Picoammeter Design

[Gyro]

Hello

I don't know if i'm going out of topic.

I'm trying to modify this desing making it multi-range, using a push-pull buffer inside the feedback to achieve higher current capability when reading milliAmps.
When i try to use the 1GOhm range, so using the circuit almost as it was design(except for the current buffer) i get a nasty saturated 50Hz output.
do you have any ideas why it happens? I shoud say that i air-wired only the part in the red area in the picture. Can this be the issue?

Not off topic as such (although by the time you get to 1R feedback resistors you're a long way from pA. )

Instinct says that you are going to need very different (incompatible?) switching mechanisms to tackle dry switching and stray leakage currents on the 1G range and the nasty contact resistance dependency of the 1R range. Do you actually need to cover this entire range on a single input? Maybe you could split them.

You might be better to bump or cross refer to (I guess I've done that now) to your https://www.eevblog.com/forum/projects/poor-linearity-of-a-opamp-push-pull-buffered/ regarding the issues around the push-pull MOSFET follower, and your https://www.eevblog.com/forum/beginners/line-filtering-in-transimpedance-amplifier/ regarding the 50Hz issues and filter attempts so that other members have some context of what you have already done and the existing replies.

[RawCode]

Thank you for all the replies, very very kind of you

The 50Hz interference behaviour is quite strange: depending on the load, there is some kind of a voltage/current "threshold". If the voltage/current exceeds this "threshold" the output signal is fine, almost without any 50Hz interference. If the voltage/current on the load is below that variable threshold though, in the output there is a 50Hz square wave(probably a saturated 50Hz sine). I mean, it's not always present, just when the current/voltage on the load is very small(200/500mV or so across the load). Could this behaviour be an issue related to the MOS current buffer? If yes, a BJT/LT1010 current buffer can solve the issue?

Yes this circuit should have this large current sensitivity, because i don't know which kind of device will be tested. To be honest I was taking inspiration from this circuit and the Keithley 220, which seems that can handle a large current range.
Watching at the schematics (i don't know if i'm allowed to share them, but you can find them in the 220's manual) it seems that the highest resistor is always connected, as Cerebus suggested, but how can i be sure about the value of the resulting parallel resistor? i mean, yes there is the formula, but how is it reliable considering both resistor tollerances? probably the best way is measuring the actual value of the resistors, but how can i do it without a specialized and very sensitive ohmmeter(considering that there is a 1GOhm resistor)?
Also, there are practically no capacitor in parallel with the sense resistors in the Keithley's schematic. Would just one capacitor (330pF) suffice for the entire range of resistors or are necessary different capacitor for each resistor?

Yes, I'm using mechanical switching(these relays, which should have at least 1.5TOhm insulation resistance: DIP12-1A72-12L). I'm assuming that you are referring to dry switching to the phenomenon where the relay's contacts get oxidised. Am I right? If not, what do you mean by dry switching?

Also, in passing, if any of your switches are mechanical (including relays) there is a real risk of not pushing enough current through to overcome 'dry' contacts.

Do you suggest the use of some kind of bootstrapping for the relays' coil, like a capacitor in parallel to a the series resistor with the coil to get a momentary current kick?

Instinct says that you are going to need very different (incompatible?) switching mechanisms to tackle dry switching and stray leakage currents on the 1G range and the nasty contact resistance dependency of the 1R range. Do

In the Keithley's schematic there is something strange, like a hybrid relay-NJFET switching mechanism, but i don't quite understand how it works. What about JFET's parassitic capacitances, leakage current? How just one NJFET can allow bidirectional current flow? Sure they are bidirectional unlike BJTs, but should it be necessary a PJFET in parallel to all of those NJFET to handle negative signals? The 220's circuit is quite complex, especially the circuit related to U319, and i don't quite understand what is going on to be honest, so please sorry if i'm saying something stupid: my electronics knowledge is still quite limited 

[Cerebus]

Watching at the schematics (i don't know if i'm allowed to share them, but you can find them in the 220's manual) it seems that the highest resistor is always connected, as Cerebus suggested, but how can i be sure about the value of the resulting parallel resistor? i mean, yes there is the formula, but how is it reliable considering both resistor tollerances? probably the best way is measuring the actual value of the resistors, but how can i do it without a specialized and very sensitive ohmmeter(considering that there is a 1GOhm resistor)?

