Picoammeter Design

[Gyro]

The Eval board would be an excellent candidate aside from the eye-watering $250 price.

[Alex Nikitin]

It does raise the question of how good it could have been, ie. how many of the remaining artifacts are due to package / unavoidable PCB spacings vs die, also improved width of guard tracks, more space for via fence in terms of PCB bulk leakage.

Well, the graphs in the datasheet show the input leakage below 0.1fA for <60% RH, that is about 600 electrons per second. For my application it is actually useful to have a small SMD package with this kind of performance.

Cheers

Alex

[plesa]

I'm thinking about getting one of these eval boards to make an electrometer, which is one of the really good ways to measure very high value resistors accurately.  Probably, I will use SHV connectors on the "business end" of the device.

Why are you considering SHV and not some triaxial connectors with guard?

[Alex Nikitin]

The Eval board would be an excellent candidate aside from the eye-watering $250 price.

If you look at the docs for the eval board, you will see that this is a special PCB with teflon board material laminated to both sides of an FR4 board.  *THAT* has got to be rather expensive, and there are proportionally not many board houses that can even do that-- so the price reflects that.

Add to that the rather expensive chip, and all the other parts, plus labor, *very* special cleaning schedule, etc. -- and I think you can see that the price of the eval board is actually a bargain!

I'm thinking about getting one of these eval boards to make an electrometer, which is one of the really good ways to measure very high value resistors accurately.  Probably, I will use SHV connectors on the "business end" of the device.

The evaluation board already uses a very good (and very expensive) triaxial connector.

Cheers

Alex

[Gyro]

1000V through 1G ohm (or 100V through 100M) is 1uA. I don't think you are going to need an electrometer for that, it would register on a 50uA moving coil meter.  (Edit:) or more practically, pretty much any opamp, bipolar or Fet.

I think you are a few orders of magnitude out on your measurement voltage and target resistance values. I'm using a 1G ohm as feedback on my Picoammeter and getting 1mV / pA!

P.S. Maybe you should suggest a group buy to them Alex! Glad I had the luxury of using air though.

[plesa]

For the far end of the high-value resistor [not the input to the electrometer amp-- the connector it has is OK already].  If you are applying 1000V to the far end of the high-value resistor [say, 1G-ohm], then a good, safe, HV connector is needed.  On the low-voltage end, the triaxial connector is just fine.  Probably I would follow the output with a gain stage, so I can get 10V out [using a 10M-ohm feedback resistor] with 1000V into the high-value resistor.  [100V is I'm measuring a 100M-ohm resistor].

Oh, I get it:)

[Gyro]

Sure, I understand, but I still think you're making things difficult (and expensive) for yourself. 1T ohms is highest you're likely to get in practice, at 1000V that is 1nA (or 100pA at 100V). A low bias opamp (such as a $1 LMC662 at ~ 3fA) is easily going to give you much more accuracy than you need. It's going to be very difficult to maintain a 1T ohm standard resistor at any sort of precision. For lower resistance values, the opamp bias becomes even more insignificant.

[dreaquil]

Those triax cables are exorbitantly expensive. 120eur for a 1m cable or so. Couldn't do without it or it would render the keithley 6487 useless.

[plesa]

Those triax cables are exorbitantly expensive. 120eur for a 1m cable or so. Couldn't do without it or it would render the keithley 6487 useless.

It depends if you really needs low noise with graphite coated insolation.
Quite good quality cable (including triaxial) are made by Habia.
Belden 9220 yellow triaxial is not low noise but it is also relativelly cheap (Pomona use it for their triax-banana adaptor)
http://www.belden.com/techdatas/english/9222.pdf
For connectors check ebay or other surplus, you can find them quite cheap.

BTW with Keithley 6487 you will receive low noise triax-banana cable.

[dreaquil]

Those triax cables are exorbitantly expensive. 120eur for a 1m cable or so. Couldn't do without it or it would render the keithley 6487 useless.

It depends if you really needs low noise with graphite coated insolation.
Quite good quality cable (including triaxial) are made by Habia.
Belden 9220 yellow triaxial is not low noise but it is also relativelly cheap (Pomona use it for their triax-banana adaptor)
http://www.belden.com/techdatas/english/9222.pdf
For connectors check ebay or other surplus, you can find them quite cheap.

BTW with Keithley 6487 you will receive low noise triax-banana cable.

Unfortunately the cables were missing with the 6487. The pomona one was specifically the one I bought from Mouser. 122Eur excluding shipping or VAT -.- http://eu.mouser.com/ProductDetail/Pomona-Electronics/5342/?qs=sGAEpiMZZMv8kklI404QlRE%2fFt1BM97a

[dreaquil]

Does anyone know of any board connectors i can use which are known to be low leakage which I can use to run some single strange PE wires to my front panel (where my connectors will be). The connectors will make life easier in dismantling the board from the connectors for debugging and troubleshooting.

EDIT: I meant to write PE not PTFE.

