Picoammeter Design

[iMo]

Sure, this is just a smoke test, it must be perfectly shielded (and it was originally, afaik it was a part of a particle detector put into a metallic tubing, and that all was inside a metallic tank filled with some noble gases). Therefore they limited the number of parts on the pcb (ie none decoupling capacitors). The connector's pins go through glass. The capacitor is marked 100J (100p) for 500V in ceramic or glass, definitely something special. On the first glance the responses are much faster than the 500G||100p would allow, that is because the rather high contamination, imho (a lot of people who do not read Metrology section were touching it)

[Kleinstein]

The capacitor marked with 100J could be 10 pF as 10 * 10^0 pF. That would make more sense speed wise.

[iMo]

I've found an info on ebay 
10pF 500v 5% Vitramon Porcelain Axial Capacitor CY13C100J MIL-SPEC HiRel Ceramic  ($15) ..
Vitramon now part of Vishay

[iMo]

I put the probe only into a small metallic cookie box (A5 format) and grounded the box to GND at the test pcb (outside the box). Set the zero with the pot to -0.1mV. After opening the short after aprox 50-60seconds it settled at aprox +4mV DC, with noise aprox 2.5mVpp.
Thus those +300mV offset disapeared (??). While observing the stuff from a 3m distance the noise was still the same, there was a small DC move up by 2mV during first aprox 5minutes. Still sensitive to touching the box, however it jumps by aprox 50+mV.
I will continue later on with some better measurements, but have to organize the experiment, most notably grounding/shielding, there is an o-ring on the probe which may isolate the probe's body while laying in the box (the front aluminum part with by teflon isolated needle is floating). Promising results, indeed.

[iMo]

I arranged the measurement somehow with better grounding inside the box, I put the outside pcb into the box as well. Outside left are the voltage sources (723 based floating voltages). I added 330nF foil at the OUT1 (thus 1k/330nF low pass at the output). Still a little bit sensitive and picking noises from outside via the wires, though (touching the box causes 3-4mV peak).
This setup is a monster, I would say

Below logged a short measurement from yesterday - with 34401A, 100NPLC (4.012 secs sampling period). The DC did not move, moreover I actually measured the Vcc's 723 voltage reg stability (PS: and noise as well), thus the DC drift has little sense in this measurement, imho.

The lowest noise I saw was 20uV stddev (running over 100 samples), the average in this monster setup in peaceful periods around 30uV, afaik. I tried to calculate the noise in pA/fA but as a novice I got weird results.. 

So the experts may help here - the input free floating in the box, 500G||10pF, 4.012sec sampling period, and say 20uV RMS best case at the output.

[magic]

As a sanity check, you could try with shorted relay to measure noise of the electronics alone. At least their voltage noise, because gate current noise of the source follower (if there is meaningful leakage) will not be accounted for in this configuration.

Not sure how much shielding you have applied around the high impedance electronics (the "probe"). It really should be enclosed in a grounded can. A larger box may not be enough, any wire entering the box which carries voltage noise or is high impedance (has a potential to pick up noise from outside the box) may inject noise into the probe circuitry.

The large bursts of noise seem to be something external being picked up.

The source follower adds that 2.855V offset and so your IN+ is not GND. Any noise on IN+ will directly appear on the output; hopefully your filtering sorts this. This noise would of course also be included in the "shorted relay" test.

[iMo]

Yep, this setup is a nonsense.. The filtering at the IN+ is not sufficient, there is 4k7/330n low pass only. As I wrote above I made the 723 voltage regulator noise measurement actually.. This TIA architecture would require a low noise reference voltage at IN+, the better cleaning, shielding, the Vcc/Vss inside the box. I may try to power it from my 20V battery, with a rail splitter (the reed relay consuming the most current), but my motivation is low.
Btw shorting the relay contacts moved the DC up by some 700uV, and the noise so far is the same (around 35-40uV stddev so far).
Not sure whether it has sense to mess with this monster further on.. Best would be to desolder the 500G resistor and the 10pF capacitor and the rest trashed..

[magic]

Btw shorting the relay contacts moved the DC up by some 700uV, and the noise so far is the same (around 35-40uV stddev so far).

Bad news: the electronics are noisy for some reason.
Good news: it's easier to hunt noise sources in low impedance circuitry like it's now

The only difference the relay makes is shorting the 500GΩ, so 700μV shift implies that there was 1.4fA flowing through it and that's your input leakage current. Not too bad.

edit
It has occurred to me that the probe contains a bipolar opamp, so there is IN+ current noise which may or may not be a problem. Either calculate what's the resulting voltage noise when it flows through your 4k7||330n impedance or throw a larger cap there or short IN+ to ground and AC couple the output for a quick test.

