8.55V. The temperature of the meter (and the ambient) was about 0.6C higher at that time. With 100G in series AZERO ON did not noticeably change the readings.

Cheers

Alex

[TiN]

Interesting. Can you also measure current on 3458A's GOHM range? I wonder how stable that is.
Also does guard switch to LO or to open (with connection to electrometers earth?) changes DCV current?

[Alex Nikitin]

Interesting. Can you also measure current on 3458A's GOHM range? I wonder how stable that is.
Also does guard switch to LO or to open (with connection to electrometers earth?) changes DCV current?

1) The current through the DUT on 1G range is not well defined  . On 1Gohm and 100Mohm ranges the current source used is the same as for 10M range - about 0.5uA, but with an additional 10Mohm resistor connected in parallel to the DUT. As the result, the current near the top of the range, i.e for 1Gohm resistor, is very small, and the short circuit current is the same on all three ranges.

2)  The measurements were performed with the guard to the LO. If I connect the guard to the electrometer earth the current for 10V input increases by about 0.7pA (from -14.7pA to -15.4pA) .

Cheers

Alex

[Alex Nikitin]

Here is a 16 hours run of 1pA current sourced by Keithley 263 and measured by Keithley 617. Temperature shown on the graph is measured by a thermocouple attached to the case of the K263 near the electrometer amp position. The visible variation with temperature is mostly due to the change of the electrometer input current on the K263. If I suppress the "zero" current again, as I've done at the beginning of this measurement, this variation is reset almost completely. This "zero" current at the moment, with the ADA4530-1 installed, is about +5...+10fA, depending on the temperature. This graph shows a very good performance from both units, essentially variations of about 5fA or 0.5% from 1pA value over 16 hours.

Vertical scale is 1fA/div. Measurements are made every 20 seconds but for a single sample with 330ms conversion time on the K617.

Cheers

Alex

[Alex Nikitin]

Another measurement. I plan to do the same measurement on a number of diodes and JFETs connected as diodes. Here we have BF861 JFET connected as a diode. I've used these for the Keithley 2015 repair and happy to see that the leakage is good. The voltage was supplied by the Keithley 263 and the current measured by the Keithley 617. The JFET was placed in a metal enclosure with two BNC connectors. The residual leakage was less than +/-2fA and the enclosure temperature was 24.6C .

Vertical scale is 0.5pA/div, the second graph is a zoomed in part of the first.

Cheers

Alex

[VK5RC]

Can I ask how are you guys cleaning the wires / connectors / components  after handling / soldering etc to avoid surface conduction?
IPA?
What sort of cloth / tissue / q tip??
Thanks Robert

[Alex Nikitin]

Can I ask how are you guys cleaning the wires / connectors / components  after handling / soldering etc to avoid surface conduction?
IPA?
What sort of cloth / tissue / q tip??
Thanks Robert

Hi Robert,

A good description of a cleaning procedure required is available in the data sheet for the ADA4530 evaluation board from AD (page 5). It is also worth noting that it is best to avoid contamination if possible, using minimal amounts of flux and solder, handling all components with clean tweezers only and so on.

Cheers

Alex

[VK5RC]

Thanks Alex,
Thanks for the reference. Data sheets can be a goldmine.
I have done a few trials of some off the shelf high value resistors (>10 GOhm) and a Keithley 619. The techniques to get reliable and accurate readings would appear to be very fastidious in many directions.
I am pretty sure my IPA is not 'clean room grade' hence I have not used it yet - but the no-touch technique is not new to me.
Robert

[Vtile]

These Metrology threads have been eye opening. Thank you all the ones who are participated, especially the certified voltnuts. 

[Alex Nikitin]

Here is the Keithley 617 measuring the Keithley 263 output over about 18 minutes time - first the zero baseline, then +50fA, -50fA and back to baseline.

Cheers

Alex

[VK5RC]

Hi Alex, got to be happy with those results.
I found an old HP "Test fixture" at a price I thought was about the costs if I made one, BUT my Keithley 619 has 2 lug 'triax' connectors and the HP 3 lug.
I took a few brave pills and bought some 2 and 3 lug 'triax' connectors ( I phoned my Bank manager first!) and built an adaptor cable. I had huge doubts that it would be good enough esp with all the touching of the various insulators during the mounting of the connectors on the triax. I washed my hands and tools with IPA before starting and tried to minimise contact but mounting a triax connector I did not find easy.
Below is a quick test measuring a 100G ohm resistor (1%) inside the test fixture, amazing the difference lifting the lid makes. Hopefully my adaptor cable can handle higher resistances accurately.
Robert
PS I have found some Chemtronics clean room IPA pre-soaked foam swabs (Digikey PN = CS25-ND) for not crazy money, which I will try if I think something has got contaminated until then just no touch and keep resistors etc in plastic bags.

[Alex Nikitin]

Hi Robert,

It looks good with that 100G! Here is a comparative measurement of five different makes of the BAV199 "low leakage" diode. As this data was collected at work, I won't specify the manufacturers . As you may see, the spread is quite large, and a lower leakage in reverse direction does not necessarily corresponds to a lower leakage in forward direction. The best samples manage below 10fA at +/-50mV.

Cheers

Alex

P.S. - to put things into perspective, I've added two comparative graphs for the 1N4148 diode and the worst device out of the BAV199 bunch  .

