Beginner level: DIY an accurate picoammeter (measure picoamps!)

[SilverSolder]

[...]

Connect the input to the -Ve output to zero the 662 offset voltage. When you remove the link, it will then just read the input bias current (hopefully single digits fA).

[...]

So how do you use that result -  do you make a note of the number,  and take it as the real zero pA reading?

[NeverDie]

You zero it using the trim pot.

[Gyro]

Yes, it's better to zero the LM662 offset voltage and then take account of any residual bias current and its polarity when making very low level measurements.

If you try to 'fudge' the overall (offset voltage + bias current) reading to zero using the offset voltage adjustment, you are likely to run into problems where the source voltage is low.

With good insulation and a nice clean LM662, the bias current should be right down in the low fA region anyway, with noise dominating (mine reads zero with open circuit (but screened) input to the 10s of microvolt level after offset voltage trimming).


EDIT: I've just been and checked mine. Its output has shifted by about 10uV from zero at least 2 years after zeroing the offset voltage, that is most likely down to the Vos changing with battery voltage rather than any change in bias current. Again, that's with open circuit but screened input. Noise is about 10uV pk-pk.

[NeverDie]

Yes, it's better to zero the LM662 offset voltage and then take account of any residual bias current and its polarity when making very low level measurements.

If you try to 'fudge' the overall (offset voltage + bias current) reading to zero using the offset voltage adjustment, you are likely to run into problems where the source voltage is low.

With good insulation and a nice clean LM662, the bias current should be right down in the low fA region anyway, with noise dominating (mine reads zero with open circuit (but screened) input to the 10s of microvolt level after offset voltage trimming).


EDIT: I've just been and checked mine. Its output has shifted by about 10uV from zero at least 2 years after zeroing the offset voltage, that is most likely down to the Vos changing with battery voltage rather than any change in bias current. Again, that's with open circuit but screened input. Noise is about 10uV pk-pk.

Aha, I guess by your definition of fudging, I had been fudging it then.    I don't see any other way though.  How is it that you are supposed to zero just the offset voltage but account for the bias current (and its polarity) separately?  I've been treating this as a black box, so maybe I now need to delve under the hood....

[Gyro]

Due to the very low bias current of the LM662, you should hopefully find that you reach very close to the same result (zero setting), its offset voltage should dominate the adjustment    The difference at the output should be in the low uV range.

Luckily, with the input shorted to the -ve output, you don't need to worry about screening - you are effectively creating a low impedance voltage follower which follows the offset adjustment pot wiper, and then zeroing the offset voltage relative to ground (the +ve output).

[magic]

How is it that you are supposed to zero just the offset voltage but account for the bias current (and its polarity) separately?  I've been treating this as a black box, so maybe I now need to delve under the hood....

By bypassing the feedback resistor and shorting IN- to OUT. This reduces feedback resistance and therefore detection sensitivity a billion or trillion times, so the output no longer reacts to input current and only offset voltage remains.

[NeverDie]

You can probably dispense with the 1k resistor on the +Ve output and use a non-isolated BNC for the output too. I put the resistor there as a precaution but it shouldn't affect stability as long as the one on the -Ve terminal (centre pin) is still present.

Should I use a shorting cap on the input BNC when doing the zero calibration on the picoammeter?


Or should I use a 50 ohm BNC terminator instead?

You're right, 50R BNCs are most common and a sensible choice.

No, never use a shorted input or terminator to zero the picoammeter - it won't do any damage, but the output will go to one rail and stay there (it basically becomes a voltage comparator circuit, biased by the opamp offset voltage). The rule is to zero voltmeters with a shorted input and ammeters with an open input.

Connect the input to the -Ve output to zero the 662 offset voltage. When you remove the link, it will then just read the input bias current (hopefully single digits fA). If you want to shield the input to make this measurement as stable as possible, then either put something across the end of the BNC - copper tape, coin, BNC plug with centre pin removed.

OK, after re-reading this, I think I get it now. 

Could I use a tilt-switch, perhaps in series with a reed switch to make a double trigger, on the inside of the enclosure to do the shorting?  By shorting inside the enclosure, I'm thinking that it would accomplish the goal of "shield the input to make this measurement as stable as possible."  On the other hand, maybe having that wiring "flapping in the breeze" when the double trigger isn't engaged might cause different problems.  Not sure.

[NeverDie]

Luckily, with the input shorted to the -ve output, you don't need to worry about screening - you are effectively creating a low impedance voltage follower which follows the offset adjustment pot wiper, and then zeroing the offset voltage relative to ground (the +ve output).

By "you don't need to worry about screening," do you mean that shielding during the zeroing isn't important after all?  See post directly above.

[NeverDie]

Due to the very low bias current of the LM662, you should hopefully find that you reach very close to the same result (zero setting), its offset voltage should dominate the adjustment    The difference at the output should be in the low uV range.