You just have to work it out the standard way, and account for tolerances in the calculation. In real life tolerances tend to add in your favour (i.e. tend to cancel because statistics, but one can't rely on that).

Also, there are practically no capacitor in parallel with the sense resistors in the Keithley's schematic. Would just one capacitor (330pF) suffice for the entire range of resistors or are necessary different capacitor for each resistor?

The RC product (i.e. time constant) needs to be the same for each range. Again, in practice, this can often means that stray capacitance in combination with high value resistors like 1G can result in too big a time constant without a compensating capacitor at all. It gets messy. Look at the Keithley 220 schematics and try to figure out how they did the compensation.

Yes, I'm using mechanical switching(these relays, which should have at least 1.5TOhm insulation resistance: DIP12-1A72-12L). I'm assuming that you are referring to dry switching to the phenomenon where the relay's contacts get oxidised. Am I right? If not, what do you mean by dry switching?

Also, in passing, if any of your switches are mechanical (including relays) there is a real risk of not pushing enough current through to overcome 'dry' contacts.

Do you suggest the use of some kind of bootstrapping for the relays' coil, like a capacitor in parallel to a the series resistor with the coil to get a momentary current kick?

No, nothing to do with the coil currents and all to do with the formation of a high resistance layer on the relay contacts. Pretty much all relays are problematic when you're trying to switch picoamps with them - leakage currents, dry contacts and so on. The old way of getting around the dry contact problem was to use mercury wetted relay contacts but those are a thing of the past. Leakage currents generally calls for expensive relays that take this stuff into account (e.g. Coto) and provide you with contacts for guard rings etc.

[GerryR]

I have read through this thread and thought the information presented here was worth the "bump."  Enjoy the journey!

[Terry Bites]

Some more hints


https://www.analog.com/media/en/technical-documentation/user-guides/ADA4530-1R-EBZ_UG-865.pdf
https://www.analog.com/media/en/reference-design-documentation/reference-designs/CN0407.pdf

[GerryR]

Another op-amp candidate for this project, oldy but a goody:
(20fA bias current) 

[magic]

I found the cheap TS27L2 to be surprisingly decent.
I don't remember the exact number, but my sample's input leakage was something like either 10fA or 20fA, not far behind LMC662.

[Gyro]

Another op-amp candidate for this project, oldy but a goody:
(20fA bias current) 

Not a bad choice.

Unfortunately, when you get to the fA level, you get into the fuzzy and dangerous area of 'Typical' specs at room temperature rather than limit values, taking into account the upper temperature spec - hence the industrial temp LMC6482 shows 4pA max while the commercial temperature spec variant of the LMC662 shows 2pA (and 4pA for the industrial spec part).

It is generally believed (I don't think it has been confirmed) that the LMC662 achieves its particularly low (2fA Typ) input current by bootstrapping its input ESD protection diodes from internal guard supplies. I don't know if the LMC6482 [EDIT: or the TS27L2] have the same or not. It would be nice to have some die pictures to know for sure. [EDIT: Alex Nikitin would probably has the best placed on this stuff as he designs Picoammeters commercially and gets some inside IC info].

As I say Typical / headline figures vs guaranteed limits are a dangerous area, at least for volume design. For one-off projects you can take few more liberties with individual samples.

As the two parts are pin compatible it would be interesting to directly compare if you have some in stock. Not quite as quick and simple as simple socket swapping due to the need to air-solder the inverting input pin (if you chose to go that way) and subsequent cleaning and drying time.

[GerryR]

I've had the 6482's "in stock" for 20+ years.  When going through my stock data sheets, I came across the 6482's and it was like Christmas!  I'm waiting for the 662's to get here, along with the 1 G resistor and then I will have at it.  I did intend on comparing the two.
Cheers,
GerryR

[magic]

It is generally believed (I don't think it has been confirmed) that the LMC662 achieves its particularly low (2fA Typ) input current by bootstrapping its input ESD protection diodes from internal guard supplies. I don't know if the LMC6482 [EDIT: or the TS27L2] have the same or not. It would be nice to have some die pictures to know for sure. [EDIT: Alex Nikitin would probably has the best placed on this stuff as he designs Picoammeters commercially and gets some inside IC info].