P.P.S. - all measurements done with the diodes temperature between 24C and 27C .

[VK5RC]

It is interesting to see what really happens to components when you can measure at very low/high  levels, assumptions go to the wall.
That data looks like it came from a nice SMU - I would like one but can't justify the extreme $, in what is for me, a pastime.
I haven't really thought much about diodes - every time I do (mainly as an RF 'mixer') my brain hurts! HIHI
Robert

[Alex Nikitin]

That data looks like it came from a nice SMU - I would like one but can't justify the extreme $, in what is for me, a pastime.

Hi Robert,

The data was taken with the Keithley 617 electrometer, used both as the source and the meter, controlled over GPIB. It takes quite a bit of time to get a good accuracy at low currents so each point was taken after a 30s settling time and averaged for 10 consecutive electrometer readings.

Cheers

Alex

[doktor pyta]

@Alex Nikitin
thanks for nice charts.
Could You please type units (V,mV) on the horizontal axes of last plots?
How many measuring points are on these plots?
All were taken for reverse bias ?

[Alex Nikitin]

@Alex Nikitin
thanks for nice charts.
Could You please type units (V,mV) on the horizontal axes of last plots?
How many measuring points are on these plots?
All were taken for reverse bias ?

Thank you. On all graphs the X-axis is in Volts and Y-axis is in Amps. 0.05V to 100 V in reverse bias (24 points) and 0.05V to 0.5V in forward bias (10 points).

Cheers

Alex

[Conrad Hoffman]

Jim Williams advocated the use of diode connected JFETs for low leakage. Every tried those?

[Alex Nikitin]

Jim Williams advocated the use of diode connected JFETs for low leakage. Every tried those?

I've posted measurements for one JFET (BF861) earlier in this thread, taken in a somewhat different setup (K263 as a source and K617 as a meter). JFETs are much better than common signal diodes like 1N4148, however very few would come close to the performance of even the worst BAV199 devices and none will survive 100V reverse voltage or have leakages similar to the best of BAV199. I plan to measure some more JFETs and BJTs soon (the Keithley 617 has some selected BJTs as input protection diodes by the way) and will post results here.

Cheers

Alex

[David Hess]

I've posted measurements for one JFET (BF861) earlier in this thread, taken in a somewhat different setup (K263 as a source and K617 as a meter). JFETs are much better than common signal diodes like 1N4148, however very few would come close to the performance of even the worst BAV199 devices and none will survive 100V reverse voltage or have leakages similar to the best of BAV199. I plan to measure some more JFETs and BJTs soon (the Keithley 617 has some selected BJTs as input protection diodes by the way) and will post results here.

Cheers

Alex

BAV199 diodes are only tested and guarantied to be below 5 nanoamps.  A 4117 JFET is guarantied to be 10 picoamps or lower.

Of course if you need a higher peak inverse voltage than a JFET gate will support, then you have to use the BAV199 or maybe the collector-base junction of a transistor but in both cases in order to have guarantied low leakage, you have to grade them yourself.

What the BAV199, JFET gate junctions, most transistor junctions, and real low leakage diodes have in common is no gold doping which would significantly increase reverse leakage current.  For the BAV199, this is revealed by its relatively high reverse recovery time of up to 3us.

[Assafl]

I don't know if this is relevant but stumbled upon this while looking for ways to stabilize electrometer readings...

The students were trying to measure electron transport in polymers induced under cosmic radiation - using a 1970's vintage electrometer. Their resolution was 400aA - which IMHO is more the crap physicists deal with than electrical engineers - 100's to 1000's of electrons.

The guarded measurement is done in a constant voltage vacuum chamber (duh).

http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1075&context=mp_presentations

This was presented as a poster session in some physics gathering...
http://meetings.aps.org/Meeting/4CF09/Session/D1.3
http://digitalcommons.usu.edu/mp_presentations/75/

[David Hess]

Bob Pease wrote an article about the problems they had testing input bias current at the femtoamp level.  The standard method was to configure the operational amplifier as an integrator but the physically large low leakage air capacitor had enough capture area that incoming cosmic rays were a significant problem.  Their solution was write the test software to distinguish cosmic ray events but now I wonder if a vacuum capacitor would have been better or at least feasible.

[Assafl]

Bob Pease wrote an article about the problems they had testing input bias current at the femtoamp level.  The standard method was to configure the operational amplifier as an integrator but the physically large low leakage air capacitor had enough capture area that incoming cosmic rays were a significant problem.  Their solution was write the test software to distinguish cosmic ray events but now I wonder if a vacuum capacitor would have been better or at least feasible.

Pease porridge columns were awesome reads indeed... I think he even had one of his National Semiconductor "Bob Pease" Show demonstrating that measurement setup. If I recall it was a very temperamental setup they had developed for production testing... completely encased in a box made of Copper PCBs - I think it then had another box around it....

Edit: Found it -

Femtoamp stuff starts at 13:00. Minute 38:45 shows the rather simple integrator setup with the 10pF Teflon cable as integrating capacitor with the shielding as guard (small size to reduce cosmic rays and the shape is self guarding).

[BFX]

Something arrived at Friday   
I'm curious how good is it since I don't believe that plastic box. But I should wait for triaxial cable and connector for my Keithley 602.
 

[Assafl]

A calibration box for a hi-pot tester?

http://www.sefelec.com/en/calibration-kit-MG-91