I think to date that's why I haven't noticed anything other than 0 volts output after completing the zeroing: I wasn't looking for anything in the low uV range.  What would that low microvoltage represent though?  Does each microvolt represent 1 pa of bias current, which, theoretically, I would need to add to the final picoamp measurement (assuming single digit pa measurements are possible), measured in the millivolt range?   

[NeverDie]

Ok, I modified the picoammeter to use BNC connectors:



I blocked off the unused hole with a piece of single sided copper PCB, held in place with scotch tape.  Meh, not ideal, but hopefully it will do for the short-term.

[NeverDie]

[NeverDie]

[NeverDie]

Unfortunately, I won't be able to test this new build until Friday, which is when I receive the BNC-to-BNC patch cable for hooking the output up to the oscilloscope:

https://www.amazon.com/gp/product/B079GSYT4M/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1

In fact, I'm not even sure if those are the proper patch cables for doing that (?).  I'd also use it for zeroing the picoammeter.

For hooking up the picoammeter to a DMM, I ordered these:

https://www.amazon.com/gp/product/B07SQPVMLD/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1
though I'm not sure whether they would be better or worse than just using a patch connecting cable together with one of the BNC-to-Banana-Plug adapters, like the kind that SilverSolder showed in his post above.

If this together with everything else (shielding the device under test within another conductive enclosure and possibly power the Rigol 1054Z oscilloscope from DC battery voltage instead of mains AC) doesn't get rid of the of the 60hz common mode noise, then the only other thing I can think of to try would be to use tightly twisted pair cables and have them feed a unity gain differential op-amp, whose output would then feed into the existing picoammeter's input.

[magic]

Could I use a tilt-switch, perhaps in series with a reed switch to make a double trigger, on the inside of the enclosure to do the shorting?  By shorting inside the enclosure, I'm thinking that it would accomplish the goal of "shield the input to make this measurement as stable as possible."

Yes, but its off resistance will appear in parallel with the feedback resistor so you need a good, low leakage switch.
You don't need shielding for zeroing because the whole circuit is low impedance. An external cable will do.

[Gyro]

Agreed, don't risk a reed switch, it will probably ruin your accuracy. You'll probably find that the offset only needs zeroing every few months at worst (it drifts only slightly as the battery voltage goes down, but you won't see it if your meter resolution is at the mV level).

Due to the very low bias current of the LM662, you should hopefully find that you reach very close to the same result (zero setting), its offset voltage should dominate the adjustment    The difference at the output should be in the low uV range.

I think to date that's why I haven't noticed anything other than 0 volts output after completing the zeroing: I wasn't looking for anything in the low uV range.  What would that low microvoltage represent though?  Does each microvolt represent 1 pa of bias current, which, theoretically, I would need to add to the final picoamp measurement (assuming single digit pa measurements are possible), measured in the millivolt range?   

No, you probably won't notice a difference - the external zeroing link just makes it much easier to get the offset voltage spot-on zero, without having to worry about noisy readings or the shielding.

Microvolt outputs represent FemtoAmps (pA/1000)    The typical spec for the LM662 bias current is 3fA. With the way I restricted the adjustment range of the offset pot, it's possible to zero the opamp to 0uV (assuming that you have a meter with that resolution). The reading is always 1pA/mV.

Luckily, with the input shorted to the -ve output, you don't need to worry about screening - you are effectively creating a low impedance voltage follower which follows the offset adjustment pot wiper, and then zeroing the offset voltage relative to ground (the +ve output).

By "you don't need to worry about screening," do you mean that shielding during the zeroing isn't important after all?  See post directly above.

As above, if you have the external link in place, you're bypassing the 1G resistor with a short, so it doesn't care about shielding any more - it's not seeing fA or pA in that state.

Ok, I modified the picoammeter to use BNC connectors:

I blocked off the unused hole with a piece of single sided copper PCB, held in place with scotch tape.  Meh, not ideal, but hopefully it will do for the short-term.

Looking good. 

[NeverDie]

Here's the present configuration for zeroing the picoammeter:


In retrospect, maybe it would have been better if I had positioned the BNC female connectors so that I could have used a more elegant connection, such as:

for zeroing the picoammeter using a connected DMM.  I made this adapter by hooking together two separate adapters.  I tried looking for a single adapter with this type of functionality, but I couldn't find any that connected the input from two male BNC connectors while also feeding a female BNC (for connecting to a DMM or an oscilloscope).

As for the cabling itself, I just now became aware that there exists something called "Low Triboelectric" cable, which claims to be lower noise than regular cabling.  Pomona claims that it is "ideal for low power, low frequency measurements."  https://www.mouser.com/datasheet/2/159/d4964_1_02-1514307.pdf
Would that be the best kind of cabling for connecting the DUT to the picoammeter, or the picoammeter to an oscilliscope?  Or does there exist even better cabling for that purpose?  Or, does it make no difference?