It has also occurred to me that this level of noise is perhaps acceptable, considering the ridiculous sensitivity of this thing? It's 500μV per 1fA, if my math is right.

[Kleinstein]

The noise level is actually quite low for the TIA when taken into account the sensitivity. A large resistor naturally has some noise and also the FETs are not super low in noise. So there is no need for super low noise for the offset / bias at +in.

The fast variable parts still look a bit strange, like interference or maybe something oscillating. Some amplifier don't like capacitive loading (e.g. shilded cables or a DMM). So it may be be a good idea to have some 100-500 ohm series resistance at the amplifiers output.

[Gyro]

...
Not sure whether it has sense to mess with this monster further on.. Best would be to desolder the 500G resistor and the 10pF capacitor and the rest trashed..

I still think that would be a crying shame. That thing is insanely sensitive and the 1.4fA input leakage current is very respectable, it might be even better after cleaning if it has been lying around in a junk box and you've been touching the insulatoror other parts with your fingers. The noise level doesn't sound that bad (for DC measurement use). I'm not clear on the source, but it might be possible to substitute a more recent, lower noise opamp. The physical construction is good and would take some significant effort to reproduce.

Before pulling the resistor, you need to consider what purpose you want to use your picoammeter for. 0.5mV output per fA is going to limit you to a very few applications - it would be useless for measuring the leakage of most components, reverse leakage of semiconductors, where the output would immediately peg to the supply rail.

Having built a 1mV/pA picoammeter (reply #23), I have found this perfectly adequate for all my leakage measurement needs (even evaluating insulating sleeving). I would still find a 1G, or maybe 10G resistor if you're feeling the need for a little more sensitivity (and noise).

As for the capacitor, I hadn't realized that Porcelain capacitors were actually that low leakage, but it is something that could be easily matched with an axial Polystyrene capacitor with a lower value feedback resistor - you would want a higher value anyway (330pF in my case).

If you have low motivation for this unit at the moment, I would wrap it carefully and store it in dry conditions. Either that or pass it on to another member. I certainly wouldn't trash any of it, the mosfet for one, has value for other instrument repairs as previously mentioned.

Just my tuppence worth anyway.

[magic]

I wonder if leakage current could be reduced by biasing the drain to a lower voltage, maybe -3V or thereabouts to be opposite to source voltage. Like all bias cancellation tricks it will probably increase noise current, but that is maybe still lower than other noise sources. The feedback resistor alone has 0.18fA/rtHz Johnson noise density.

You could try different bias currents on the FET.


To hunt for noise sources, short things out. Short IN+ to ground to eliminate opamp current noise from the equation. Close the relay, short the 39k2 resistor (WTF is even its purpose anyway?) and then short the FET gate to source to eliminate the FET and see voltage noise of the opamp alone. Always keep in mind that 1mV is 2fA - this puts things in perspective.

Ideally, you want noise to be dominated by the resistor (and maybe leakage currents flowing through it). Which is to say, overall noise should decrease when the relay is closed.


Thermal drift of the FET's uncancelled Vgs may become another annoying factor. It may be possible to compensate for it by manipulating IN+ bias; decent thermal tracking between components is necessary, of course. Drift may also become zero at one particular drain current, possibly impractically large.

[iMo]

The last attempt: here is the schematics of the testing setup after some changes - the IN+ filter is now 33k/470u and I split the source resistor as well (6k8/5k6/220u). I set the zero to some +4mV two hours back and now I read +17mV out at this moment and waiting till it stabilizes somehow (because of the large leaky aluminum caps). Just now I see 35uV stddev (shorted contacts). I will report later on with some graphs.

@Kleinstein: I have the 1k output resistor at the opamp's output from the very beginning .

[David Hess]

I wonder if leakage current could be reduced by biasing the drain to a lower voltage, maybe -3V or thereabouts to be opposite to source voltage. Like all bias cancellation tricks it will probably increase noise current, but that is maybe still lower than other noise sources. The feedback resistor alone has 0.18fA/rtHz Johnson noise density.

Bootstrapping the drain to the source is commonly done to control and reduce JFET gate current.  (1)  The JFET and signal source are likely so noisy that any increase in noise from the bootstrap circuit will be insignificant.

I do not know if it would improve the gate leakage of a MOSFET; I have not tried it.

(1) JFET gate leakage increases with drain voltage through impact ionization?  Pease mentioned it in one of his articles after he and a friend rediscovered this phenomena.

