Picoammeter Design

[dreaquil]

Glad to see this thread has come back! I'm in the process of setting up my one. Before I fried one of the opamps (waiting for that to arrive) I measured 0pA input offset at around 350fA with an LMC6462 ( I stuffed up the design by not having the power rails large enoguh for the LMC662). I'll get back to you when I have more results.

For anyone looking for a good feedback resistor I'm using an Ohmic 10G MOX-750231008FE.

[Gyro]

Hi dreaquil, nice to see you back. I was wondering how you were getting on with yours.  I seem to have hijacked your thread a bit in the meantime, sorry. I'll be interested to see how low you can get with your guarded PCB...and the LMC6462. The air wired approach works very well but isn't as duplicatable (new word ) as a PCB.

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!

Many thanks Alex, that's really good to know. I've left it a few hours and I'm hard put now to see much of an offset underneath the noise. It needed a bit of copper tape over the BNC to get there in the end.

Chris

[splin]

Funnily enough whilst reading this thread I just received an email from Analog devices announcing the ADA4530-1 electrometer amp with impressive specs -

  Ib  <1fA typical,  20fA max (-40C to 85C). (Production test 20fA max at 25C)
  Voffset 9uV typical, 70uVmax (0C to 125C)
  Voffset drift .13uV/K typical .5uV/K max.
  Input resistance 100Tohms
  Supply .9mA, 4.5V to 16V


Its a bit noisy at 4uVp-p 0.1 to 10Hz and pricey at $15, 100+ (plus distributer margins).

It also has a built in guard amplifier and esd protection diodes up to 10mA. Don't know when they will be available though.

http://www.analog.com/en/products/amplifiers/operational-amplifiers/ada4530-1.html#product-overview

[EDIT] Added input resistance and supply current
[EDIT] Corrected part number

[Gyro]

That's an impressive looking device! Actually all the application information (also the app note) is as interesting as the device itself. Lots of detail.

Edit: The eval board details too.

The input bias current at 85'C is stunning - the LMP7721 is 900fA at the same temperature. It's a neat trick bringing out the protection diode guard buffer, the 662 and 7721 presumably have one, but only internal. It's surprisingly noisy as you say, the LMP7721 is much quieter whilst having a higher GBP.

Time to start saving the pennies... and for an ultrasonic cleaner that's safe to use with IPA (see app note). I though that was a no-no due to cavitation and flammable fogs.

Edit: Damn, the eval board is $250 and doesn't have anywhere for a protection resistor (inside the feedback loop anyway).

[plesa]

Nice find of LMP alternative.
Ultrasonic is good for dirty stuff, you can clean components by vapor or only wash them and dry them with hot air gun.
So I will rather buy the LMP or the AD dev kit. instead of ultrasonic
Some components like ceramic caps I needs to bake for several days to stabilize their leakage current.
BTW there are other cleaning methods but these are more industrial like cleaning with plasma - this will decompose the fingerprints, flux and oil residues.

[exmadscientist]

Just a few notes on cleaning from an ex-scientist:

When I worked in an clean-room environment, there were three pieces to our cleaning strategy. It's rather "low-tech" and uncontrolled -- this wasn't exactly a semiconductor fab -- but you'd be amazed at how well you can do with some basic stuff:

• Ethanol was the backbone of our cleaning regimen for stuff that just had to be "regular clean". It is significantly more aggressive than isopropanol and goes after grease and organics very effectively.
• We used ultrapure (DI) water for all critical cleaning and rinsing. It's quite aggressive chemically, particularly in going after ionic anything. (I've seen it rust 316 stainless steel, which some poor, misinformed "experts" say cannot ever happen! Granted, it was slow corrosion and in extreme circumstances, but it's possible.)
• Our only detergent was Alconox laboratory powder detergent. It's available on Amazon (among other places) and can clean just about anything off of anything. It's gentle yet powerful. Two warnings: don't ever do laundry with it (it pulls dyes out of fabrics), and don't use it in automatic washers (it foams). There's a low-foaming version called Alcojet, but in my experience Alcojet is significantly less effective and should only be used when Alconox can't be.
• After cleaning with Alconox, we rinsed critical items with ultrapure water until the rinsate conductivity fell to zero on a small handheld conductivity meter. That was usually less than a few minutes even for larger items.
• Ultrasonic cleaners with an Alconox bath were our first choice for cleaning anything small enough and robust enough to go through them. This is how we cleaned a lot of things that had to be seriously clean.
• Adding citric acid to the Alconox bath will clean metal surfaces, particularly copper ones, to just about any degree of cleanliness you could need.