More testing to do, but the initial tests are demonstrating that the amount of measurement noise showing on the DMM is greatly reduced.   

[NeverDie]

BTW, probably the least expensive project box that leaves plenty of space for this picoammeter project is a simple tin box, such as:

https://www.amazon.com/gp/product/B07DQFP7XM/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
which, IIUC, is made from tin plated steel.    If you buy a few the cost per box is less than $2.

[Kleinstein]

For the zero adjustment (with the input connected to the output) the cable / input connector is not critical. The OP is working as a times 1 buffer, thus no more pA anymore. The stability of the offset is more like in the 10 - 100 µV order of magnitude anyway.  So the BNC connectors and normal cables are OK.

The critical part is when actually measuring small currents - than the cable type can make a difference and ideally there would be no extra cable. One could consider directly a 2 nd box on top for the parts to test.

The is an alternative way to do the zero adjustment: connect a higher value resistors (e.g. 1 M - exact value does not matter) to connect the input to ground. The OP than works as amplifier for the offset. This way may be a little more sensitive to stray currents and shielding however, but it can work without a sensitive voltmeter at the output.

[NeverDie]

One could consider directly a 2 nd box on top for the parts to test.

How would the picoammeter input be wired to the "2nd box on top for the parts to test"?

[NeverDie]

Would connecting the picoammeter to a box with the DUT using this:

be better than using a cable?

[Kleinstein]

For the test box I would consider some connector / feed through and maybe a clip to hold the parts to test.
So the 2nd box directly connected and only one connector, e.g. the BNC output inside the 2nd box - so no additional cable or adapter. If a cable would be used for some external current, leave the 2nd box open.
 
To get test signals in (e.g. the an external DC voltage to test leakage) have some connectors at the outside and a cable with clip in the inside.  So the 2nd box could be closed and the test signal  (usually the other side of the part to test) applied externally.

[SilverSolder]

Here's the present configuration for zeroing the picoammeter:
(Attachment Link)

In retrospect, maybe it would have been better if I had positioned the BNC female connectors so that I could have used a more elegant connection, such as:
(Attachment Link)
for zeroing the picoammeter using a connected DMM.  I made this adapter by hooking together two separate adapters.  I tried looking for a single adapter with this type of functionality, but I couldn't find any that connected the input from two male BNC connectors while also feeding a female BNC (for connecting to a DMM or an oscilloscope).

As for the cabling itself, I just now became aware that there exists something called "Low Triboelectric" cable, which claims to be lower noise than regular cabling.  Pomona claims that it is "ideal for low power, low frequency measurements."  https://www.mouser.com/datasheet/2/159/d4964_1_02-1514307.pdf
Would that be the best kind of cabling for connecting the DUT to the picoammeter, or the picoammeter to an oscilliscope?  Or does there exist even better cabling for that purpose?  Or, does it make no difference?

More testing to do, but the initial tests are demonstrating that the amount of measurement noise showing on the DMM is greatly reduced.   

The meter (and the scope) can be considered "half deaf" compared to the picoammeter, so they won't notice the quality of the cables unless they are truly terrible.  The connection between the DUT and the picoammeter needs to be good, though.  Short, and good.

[NeverDie]

Here's the present configuration for zeroing the picoammeter:
(Attachment Link)

In retrospect, maybe it would have been better if I had positioned the BNC female connectors so that I could have used a more elegant connection, such as:
(Attachment Link)
for zeroing the picoammeter using a connected DMM.  I made this adapter by hooking together two separate adapters.  I tried looking for a single adapter with this type of functionality, but I couldn't find any that connected the input from two male BNC connectors while also feeding a female BNC (for connecting to a DMM or an oscilloscope).

As for the cabling itself, I just now became aware that there exists something called "Low Triboelectric" cable, which claims to be lower noise than regular cabling.  Pomona claims that it is "ideal for low power, low frequency measurements."  https://www.mouser.com/datasheet/2/159/d4964_1_02-1514307.pdf
Would that be the best kind of cabling for connecting the DUT to the picoammeter, or the picoammeter to an oscilliscope?  Or does there exist even better cabling for that purpose?  Or, does it make no difference?

More testing to do, but the initial tests are demonstrating that the amount of measurement noise showing on the DMM is greatly reduced.   

The meter (and the scope) can be considered "half deaf" compared to the picoammeter, so they won't notice the quality of the cables unless they are truly terrible.  The connection between the DUT and the picoammeter needs to be good, though.  Short, and good.

That makes sense. 

What does he mean by "feed through"?  Something like this?

[NeverDie]

What do you mean by "feed through"?

[NeverDie]

I just now received this:


My plan is to connect it to the picoammeter using:


and to put the DUT inside it.