LMC6001 is on zeptobars. It is believed to be one half of LMC662 or something very similar to that and indeed its die contains transistors for two channels, but only the left one is wired up.

Protection structures are near the input pads (left edge) with connections to GND and VCC and they look quite small but I will not pretend to understand how they work. Other than that, there are only polysilicon (green) traces going from there straight to the input stage (16 circular transistors in the center). These are better visible in the right channel, where metal doesn't obscure them.

There is a paper about LMC660 written by Monticelli which I linked in Noopy's opamp thread recently, but IIRC there was nothing about input protection there.

As the two parts are pin compatible it would be interesting to directly compare if you have some in stock. Not quite as quick and simple as simple socket swapping due to the need to air-solder the inverting input pin (if you chose to go that way) and subsequent cleaning and drying time.

One could bend the important pin sideways and attach some sort of single-pin socket to it. That's how I did it, with the rest of the circuit literally running on a breadboard and the whole experiment enclosed in a grounded metal box.

I verified input leakage by hanging an appropriately bent wire on the leads of the air-suspended feedback resistor to short it out.

[David Hess]

How just one NJFET can allow bidirectional current flow? Sure they are bidirectional unlike BJTs, but should it be necessary a PJFET in parallel to all of those NJFET to handle negative signals?

Only one polarity of FET, whether a JFET or MOSFET, is required to switch a signal.  CMOS multiplexers use both in parallel so that their signal range includes the entire supply voltage range.

Bipolar transistors actually are bidirectional but were seldom used that way because of various deficiencies.  They used to make symmetrical bipolar transistors for chopping applications but they were quickly replaced with JFETs and then MOSFETs when they became available.

Below is an example from the Tektronix 7T11 sampling sweep using 2N3904s for Q400 and Q402.  I have no idea why Tektronix did not use a JFET since they obviously had them available.  I have to assume that the bipolar transistors, 2N3904s in this case, actually performed better.

[David Hess]

LMC6001 is on zeptobars. It is believed to be one half of LMC662 or something very similar to that and indeed its die contains transistors for two channels, but only the left one is wired up.

The LMC6001 is an LMC6081 tested for 25 femtoamp input current.

I used a lot of LMC6081s for low drift high temperature integrators and measured many of them at 2 femtoamps.

[Gyro]

LMC6001 is on zeptobars. It is believed to be one half of LMC662 or something very similar to that and indeed its die contains transistors for two channels, but only the left one is wired up.

Protection structures are near the input pads (left edge) with connections to GND and VCC and they look quite small but I will not pretend to understand how they work. Other than that, there are only polysilicon (green) traces going from there straight to the input stage (16 circular transistors in the center). These are better visible in the right channel, where metal doesn't obscure them.

Thanks, I hadn't seen that. No, I wouldn't pretend to understand the fine detail without Noopy's closeups and interpretation either.

Another interesting comparison might be the LMP7721, which (similar to the LMC662) claims 3fA typ input current (20fA max @25'C), and from the datasheet/app note, definitely has internal guarding. I can't find it now, but I'm sure I saw a reference to its input protection being able to withstand 10mA continuous rather than the usual 5mA too.

It still seems to be a strange anomaly that the low cost LMC662 has a claimed 2fA typ input current, similar to the LMP7721, when others, such as the LMC6001 / LMC6081 and LMC68482 are all in the 20-25fA range (ok the LMC6001 is the tested limit rather than a typical). The LMC662 does seem to get used in commercial picoammeter input stages though, as indicated in Alex Nikitin's earlier posts, and observed in several equipment teardowns on the forum. It would be rather amusing if it all came down to the datasheet author having accidentally missed out a zero.

[magic]

The original spec for LMC660/662 was 40fA. It's in the Monticelli paper and the 1989 NS linear databook. So those other parts may have been an improvement at the time of their introduction, or at least on paper.