[Alex Nikitin]

Funnily enough whilst reading this thread I just received an email from Analog devices announcing the ADA4530-1 electrometer amp with impressive specs -

  Ib  <1fA typical,  20fA max (-40C to 85C). (Production test 20fA max at 25C)
  Voffset 9uV typical, 70uVmax (0C to 125C)
  Voffset drift .13uV/K typical .5uV/K max.
  Input resistance 100Tohms
  Supply .9mA, 4.5V to 16V


Its a bit noisy at 4uVp-p 0.1 to 10Hz and pricey at $15, 100+ (plus distributer margins).

It also has a built in guard amplifier and esd protection diodes up to 10mA. Don't know when they will be available though.

http://www.analog.com/en/products/amplifiers/operational-amplifiers/ada4530-1.html#product-overview

[EDIT] Added input resistance and supply current
[EDIT] Corrected part number

A very nice chip (speaking from personal experience  ).

Cheers

Alex

[splin]

Funnily enough whilst reading this thread I just received an email from Analog devices announcing the ADA4530-1 electrometer amp with impressive specs -

  Ib  <1fA typical,  20fA max (-40C to 85C). (Production test 20fA max at 25C)
  Voffset 9uV typical, 70uVmax (0C to 125C)
  Voffset drift .13uV/K typical .5uV/K max.
  Input resistance 100Tohms
  Supply .9mA, 4.5V to 16V


Its a bit noisy at 4uVp-p 0.1 to 10Hz and pricey at $15, 100+ (plus distributer margins).

It also has a built in guard amplifier and esd protection diodes up to 10mA. Don't know when they will be available though.

http://www.analog.com/en/products/amplifiers/operational-amplifiers/ada4530-1.html#product-overview

[EDIT] Added input resistance and supply current
[EDIT] Corrected part number

A very nice chip (speaking from personal experience  ).

Cheers

Alex

I was pretty sure you would know all about them 

Presumably you've got engineering samples - any idea when the general public will be able to buy them?

[Gyro]

I was pretty sure you would know all about them 

+1 

Just been reading a bit more... 3 HOURS ultrasonic cleaning with IPA for No-Clean Flux 

[Alex Nikitin]

I was pretty sure you would know all about them 

Presumably you've got engineering samples - any idea when the general public will be able to buy them?

I do have some engineering samples, however I can not give you more information, sorry*.

Cheers

Alex

* P.S. - I will have an opportunity to ask about it next week. If I'll get permission to say more, I will  .

[Gyro]

@exmadscientist,

Did you ever come across a cleaner called "Micro" by International Products Corp. I was given a sample bottle of it years ago (no idea why now) and still got it. Google shows up Micro-90, might be the same stuff because it was probably the '90s when it was given to me! 

It claimed to be very thorough but gentle (apart from removing the oxide layer from Aluminium that is!) and was suitable for standard wash, ultrasonic etc.

[exmadscientist]

@exmadscientist,

Did you ever come across a cleaner called "Micro" by International Products Corp. I was given a sample bottle of it years ago (no idea why now) and still got it. Google shows up Micro-90, might be the same stuff because it was probably the '90s when it was given to me! 

It claimed to be very thorough but gentle (apart from removing the oxide layer from Aluminium that is!) and was suitable for standard wash, ultrasonic etc.

Micro-90 rings a faint bell somewhere in the back of my mind, though that wasn't one of the cleaners we used. It's definitely a good laboratory-grade detergent and probably quite suitable for this type of cleaning. Alconox Inc. claims it's equivalent to their Liquinox product, which sounds about right to me. (It's also probably still fine even if it is from the '90s -- I think one of our Alconox cartons was at least that old, and we still trusted the stuff for critical cleaning.) I wouldn't hesitate to try it.

[dreaquil]

Wow! That ADA4530 is a beast. I'll let you know what levels I get down to. However I have a reed relay on the input also because of the specs of my application. Although it shouldn't introduce too much leakage as it has a minimum insulation resistance of 10^12ohm.

If anyone gets any info on when the ADA4530-1 is coming out please post here to let us know! It sounds like a really great opamp.

[BravoV]

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

So viewing the measurement result thru scope ? Mind elaborate more on the detail circuit, I'm just hobbyist here.

