Picoammeter Design

[dreaquil]

Hi,

Does anyone know of any reference material which I could use to design a decent picoammeter? I know of the Low Level Measurement Handbook and a few other articles but I want to know as much as I can because it's such a noise prone design.

[Gyro]

I don't know about decent but this design using the LMC662 might be a useful starting point...

http://www.vk2zay.net/article/251

Edit: Apart from that, the Keithley Low Level Measurement Handbook that you've already found is the best reference I know of.

[Cerebus]

I'll second the Keithley Low Level Handbook.

I've found digging out older service manuals for instruments, the ones that have proper schematics, to be very helpful as examples of good design. You could start with the Keithley picoammeter that Dave repaired a way back. The service manual is linked from the episode page.

[unitedatoms]

I watched a documentary about madam Curie measuring the discharge rate of pF capacitor (leak caused by radioactivity of Uranium ore) using electrometer with rotating mirror and beam of light. The electrometer was working as a nullmeter where opposite side was connected to piezocrystal, generating an accurate DC by pulling end of crystal with weight.

So for picoammeter level accurate comparative measurements one needs a piezo DC source, scale, set of weights, electrometer, capacitor with shield, clocks and earth's gravity.

This design can give so many ideas for calibration of picoammeter.

[Gyro]

In terms of practical advice....

- Put the picoammeter, or at least your sensing head containing the input amp and feedback resistor as close as possible to what you're measuring (ie, make it small).

- Don't use any cables (in line with keeping it close). Even the low noise ones are piezoelectric / triboelectric / leaky.

- Shield everything in a metal enclosure.

- Air wire everything that's high impedance - lift the IC input pin if using a PCB. Air is the best insulator and isn't piezoelectric / triboelectic.

- Use a decent input connector, BNCs are available in high voltage variants that have an extended ribbed ptfe rear insulator. Use it as one of the supports for the internal air wiring. Don't use a mating BNC plug - insert a suitably sized solder pin on wire into the center contact (you could use the guts of a BNC plug)

- Wash everything in IPA and allow to air dry, then never touch it with fingers.

The close proximity and screening ought to help keep noise under control.

[dreaquil]

Thanks for the replies guys, some very useful information. I'm going to look into all the suggestions and I'll get back to you I've got a prototype on the way but I'm looking at improvements for rev 2

[Gyro]

Good luck, I'm intending to build my own

[rvalente]

The HP 4140B is a pA meter and precision voltage source, I'd see how they did and replicate...

Here is the service manual: http://www.keysight.com/upload/cmc_upload/All/04140-90021.pdf
Datasheet: http://www.keysight.com/upload/cmc_upload/All/59528837.pdf

[Skimask]

I watched a documentary about madam Curie measuring the discharge rate of pF capacitor (leak caused by radioactivity of Uranium ore) using electrometer with rotating mirror and beam of light. The electrometer was working as a nullmeter where opposite side was connected to piezocrystal, generating an accurate DC by pulling end of crystal with weight.

Was that the one on PBS recently, like in the past week or so?

[unitedatoms]

I watched a documentary about madam Curie measuring the discharge rate of pF capacitor (leak caused by radioactivity of Uranium ore) using electrometer with rotating mirror and beam of light. The electrometer was working as a nullmeter where opposite side was connected to piezocrystal, generating an accurate DC by pulling end of crystal with weight.

Was that the one on PBS recently, like in the past week or so?

Yes, it was on PBS yesterday. Lots of nice stories about discovering chemical elements. I was impressed what a person can do in a lab with few glass bottles, wires, pencil and paper. Madam Curie measured Radium atomic mass to 3.5 digit accurate with basically bare hands.

So considering that capacitor was about 0.1pF, voltage may be in range of 100V, time constant in ballpark of 100 sec. The resistance could be 10^15 Ohm. So currents were around hundred femtoAmperes. May be it is possible to estimate the electrons count per second, like divide current in Amperes by elecron's charge in Cuolombs and estimate the decay rate of ore. But I am too lazy. May be thousands-to-millions of electrons per second.

Edit: OK wolframAlpha answer to "electrons per second in 100 femtoamperes" is one electron per 1.6 usec.

[fcb]

The Signal Path did a great teardown/repair of a Keithley 220 - you'll get some great hints from this video (teflon standoffs etc...). And you get to see Shahriar wearing a natty little hat.

https://youtu.be/pMYK5qoQvYo

[plesa]

Look on how Keithley did it in 6485 https://www.eevblog.com/forum/testgear/keithley-64856487-teardown/
They used LMC662, LMC6081.
Or consider buying or check at least layout on LMP7721 dev kit.
In other thread was mentioned some discount code on TI development boards,maybe it can be used for this kit as well.

[Gyro]

What strikes me, looking at the teardowns, is the tremendous advantage you have in making a one-off home unit - air wiring, isolated battery power etc, that just wouldn't be feasible in a production unit (not to mention losing that dust generating cooling fan!)

The LMC662 in dip pkg looks a very good bet for avoiding a PCB and associated leakage, also availability and low cost. The LMP7721 might spec a little better (well for input protection, not necessarily i/p current) but would be a pain to mount and more importantly clean to keep package surface leakage low. Input protection on the LMC662 shouldn't be an issue because you can put a large value series resistor directly on its input (I think the i/p protection diodes can still manage 5mA... versus 10mA for the LMP7721).