[magic]

IN+ filter is now 33k/470u and I split the source resistor as well (6k8/5k6/220u). I set the zero to some +4mV two hours back and now I read +17mV out at this moment and waiting till it stabilizes somehow (because of the large leaky aluminum caps)

I suggest methodically locating noise sources rather than random component swaps

Bootstrapping the drain to the source is commonly done to control and reduce JFET gate current.  (1)  The JFET and signal source are likely so noisy that any increase in noise from the bootstrap circuit will be insignificant.

I do not know if it would improve the gate leakage of a MOSFET; I have not tried it.

I generally expect all things to have non-zero, and usually positive, admittance. The logic with MOSFET is of course that the gate and its dielectric covers the whole length of the channel, so if one end is positive and leaks positive current (as we found) taking the other end negative will hopefully make it leak negative current and balance things out.

I don't know what's the drain breakdown rating of this FET.

[iMo]

Btw the 776's supply current according to the Fairchild's DS is set to some 40uA with the 4M7 resistor, provided I identified the resistor properly (I saw around 5M).

[iMo]

.. I don't know what's the drain breakdown rating of this FET.

Here are some params..
I set the Ids to 1mA, it could be it is a wrong setting, provided the 776 runs micropower..

[ch_scr]

If it was meant to be in high vacuum, the parts will have a lot harder time cooling down, with no air convection.
Maybe originally the FET power dissipation was set similar to the 776, or in the ballpark at least?

[iMo]

This is the latest 4hours run, shorted relay contacts, as per latest schematics above.
STDDEV aver ~35uV, min ~20uV.
Drift some 200-250uV up.

[iMo]

I put the probe into a plastic tube covered with Al foil (the needle is not touching the foil nor the plastic), the pickup is much lower, the noise seems still the same on the first glance, I will do some measurements later on during night.
In the meantime I tried with opening/closing the relay (btw the relay's coil is the major contributor to the pickup) - see below. The DC with shorted contacts moved a little bit up and down as I put the stuff on the top of my HP meter, then I replaced it while still sampling data (as the meter creates heat, obviously)..
I also changed the fet's source resistor to 68k.
The best case difference between opened/closed is aprox 1.5mV in this measurement (I saw below 1mV already) after 20-30minutes.
Experts here may judge on other params from that charging shape..
PS: the time axis is in HH:MM

[zrq]

The minutes scale slow recovery after releasing the relay (like switching off Zero Check on Keithley electrometers) looks like dielectric absorption.
Maybe from the feedback capacitor or the resistor, charged by the thermal emf of the relay. Wrong, that would be much smaller.

[David Hess]

Bob Pease mentioned using relays which have electrostatic shielding between the coil and contacts, and driving them with the minimum change in voltage needed for operation, to minimize coupling from the drive signal to the switched signal.  Eventually he used a mechanical switch with a plastic rod for actuation.

I would have tried using a reed switch with an electrostatically shielded coil however I assume it would perform poorly because this does not seem to be common.

[iMo]

The minutes scale slow recovery after releasing the relay (like switching off Zero Check on Keithley electrometers) looks like dielectric absorption.
Maybe from the feedback capacitor or the resistor, charged by the thermal emf of the relay. Wrong, that would be much smaller.

While shopping I was thinking on that long recovery - I think it comes from the capacitor created by the needle and the outer AL foil. The needle is floating in air (it goes through the teflon isolation), then there is a plastic cap about 1cm over the needle (such the needle does not hit the AL foil), then air, then the plastic pipe and finally the grounded AL foil. That creates the capacitor.
The first sharp fast peak comes from the 10pF FB porcelain capacitor, imho, the slow recovery part from that "pipe capacitor", I would say. I will try to simulate that..

PS: the front part of the probe made of AL with teflon is floating, not grounded, perhaps it carries a charge too..

[Kleinstein]

The input see a relatively low impedance. So the input capacitance discharges relatively fast.

The fast spike is likely due to capacitive coupling from the relay coil or drive part to the input in some way. It only need a tiny bit of coupling capacitance (0.05 pF range) to charge the 10 pA to -35 mV.
The parasitic capacitance may also be responsible for the dielectric absorbtion to cause the slow settling. To test this one could use different times of the relay turned on.

[iMo]

..The fast spike is likely due to capacitive coupling from the relay coil or drive part to the input in some way. It only need a tiny bit of coupling capacitance (0.05 pF range) to charge the 10 pA to -35 mV.
The parasitic capacitance may also be responsible for the dielectric absorbtion to cause the slow settling. To test this one could use different times of the relay turned on.

You are right again .. (.. how this guy does it )
Where to put the parasitic capacitance to simulate the slow part then?

[Qmavam]

Some will find this 19th century Galvanometer interesting.

                      Mikek