Picoammeter Design

[krivx]

Looks good, but the point where the Zeners meet looks very close to the +Battery node. Maybe move that.

[SeanB]

Hopefully you did not wash the ps cap with IPA, as it will have degraded it rather badly. Styrene dissolves in alcohol, so hopefully it was only exposed to the vapour and the long stabilisation diffused it back out without degrading the capacitor.

Nice work, and you can still see the milling marks on the box lid, and the extra sleeve on the Duracell is a nice extra touch. I would replace the zeners with reverse biased bipolar transistor BE junctions, less sensitive to light and lower leakage. 6-8V breakdown as well.

[Gyro]

Thanks for the feedback folks 

Glad you like to battery sleeve - I wanted to provide a bit of extra insulation as the battery is 'floating' +and - from the case, then found that the plastic blister pack makes an excellent sleeve if you cut it a bit and fold it in on itself. Nice and springy, it holds the battery nicely in place without additional padding. One to remember before throwing the packaging away.

The zeners (which are clearly a rather embarrassing afterthought!) are actually well clear of the board, up on the ends of the 4mm sockets. I think the sockets look shorter because I put some bits of silicone rubber tube around them (bodge battery buffers). I should really have put them on the board before the 1k resistors to give better protection, but I didn't want to desolder the input and pull the board out again. Good idea about the reverse biased b-e junctions, but I checked the zeners and there's no significant leakage at max. 4.5V (for a 2k source impedance anyway). Just thought, I am now actually able to test transistor b-e junction leakage  I've already found that 1N4148s do actually leak about 3nA at 10V (spec 25nA max @20V) I expected much lower for some reason.

Interesting point on the polystyrene capacitors - I did some googling beforehand and there seemed to be a consensus  that polystyrene is only mildly soluble in IPA after sustained immersion testing. After I finished the unit a couple of days ago I tested another capacitor after a quick swab with IPA... After a quick air dry, leakage hit the end stop at 4.5nA and was still the same an hour or so later (though I'd killed it). Next morning it was down to very sub pA again. No physical signs of dissolving or crazing. I can only think that the IPA draws some moisture into the surface, which takes a long time to evaporate again. It certainly explains the stabilization time anyway!

[Marco]

Would a plain high voltage ceramic capacitor suffice too?

[SeanB]

1N4148 leakage is a worst case, typically at 125C, but it suffers rather badly in being sensitive to light, IR ( it is after all a silicon photodiode in a glass package) and to temperature and stress of the leads. Plastic packages TO92 devices should be a lot lower, but the SMD versions are not as good, just from the smaller package.

nice that you connected the PS cap correctly, with the outer foil to ground. If you get a larger value ( I have some 1uF polystyrene caps around somewhere, which are pretty big, even at a 50V rating device) you will see just how badly they perform with contamination, though they are very good and stable otherwise, only PTFE and vacuum is better. Ceramic, even NPO, is terrible in comparison, it drifts too much in value with both time, applied voltage and has a lot of dielectric absorption in comparison, even a NPO type. Only advantage is that it is very cheap and will handle temperature over 70C.

[krivx]

Do you have part/series numbers for the cap, connector and 1G resistor for anyone wanting to take a crack at this?

[Gyro]

@Marco

Would a plain high voltage ceramic capacitor suffice too?

Somewhere between maybe and no. I just tried a fresh from bag 220pF 1kV Murata ceramic. It reads around 300fA at 10V, surprisingly low  but an un-cleaned polystyrene 330pF from the parts box comes out at around 100fA or less (not sure how I'm going to clean them now!). A nice clean 2n2 @ Y1 (5kV test) ceramic reads over 6pA at 10V. So a 330pF 5kV part might get near the ballpark but probably still not as good as a clean low voltage Polystyrene.

@SeanB

I shielded the 1N4148s from light as much as possible but not in dark condition. I did try sliding a piece of heatshrink over one and the leakage shot up to over 1nA  Not the world's greatest insulator then! Haha, yes the end marking of the PS survived its wash. As I said, I'm worried about how to clean them now. The highest value I have is 100nF, so 1uF must be big!

[Vgkid]

What about using a jfet as the protection diode, especially the to-18 canned variants.

[SeanB]

Fist sized, and they are pretty heavy for a part with such thin leads. You also discovered heatshrink is conductive ( surprise surprise carbon black is a conductor), and to clean the ps caps you wash with deionised water and dry with cool air. Solder in using a heat shunt as well, there were only a few series of potted Murata polystyrene capacitor that would survive board cleaning at all, or reflow soldering. Du Pont stopped making the polystyrene film used in them a few years ago, which was the end of them being made new by most manufacturers, only the cheapest off brands are using Chinese film.

[Gyro]

Do you have part/series numbers for the cap, connector and 1G resistor for anyone wanting to take a crack at this?