A keep it simple approach eg. Using an existing external voltage source should help a lot too.

[Kleinstein]

A TIA can cover a rather large current range. So ranges could be in 1:100 steps or similar, unless very high resolution is needed.
So it might be easier to build a complete separate input stage just for the lowest range - this way saving on the switches, with all the trouble with extra leakage. Also consider battery power, as the very low bias OPs usually don't need mich current.

The higher ranges are far less critical.

[dom0]

What strikes me, looking at the teardowns, is the tremendous advantage you have in making a one-off home unit - air wiring, isolated battery power etc, that just wouldn't be feasible in a production unit

Keithley 640 :-)

[plesa]

Or use decent DMM with known input impedance  (10M) on lowest DCV range. There was not mentioned sensitivity level required :-)
100pA will be 1mV.
Or check uCurrent from Dave.

[Gyro]

Keithley 640 :-)

Well I think the vibrating capacitor might be a bit challenging for home construction 

Or use decent DMM with known input impedance  (10M) on lowest DCV range. There was not mentioned sensitivity level required :-)
100pA will be 1mV.
Or check uCurrent from Dave.

I think a Picoammeter worthy of the name ought to be able to resolve at least single digit pA, should be easily possible with the single digit fA input current of an LMC662. That would open it up to serious insulation evaluation, home made ionisation chambers etc. (the fun stuff  )

[plesa]

Oki, so for your application will be enough to build not whole picoammeter with multiple ranges, but rather TIA. So it is much more simplier ;-)

[Gyro]

I think it's still practical to do multiple ranges by using the LMC662 in standard inverting mode using a single high value feedback resistor switched from different proportions of the op-amp output. That way the range switching is 'low' impedance and the only high impedance node is one end of the feedback resistor/input connector/LMC662 -ve input. (ok, I will include a high value resistor between this node and the LMC662 input pin for input protection.

This style:

http://www.tradeofic.com/uploadfile/ic-circuit/200972431748582.gif

... but voltage output to an external DVM.

[dreaquil]

I got my first prototype back today and tested it out. Worked pretty well for a rough estimate .. used it to calculate the insulation resistance of a cable at 2G and 500G. It's in the range and the readings are repeatable. Now I managed to get my hands on a 6485 Picoammeter for calibration and reference I'll tell you how it goes.

Just so you know, I didn't air solder. I did a surface mount construction with a guard ring surrounding the input. Removed the top solder mask to avoid static build up and used a 1% 1G resistor with an LM6462 (150fA input bias). I know I can improve this with an LM662. My application wasn't precision measurement but rather ball park measurement so it works perfectly so far however I would like to get it to a precision stage.

EDIT: The capacitor is there to limit the bandwidth due to the Johnson equation.

[Gyro]

Wow, that was quick! 

Just one thing - I'd be tempted to put a resistor (10-100k?) directly in series with the input of the op-amp (pin 6), after the feedback parts to give its protection diodes a better chance at fighting off ESD strikes.

[Kleinstein]

Its better to have the resistor directly at the input: this way it also prevents oscillation in case of a highly capacitive source, like a long cable.
At 100 M in the Feedback,  a 100 K resistor can give a maximum of about 5 mV drop, before the OP goes into saturation. So no real need to worry about this.

[dreaquil]

Yeah that would probably be a smart move. I'm curious to how it stacks up against the picoammeter. I have a number of improvements for rev 2.

[Gyro]

I've finally got around to putting my own Picoammeter together. It uses the LMC662 as previously discussed.It seems to work pretty well and readings seem to be stable and repeatable. Maximum current is around +/-4.5nA, that will drop to +/-2.5nA at battery end of life voltage, assuming that I risk leaving it in there that long. Supply current is just under 1mA, even adding the most frugal of power LEDs would double this, so I haven't bothered.

Accuracy seems to be fine - within the limits of my test capability anyway. Tested with my only 10G ohm resistor (also 2%) it reads 1.022nA at 10V, close enough. Input voltage offset is less than 60uV using a randomly chosen LMC662.

EDIT: Ignore the 60uV, that was due to me not having nulled the Vos properly the other day. 



I've included a protection resistor within the feedback loop which should be good to +/-500V on the input, that's with 0.5mA through the LMC662 protection diodes (spec limit 5mA) so in practice is limited by the breakdown voltage of the resistor (I used a 1M large bodied carbon film).

I've also included a null adjustment pot for the op-amp offset voltage (range approx 1.35mV).  I've successfully nulled it to 1uV on my bench DVM but in practice it's ridiculously sensitive to external influences at that (fA) level. The LMC662 Vos also drifts over the supply voltage range by a few 10s of uV, so in practice it's fine for zeroing at the 100fA level (see pic). Vos TC is only 2uV/'C so not an issue.

I've included rudimentary output protection, the two 1k resistors are to ensure stability against capacitive loads (oscillation shows up as a few mV of offset).



Construction is my favorite double sided Manhattan style, using single sided copperclad. The input circuit air-wired and parts cleaned with IPA (it took several hours to stabilize afterwards - mainly the polystyrene cap I think).

[Vgkid]

Nice job  .