Measuring nanoamps and below like a Ninja

[Zucca]

FYI, in case you need an electrometer... Keythley 617....

https://www.eevblog.com/forum/buysellwanted/(fs)-(eugermany)-keithley-617-programmable-electrometer-500-ship/

[Martin.M]

or with a Nixie-Nano 

greetings
Martin

[CalMachine]

These are all pretty awesome!! 

The lowest I've been able to measure with a fair amount of stability was in the pA range utilizing our Fluke 732B and a few of our TOhm standards.

[VintageNut]

I recently acquired a KE487 and a KE8002A. There is a 10G in the enclosure. Voltage source is a Fluke 731B. The resistor appears to be about 0.25% low. The 487 spec is 0.3% of reading so a better picoammeter would be required to make this measurement. The 6517B is only slightly better at 0.2% for the 2nA range. The 6430 is 0.05% on the 1nA range. The 6430 costs more than a 3458A.

This is probably why high resistance bridges remain in production. 

[TiN]

Well, it took bit more hours than I expected to get 4142B going with Ta wet-slug XTV cap as DUT.
Had to add 2KOhm in series, as SMU output is not rated >10uF load.

Connection is simple:

4142B(41421B SMU module) guarded force triax -> CNMC low-noise triax -> die-cast box -> 2K resistor -> XTV 1300uF 30V cap -> twinax cable -> 4142B GNDU sink.



As usual, linux-gpib is used to control ATE box, running on Terasic DE1-SoC FPGA board (linaro Linux with 3.18 kernel) + Adafruit BME280.
Will see if new setup able to reach 3.2nA value, achieved using K2400 and regular coax cable.

[Alex Nikitin]

I recently acquired a KE487 and a KE8002A. There is a 10G in the enclosure. Voltage source is a Fluke 731B. The resistor appears to be about 0.25% low. The 487 spec is 0.3% of reading so a better picoammeter would be required to make this measurement. The 6517B is only slightly better at 0.2% for the 2nA range. The 6430 is 0.05% on the 1nA range. The 6430 costs more than a 3458A.

This is probably why high resistance bridges remain in production.

Looking at these numbers I now more and more appreciate the Keithley 263 calibrator I've bought recently. My 617 electrometer is specified at 0.25%+500fA at 2nA range and 1.6% +10fA on 200pA, 20pA and 2pA ranges (1 year accuracy). The 263 however is specified (as a current source) at 0.065% +100fA at 2nA range, 0.25% +30fA at 200pA range,   0.375% +10fA at 20pA range and 0.425% +10fA at 2pA range. And I've paid less for it  .

Cheers

Alex

[e61_phil]

Looking at these numbers I now more and more appreciate the Keithley 263 calibrator I've bought recently. My 617 electrometer is specified at 0.25%+500fA at 2nA range and 1.6% +10fA on 200pA, 20pA and 2pA ranges (1 year accuracy). The 263 however is specified (as a current source) at 0.065% +100fA at 2nA range, 0.25% +30fA at 200pA range,   0.375% +10fA at 20pA range and 0.425% +10fA at 2pA range. And I've paid less for it  .

These are really impressive specs. How do you want to verify them?

[Alex Nikitin]

Looking at these numbers I now more and more appreciate the Keithley 263 calibrator I've bought recently. My 617 electrometer is specified at 0.25%+500fA at 2nA range and 1.6% +10fA on 200pA, 20pA and 2pA ranges (1 year accuracy). The 263 however is specified (as a current source) at 0.065% +100fA at 2nA range, 0.25% +30fA at 200pA range,   0.375% +10fA at 20pA range and 0.425% +10fA at 2pA range. And I've paid less for it  .

These are really impressive specs. How do you want to verify them?

The interesting feature of the K263 is its self-calibrating capabilities. To calibrate the high ohm and low current ranges it uses the "Ladder" technique and the internal electrometer (quite high-performing, based on the AD549L chip). Below is the list of the calibration equipment required  . I can use the HP3458A as a multimeter, though even K2015 should be fine if recently (<1year) calibrated.

Cheers

Alex

P.S. I can additionally verify the 10M and 100M resistors directly with the HP3458A (50ppm and 0.05% 1 year accuracy respectively).

[VintageNut]

Yes the 263 is very good for current.

My quandary is that measuring a 100G or 1T resistor is not going to be much better than 1% using any current source or current measurement instrument.

1% is 10,000 ppm uncertainty. Thats like a crappy handheld DMM performance. 

[Alex Nikitin]

Yes the 263 is very good for current.

My quandary is that measuring a 100G or 1T resistor is not going to be much better than 1% using any current source or current measurement instrument.

1% is 10,000 ppm uncertainty. Thats like a crappy handheld DMM performance.