I don't know why the spec has been revised - due to an actual improvement, an improvement in their ability to measure it, or a typo. Somewhere there is a video of Bob Pease talking about their adventures building test fixtures capable of measuring Ib of those opamps, maybe it was even posted in this thread?

[Kleinstein]

In the sub pA range there can be a big difference between typical an maximum specs. Not too many DS show both typical an max values.  E.g. The LMC6462AM gives 150 fA typical and 200 pA_max. There can also be differences if there are more manufacturers and the letters at the end.

[Cerebus]

Somewhere there is a video of Bob Pease talking about their adventures building test fixtures capable of measuring Ib of those opamps, maybe it was even posted in this thread?

Probably, because I know that I've posted a link the forum more than once. But there's no harm with doing it again:

[Gyro]

Ah, yes, that was the one which mentioned the potential pitfalls of too much Teflon and cosmic rays. It ought to be pinned somewhere.

I wonder if the LMC662 headline figure partly came from that video [EDIT: No of course it can't, the datasheet pre-dates the video  ], it mentions that 99+% of devices came in below 5fA. Being an earlier device, they may have made a bigger deal of such a low typical figure (yes there's a big difference between typical and limit values, I mentioned earlier that it was a danger zone for volume production), than the LMC6001. Either that, or they got really lucky with that particular die layout.

The LMP7721 seems to be the only device where they have made the effort to optimise the pinout specifically for low leakage. Although it is only available in SO8, it has two dedicated n/c pins between the inputs and other pins, that can be included in the guarding scheme. The price seems to have come down too (at least looking at RS in the UK) it's about a third of the price of an LMC6001 (probably a result of the additional part selection step in the latter).

As an aside, I don't know how much benefit there is to be gained from using a dual device in practice. In my proto adaptor, having the rail splitter and input amp in the same package does seem to have benefited from offset voltage tracking between the two with supply and temperature, as the offset null seems to remain remarkably stable. Maybe just luck with a particular sample, tracking isn't something that appears in commodity opamp datasheets.

[Kleinstein]

The ADA4530 also has a special pinpout. It has great data but also a steep price.

[bsw_m]

ADA4530-1 input bias current measurement with 0.1Hz bandwith:
Perhaps later, I will measure the input currents for the LMC662 and LMP7721 in the same way.

[bsw_m]

ADA4530-1 input bias current measurement with 0.01Hz bandwith:

[magic]

That's nice.

On the other hand, LMP7721 is supposed to have less voltage noise than the rest of those opamps. It probably doesn't matter in this application.
The datasheet shows some dependence of leakage on common mode input voltage.

edit
Attached input protection and leakage paragraph from Monticelli.

[Cerebus]

ADA4530-1 input bias current measurement with 0.1Hz bandwith:
Perhaps later, I will measure the input currents for the LMC662 and LMP7721 in the same way.

Please forgive me for being that guy, but that chart appears to show typical readings in the region of 5x10^-17 A (50 attoamps), taken in 0.1 second samples. That's around 31 individual electrons measured in each sample. What instrument are you using that is capable of delivering valid readings under those circumstances?

[SilverSolder]

ADA4530-1 input bias current measurement with 0.1Hz bandwith:
Perhaps later, I will measure the input currents for the LMC662 and LMP7721 in the same way.

Please forgive me for being that guy, but that chart appears to show typical readings in the region of 5x10^-17 A (50 attoamps), taken in 0.1 second samples. That's around 31 individual electrons measured in each sample. What instrument are you using that is capable of delivering valid readings under those circumstances?

An abacus?   

[bsw_m]

What instrument are you using that is capable of delivering valid readings under those circumstances?

Old V7-45 electrometer, designed in MNIPI.
Edit:
Please note 0.1second per sample - this is ADC speed. Measuring interval (bandwith) is 0.1Hz or 0.01Hz.
Regarding the reliability of measurements, I'll just say that the performance verification procedure provides for checking the 20aA point, while the maximum permissible error is ± 8aA

[bsw_m]

An abacus?   

We can say that the abacus for individual electrons