I've had the glass resistor for years (stripped from an old ion gauge meter together with a 100M and 10G). I think it's a Welwyn 3800 series. RS components list several 1G resistors in non-glass packages:

http://uk.rs-online.com/web/c/?searchTerm=1G+ohm&sra=oss&r=t

 296-0667 looks a good compromise between size and cost (and not having to buy a bag of 5). It's 1% 100ppm/'C too. They also have 160V polystyrene capacitors (and of course LMC662s).

The extended insulation BNC came from my parts box too, but a clean standard one should be just as good.

[Gyro]

What about using a jfet as the protection diode, especially the to-18 canned variants.

They certainly work from the leakage point of view, Datron (among others) use them for that purpose, they wouldn't provide the Zener action for use in the output though (to allow normal output swing). The internal input protection diodes in the LMC662 are really good, they're bootstrapped to achieve the low input bias current while still being good for 5mA (the LMP7721 uses the same trick and achieves 10mA protection).

Fist sized, and they are pretty heavy for a part with such thin leads. You also discovered heatshrink is conductive ( surprise surprise carbon black is a conductor), and to clean the ps caps you wash with deionised water and dry with cool air. Solder in using a heat shunt as well, there were only a few series of potted Murata polystyrene capacitor that would survive board cleaning at all, or reflow soldering. Du Pont stopped making the polystyrene film used in them a few years ago, which was the end of them being made new by most manufacturers, only the cheapest off brands are using Chinese film.

Haha they must be. The old Plessey ones I have include a thicker wire fused into each end with the fine wire brought out and soldered externally. Probably even worse for transferring soldering heat though. Ah, I hadn't thought about the carbon black! Something to be wary of if covering high voltage connections, which it very well might be used for. It was only in very light contact with the 4148 leads too. Yes I knew DuPont had stopped producing the film - I was surprised when I just checked RS for kirivx that they still have them in stock.

I'm not sure how well just deionised water would work against finger grease. I must do some sacrificial tests with kitchen detergent (...or just not touch them in the first place  )

I must find the article Bob Pease wrote on low leakage diodes (maybe in one of his books). I think he highlighted 1N914s being a problem because they were gold bonded. They used to be used more or less interchangeably with 1N4148s in most applications.

[Gyro]

Do you have part/series numbers for the cap, connector and 1G resistor for anyone wanting to take a crack at this?

Ah, RS do have the extended insulation BNCs (for a price!) if you really want to go the extra mile.

http://uk.rs-online.com/web/p/bnc-connectors/2127400/

As I said, a standard BNC socket should be fine though. I haven't tried Farnell etc for pricing. You could try ebay for the resistor and socket too.

EDIT: Remember, the shortest leakage path is between the pins on the LMC662 dip package, nothing to be done about that apart from getting it really clean.

By the way, I made the input 'plug' from the insulator of a standard BNC plug with a barb pin pressed in as center pin and a loop test point soldered to the top. It keeps the input connector clean (long leakage path) when handling and saves pushing wires up the center of the socket.

[edavid]

I must find the article Bob Pease wrote on low leakage diodes (maybe in one of his books). I think he highlighted 1N914s being a problem because they were gold bonded. They used to be used more or less interchangeably with 1N4148s in most applications.

1N914s and 1N4148s use the same gold-doped die, so both have fairly high leakage.

I'd suggest 1N3595 as a good cheap low leakage diode.

Too bad PN4117s are out of production, but you can still get them.

[Alex Nikitin]

I must find the article Bob Pease wrote on low leakage diodes (maybe in one of his books). I think he highlighted 1N914s being a problem because they were gold bonded. They used to be used more or less interchangeably with 1N4148s in most applications.

1N914s and 1N4148s use the same gold-doped die, so both have fairly high leakage.

I'd suggest 1N3595 as a good cheap low leakage diode.

Too bad PN4117s are out of production, but you can still get them.

A good low leakage diode for this kind of circuit is FJH1100 from Fairchild Semi (RS #7729256) however it is very expensive (almost £14 each, inclusive of VAT) and light sensitive (not a problem inside a box though). On the other hand it is good down to some fA at room temperature near 0V for the input protection to ground.

Cheers

Alex

P.S. - I plan to measure leakage currents on a number of various "low leakage" diodes sometime soon, with both forward and reverse bias voltages applied. There are some potentially good protection diodes available - ESD9R from ON Semi for example.

[Gyro]

LEDs are supposed to be good too due to their high bandgap voltage. Of course that equates to a high forward voltage too and of course they are also light sensitive! I haven't tested one yet though.

[Alex Nikitin]

LEDs are supposed to be good too due to their high bandgap voltage. Of course that equates to a high forward voltage too and of course they are also light sensitive! I haven't tested one yet though.

I'll try few LEDs as well, no problem. I've just assembled a tri-axial cable for my Keithley 617, so should be able to measure leakages reliably down to about 10fA. Need to build a test box though.

Cheers

Alex

[BravoV]

Thanks for sharing  , subscribed.

Made me want to build one, problem is finding that Giga ohm resistor. 