I think that with the K263 about 0.1% at 100G should be possible, using an accurate 100V source (say, K617) and measuring 1nA by a compensation method.

Cheers

Alex

[TiN]

If you think 1% is crappy, check high-dollar Guildline 9337 10POhm resistance standard, which has amazing 25% spec . I wonder what insulator is used there.

[VintageNut]

If you think 1% is crappy, check high-dollar Guildline 9337 10POhm resistance standard, which has amazing 25% spec . I wonder what insulator is used there.

Good point. Looks like higher resistances have looser specs. I did not know that. That explains the 1% pA uncertainties for most Keithley sensitive current instruments.

I have a KE515 bridge but have not attempted an overhaul. The output connector appears to have been hacked to install a BNC. I think that it is supposed to have a triax connector.

[mrtn]

One of my first jobs was at a company that makes radon detectors. One (not so great) method to detect radon is to monitor the amount of ionized particles that are generated in a volume of radon laden air. The radon decays and produces charged particles.

How do you measure that? Well the cheapest way is to put a voltage across two parallel plates and measure the current between the two. I developed an instrument that put about 35v across two plates (area ~100cm^2) with long teflon insulators (about 2 cm between plates). The current sensor was an opamp transimpedance amplifier with a feedback resistor of around 1 gigaohm. The PCB had guard traces that were kept at the same potential as the input net. The whole thing was in a shielded box with an air path that passed through wire mesh.

It could measure into the tens of femto-amp range. Unfortunately I don't have statistical information as I'm just working from memory.
It was a fun project.

[Jay_Diddy_B]

Hi,

The most sensitive current measurement that I can make in my lab is with an HP4155B Semiconductor Parameter Analyzer.

Here is a photograph showing Id (Yellow) and Gm (Blue) for a small signal MOSFET (2n7002) versus Vgs.



The vertical scale for the current is log from 1fA (not f**k all, femto Amps) to 100mA, The lowest drain current measured was 1pA. The marker is at Vgs=400mV there is Id=16.9pA

The HP4155B has four of these SMUs.

Regards,

Jay_Diddy_B

[TiN]

Retest with 11KOhm BMF resistor in series with DUT.

1st test was for noise floor for 41421B SMU (cap not connected, resistor just hanging in the air in DUT box).
Source 10V, measure current. It happen to be ~160 fA (femtoamps) average, with noise ~400 fApk-pk. 
There are no Teflon plates or insulators in the DUT box, and also GNDU return for ground is routed using 1m long twinax cable, not a low-leakage triax. Don't have extra triax cable with two TRX ends .

Now connect 1300uF 30V wet tantalum XTV cap #1 (same one measured by K2400 at ~3.2nA Ileak before) as DUT and test again.
Leakage around 1.21 nA +/-0.3nA at 10.00VDC sourced, over >6 hours, even with 3C ambient temp change. Cap had to soak at 10V for ~20hours to stabilize at this level, just like Jim Williams observed and hinted in Linear AN124.

Next test - cap #2, same conditions, starts today

[guenthert]

I have a KE515 bridge but have not attempted an overhaul. The output connector appears to have been hacked to install a BNC. I think that it is supposed to have a triax connector.

Without derailing the thread (too much), I have a question regarding those triax connectors:  those connectors and cables are astonishingly expensive (probably due to the niche market and target audience).  Can't one just replace a triax connection with two BNC connections (one being signal shielded by guard, the other return (GND) shielded by, well, shield)?  I realize guard might be on high potential, but the preamps driving guard are limited to quite low currents, so this should be fairly safe (for humans, not necessarily for semiconductors) as long as one is not charging up CRT-sized capacitors and after being zapped for the third time, one will be more careful anyhow.  (where's the SHOCKED emoticon when one needs one?)

[VintageNut]

I have a KE515 bridge but have not attempted an overhaul. The output connector appears to have been hacked to install a BNC. I think that it is supposed to have a triax connector.

Without derailing the thread (too much), I have a question regarding those triax connectors:  those connectors and cables are astonishingly expensive (probably due to the niche market and target audience).  Can't one just replace a triax connection with two BNC connections (one being signal shielded by guard, the other return (GND) shielded by, well, shield)?  I realize guard might be on high potential, but the preamps driving guard are limited to quite low currents, so this should be fairly safe (for humans, not necessarily for semiconductors) as long as one is not charging up CRT-sized capacitors and after being zapped for the third time, one will be more careful anyhow.  (where's the SHOCKED emoticon when one needs one?)

It is known practice to use ordinary coax where the coax shield carries the guard signal. Leakage is theoretically zero in this configuration. I use this configuration for very high resistance measurements inside of a shielded ground-potential box. The guarded box resides inside of a safety ground potential box. This requires an interlock that turns off the driving instrument when the grounded box is opened.