[splin]

Does anyone know of any board connectors i can use which are known to be low leakage which I can use to run some single strange PTFE wires to my front panel (where my connectors will be). The connectors will make life easier in dismantling the board from the connectors for debugging and troubleshooting.

How strange exactly?   

How about using PTFE stand-offs with 2mm bullet connectors or similar soldered on top?

[Gyro]

What about Oxley Snaplox test point and connector....

http://www.oxleygroup.com/images/newproductspdf/126.pdf

[dreaquil]

I meant to write PE insulated wires not PTFE. Those Oxley Snaplocks look ideal, but I'm not sure ill be able to order them because I can't order from farnell and my needs are very low volume (20 max?). I came across these though http://www.digikey.com/product-search/en?keywords=182596000
They look pretty good with a 5mm pitch for higher insulation resistance and it looks like they have an air gap built in.

[edavid]

Does anyone know of any board connectors i can use which are known to be low leakage which I can use to run some single strange PE wires to my front panel (where my connectors will be). The connectors will make life easier in dismantling the board from the connectors for debugging and troubleshooting.

If they are single wires, how can the connector type have an effect on the leakage?

[Gyro]

What about using a single berg pin type female connector?  The sort they use for those cheap jumper leads to plug onto Arduino headers. As edavid says, if it's a single wire, there's no leakage unless something touches the connector or the wire. The only issue I can see is persuading a single berg pin to stay vertical while you solder it!

eg. http://www.ebay.co.uk/itm/Jumper-Leads-Cables-Pin-type-Female-to-female-20cm-length-40-wires-/301112648485?hash=item461bb66325:g:Ki4AAOSwA4dWI7qW

[Gyro]

Probably one for Alex...

I've been playing around with spark gap input protection for ESD (at some point my 1M series resistor is going to flash-over). I've tested a few telecom type spark gap tubes, the ceramic ones are better than the glass ones (probably less humidity prone) but they come in at 5pA+ after cleaning. I have a 1kV one that came from an old Solartron meter, but it's still single digit pA. I tried a small neon bulb, but higher too (anything glass is bad). My best option at the moment seems to be to make a small air-gap with a wire from the ground tag. Is it normal to include spark gap input protection on picoammeters?

Edit: Leakage tests were carried out at 10V

[Marco]

at some point my 1M series resistor is going to flash-over

Use 10 10kV (VR37) 100k resistors then ... your feed through will be a good enough spark gap at that point.

[Gyro]

Thanks Marco, unfortunately not an option due to size constraints (see photo in reply #23). I used the biggest body size I could (2W carbon film). The input connector is also an extended insulation one for low surface leakage.

I'm probably being over-cautious as resistor and op-amp input protection diodes are good for at least 500V (resistor voltage spec), probably double that in practice. I've also managed to bore out the center of a BNC connector shell to clear my input insulator and provide a total ground shield when not in use. I may implement an air gap to break down at around 1kV.

Edit: ... It looks as if anything with a package is going to compromise my (currently single digit fA) resolution.

[SeanB]

Tried soldering in a shorting wire and then snipping it with some clippers to leave 2 sharp points almost touching. 1mm gap will arc over at around 1kV, and a pair of sharp points will make the initiation easier.

[Alex Nikitin]

Is it normal to include spark gap input protection on picoammeters?

No, usually a couple of low leakage diodes to ground on the input after a large resistor is enough.

Cheers

Alex

[Gyro]

Many thanks guys.

Sean, that's the sort of thing I was thinking, I have the solder tag of the ground terminal as a fairly rigid mounting so it should be reasonably easy to maintain a small gap.

I was briefly concerned about corona / ionization current leakage across the gap until I remembered that the input is a virtual ground node, the only time it will see voltage is if the test item is shorted (must remember to include a current limit resistor in case the gap triggers).

I'm probably going overboard in that case Alex, it is a bit belt and braces but I'm not sure if I've been particularly lucky with this LM662 sample and don't want to subtly degrade it with multiple strikes.

Chris

[plesa]

For basic input protection as Alex pointed use only low leakage diodes (1N3595  or DPA1 or low leakage  JFET) and if you are going to connect voltage above 200V create additional box with resistor in series.

[Gyro]

The low leakage diodes are already taken care of - they are the bootstrapped protection diodes within in the LMC662 itself, they're capable of sustaining 5mA max. With the 1M series resistor I'm only asking them to handle 500uA at 500V input, I can't see the resistor actually breaking down until >1kV. You can see my schematic and photos in reply #23. The 1M protection resistor is within the feedback loop to avoid parasitic effects on input impedance (it's a transconductance amp). Note that the 1G ohm feedback resistor is also rated at 500V. I think I'm covered pretty well - the spark gap is mainly precautionary, I just need it to break down before the resistors in case of a major zap (which I ought to be careful enough to avoid!)

[Gyro]

Zero leakage spark gap added... and yes it is more stable than it looks! 

The gap is similar to the 1-2kV sawn spark gap protector from my junk box (came from a CRT base PCB).