[Alex Nikitin]

Thanks for sharing  , subscribed.

Made me want to build one, problem is finding that Giga ohm resistor. 

Well, you can build one without a resistor. There is a second way to measure low currents - with an integrator. Replace the resistor with a capacitor and a reset switch, add a comparator and a timer. 10pF capacitor would create a slope of -1V/s with +10pA input current.

Cheers

Alex

[Gyro]

I'll try few LEDs as well, no problem. I've just assembled a tri-axial cable for my Keithley 617, so should be able to measure leakages reliably down to about 10fA. Need to build a test box though.

Hope you have better luck than I just did (probably wasn't dark enough). I got it from Bob Pease's 'Troubleshooting Analog Circuits'. Apparently the c-b junctions of most 2N3707s are good too, as are 2N4117A jfets.

P.S. I need to build a test box too, AC pickup is a killer!

[Alex Nikitin]

I'll try few LEDs as well, no problem. I've just assembled a tri-axial cable for my Keithley 617, so should be able to measure leakages reliably down to about 10fA. Need to build a test box though.

Hope you have better luck than I just did (probably wasn't dark enough). I got it from Bob Pease's 'Troubleshooting Analog Circuits'. Apparently the c-b junctions of most 2N3707s are good too, as are 2N4117A jfets.

P.S. I need to build a test box too, AC pickup is a killer!

I design electrometers (and other precision analogue stuff) and have to deal with femtoamp current levels everyday at work  . For obvious reasons I won't disclose the details of these circuits but will be happy to help with an advice where I can.

Cheers

Alex

[Gyro]

Haha, a stopwatch and a voltmeter, my reactions are getting too slow for that one. Another alternative is to use two back to back semiconductor junctions as the feedback resistor and make a logarithmic scale meter, that would take some experimentation and calibration though!

I don't know where you are based BravoV. Your local equivalent of RS? I'm sure DIgikey must stock them too. There are lots on ebay from Hong Kong (watch the physical size though), glass ones from Bulgaria / Ukraine / Russian Federation too.

[Gyro]

I design electrometers (and other precision analogue stuff) and have to deal with femtoamp current levels everyday at work  . For obvious reasons I won't disclose the details of these circuits but will be happy to help with an advice where I can.

Interesting, thanks Alex. I don't know about advice - I'd just throw in the gems that we haven't thought of yet!     One that tripped me was when I tried shorting the input to null it. Of course when you do that with a transconductance amplifier it overrides the feedback resistor and turns the opamp into a comparator, so the optput hits one of the rails. Then I referred to the Keithley book and realized that an ammeter should always be checked open circuit (but screened).  I suppose you could also argue that feeding into a virtual earth, any thermal emf should create a very high current so the output should hit the rail anyway.

[Alex Nikitin]

Then I referred to the Keithley book and realized that an ammeter should always be checked open circuit (but screened).  I suppose you could also argue that feeding into a virtual earth, any thermal emf should create a very high current so the output should hit the rail anyway.

In your design you can null the offset by connecting the input to the output (-) with a bit of wire. That should remove the input bias component. After that you remove the wire and cap the input. Any output voltage now is the result of the input bias current. Otherwise you may actually introduce some DC offset on the input, compensating for the bias current drop on the 1G resistor! If your LMC662 clean and undamaged, you should see very little difference on the output after removing the wire (and the circuit settles down).

Cheers

Alex

[Gyro]

Ah, I hadn't thought of that, thanks. My DVM is high impedance (not 10M) so the 1k output resistor shouldn't introduce any error (unless my zeners are leakier than I thought).

EDIT: Yes that worked very well (didn't initially, then I realized I hadn't got it turned on!). I was able to null the opamp cleanly to 0uV with the link attached. With the link removed, things obviously get rather noisier but if I take the average offset it looks less than 5uV (5fA bias current through the 1G) and even the peak readings still comes out less than 10fA. I must have got the package reasonably clean then.    Actually, leaving it a bit longer, the noise seems rather more symmetrical around zero.

[Alex Nikitin]

Ah, I hadn't thought of that, thanks. My DVM is high impedance (not 10M) so the 1k output resistor shouldn't introduce any error (unless my zeners are leakier than I thought).

EDIT: Yes that worked very well (didn't initially, then I realized I hadn't got it turned on!). I was able to null the opamp cleanly to 0uV with the link attached. With the link removed, things obviously get rather noisier but if I take the average offset it looks less than 5uV (5fA bias current through the 1G) and even the peak readings still comes out less than 10fA. I must have got the package reasonably clean then.    Actually, leaving it a bit longer, the noise seems rather more symmetrical around zero.

Yes, a good result. With 1G resistor and ~0.5Hz bandwidth you have due to 330pF cap, the noise of the resistor is about 3uV RMS at room temperature so the equivalent current noise is about 3fA RMS. You are near the limit of detection for 1G resistor, well done!

Cheers

Alex