Ordinary coax has a problem with triboelectric induction during ordinary usage. The high dollar triax cable is constructed with lubricant that reduces or eliminates the effects of triboelectric induction.

It is fairly simple to set up your measurement, turn on the measurement while leaving off the source, flex the cable and see if you can measure an error induced by flexing the cable.   

[Alex Nikitin]

Ordinary coax has a problem with triboelectric induction during ordinary usage. The high dollar triax cable is constructed with lubricant that reduces or eliminates the effects of triboelectric induction.

It is not connected to the coaxial/triaxial difference. You can get a "low noise" coaxial cable with a graphite lubricant and you can get a "standard" triaxial cable without any lubricants. Unfortunately, two triaxial cables I can get easily are of the "standard" type. So far I've used the Belden 9222 which works very well as long as you are not moving it during the measurement. I am trying to get some "low noise" triaxial cable but so far without much success!

Cheers

Alex

[VintageNut]

Ordinary coax has a problem with triboelectric induction during ordinary usage. The high dollar triax cable is constructed with lubricant that reduces or eliminates the effects of triboelectric induction.

It is not connected to the coaxial/triaxial difference. You can get a "low noise" coaxial cable with a graphite lubricant and you can get a "standard" triaxial cable without any lubricants. Unfortunately, two triaxial cables I can get easily are of the "standard" type. So far I've used the Belden 9222 which works very well as long as you are not moving it during the measurement. I am trying to get some "low noise" triaxial cable but so far without much success!

Cheers

Alex

For a home hobbyist, the lubricated coaxial cable is probably good enough. The shock hazard remains. I doubt that any of us here employ a truly safe interlocked and shielded enclosure.

The OEM triax cable is low noise and safe when used with a properly interlocked enclosure. 

You can purchase lengths of unterminated triax cable from Tek.   

[plesa]

Ordinary coax has a problem with triboelectric induction during ordinary usage. The high dollar triax cable is constructed with lubricant that reduces or eliminates the effects of triboelectric induction.

It is not connected to the coaxial/triaxial difference. You can get a "low noise" coaxial cable with a graphite lubricant and you can get a "standard" triaxial cable without any lubricants. Unfortunately, two triaxial cables I can get easily are of the "standard" type. So far I've used the Belden 9222 which works very well as long as you are not moving it during the measurement. I am trying to get some "low noise" triaxial cable but so far without much success!

Cheers

Alex

I replaced all Belden 9222 by Keithley cables. You can find them cheap on eBay (new Keithley Low Noise Triax Cable 7078-TRX-5 for $40). This is cheaper than making your own because connector price.
If you needs cable only this has good price http://www.ebay.com/itm/10m-33-ft-Low-noise-Triax-Cable-50-Ohms-RF-Medical-aerospace-camera-NEW-/201706863349?
TiN is using this type ( CNMC is made by quirkwire.com), we discussed it in this thread https://www.eevblog.com/forum/repair/electrometer-output-stage-keithley-6517/
Cable looks like silver plated which can be handy, Tek/Keithley cable is difficult to solder.
The main disadvantage of Belden 9222 is that it is intend to use in video applications and not in low currents measurements.

[TiN]

Where did you see 7078-TRX-5 for 40$? I see only 80$ one, and it's very short..

[plesa]

Where did you see 7078-TRX-5 for 40$? I see only 80$ one, and it's very short..

It was offer which has been accepted, the buy now prices are ridicilous:)
BTW is the CNMC cable silver or tin plated?

[Alex Nikitin]

While I am waiting for the low noise triaxial cable to arrive from the US (as well as a couple of triaxial sockets), I've built a nice 100G+1G resistor box with plain coaxial sockets. I've measured 1G resistor on the HP3458A as 1.005G and the K617 confirms that value. For 100G value I have to rely upon the K263 and k617. This box so far works very well and it did allow me to estimate the temperature coefficient of 100G resistor at about 400ppm/C (there is no figure in the manufacturer's datasheet, only "can be as good as 50ppm/C"  . I also attempted to measure the input impedance of the K617 by measuring the voltage drop on 100G resistor in series with 19V source. Despite me using a non-guarded connection so the insulation resistance of 1m Belden cable was connected in parallel to the K617 input, only ~5mV drop was observed (after ~10 minutes), making the input impedance in-spec at ~400Tohm (0.1T x 4000).

Cheers and Happy New Year to all!

Alex

[plesa]

Nice!
I will expect the burden voltage even lower.
Attached graph is input noise of 6517.IT is close to what Keithley specify in datasheet ( 0.75 fA p-p and input bias 3fA).

[Alex Nikitin]

Nice!
I will expect the burden voltage even lower.

It should be lower with a guarded connection. I'll repeat this measurement soon when I'll put a proper triax socket on the box.

Cheers

Alex