[dreaquil]

@Gyro im getting about 1.4pA of leakage with all connectors attached , reed relay and my test voltage applied (40.96V) since I'm using it as a high impedance tester. On the other hand when I turn off the test voltage and disconnect the input from anything using a seperate reed relay I'm getting leakage on the order of 50fA. The LMC6462 has a typical input bias of 150fA which implies that theres probably some leakage thats cancelling out the bias current

[Gyro]

Micro-90 rings a faint bell somewhere in the back of my mind, though that wasn't one of the cleaners we used. It's definitely a good laboratory-grade detergent and probably quite suitable for this type of cleaning. Alconox Inc. claims it's equivalent to their Liquinox product, which sounds about right to me. (It's also probably still fine even if it is from the '90s -- I think one of our Alconox cartons was at least that old, and we still trusted the stuff for critical cleaning.) I wouldn't hesitate to try it.

Thank you. I'll give it a try on a sacrificial board first. According to the comparison chart on the website it handles Flux, Fingerprints, lubricants, mineral deposits, oxides, scale, rust, Blood etc. Unfortunately it's no good on Proteins, Eggs, Butter, Fruit Stains and Urine. As Blood IS covered I think I can probably live with those shortcomings. 

[Gyro]

@Gyro im getting about 1.4pA of leakage with all connectors attached , reed relay and my test voltage applied (40.96V) since I'm using it as a high impedance tester. On the other hand when I turn off the test voltage and disconnect the input from anything using a seperate reed relay I'm getting leakage on the order of 50fA. The LMC6462 has a typical input bias of 150fA which implies that theres probably some leakage thats cancelling out the bias current

That sounds pretty good for that opamp (at least the opamp isn't dominating the overall leakage), I know you said you had supply voltage issues with the LMC662, the  LMP7721 is spec'd down to 1.8V, but of course it's a single, not dual. Alex's method worked well for discriminating leakage from bias current.

Relay choice is an interesting one, I've seen old Solartron arrays where the coils and poles were actually spaced alongside the reed capsules both for leakage and thermal emf. I don't know if you can still get unencapsulated reed relays where you could maybe squeeze a thin wrap of ptfe tape between the capsule and coil. Hopefully it's getting you within your target anyway!

P.S. Any chance of a picture? 

[dreaquil]

It was actually the fact that the input common mode range is V+-2.3V Which I overlooked! The input common mode on the LMC6462 can take inputs all the way up to the supply voltage. This is a problem because I shifted the non inverting input to +4.096V so that it can swing negatively. Ill get you a picture when I open it up again because its in production line for testing (this is my thesis - a piece of manufacturing test equipment.)

[Gyro]

Ah yes, I understand now. I went back and had another look at your comments and schematic in reply #19. It looks like it's holding up well against your expectations then 

[pmcouto]

Wow! That ADA4530 is a beast. I'll let you know what levels I get down to. However I have a reed relay on the input also because of the specs of my application. Although it shouldn't introduce too much leakage as it has a minimum insulation resistance of 10^12ohm.

If anyone gets any info on when the ADA4530-1 is coming out please post here to let us know! It sounds like a really great opamp.

According to the information on AD site, it seems that the ADA4530-1 is now available:

Product Lifecycle
This product has been released to the market. The data sheet contains all final specifications and operating conditions. For new designs, ADI recommends utilization of these products.

[dreaquil]

Hmm not on digikey or mouser yet though.

[Gyro]

I do have some engineering samples, however I can not give you more information, sorry*.

Cheers

Alex

* P.S. - I will have an opportunity to ask about it next week. If I'll get permission to say more, I will  .

Hi Alex,

I'm not sure if you've had your meeting yet, but I have one that you might want to throw into the mix. If you're making a device with 'ultimate' specs like that, then why stuff it into a bog standard SO8 package? They seem to be making it harder for people trying to use it, with PCB layout and leakage concerns. Reading their datasheet and app note where it starts talking about advanced PCB materials (possibly laminated into a standard FR4 stack) it sounds a lot more expensive for the customer than their added cost in using a better package. Surely they could at least put it into an an S014 or SO16 with part populated leadframe? It's been done before for creepage on offline switchers etc. Is it just lack of imagination? 

Thanks,

Chris

P.S. It can't be a pin compatibility issue because I don't think anything else has the same pinout, the LMP7721 is different.

[Alex Nikitin]

I do have some engineering samples, however I can not give you more information, sorry*.

Cheers

Alex

* P.S. - I will have an opportunity to ask about it next week. If I'll get permission to say more, I will  .

Hi Alex,

I'm not sure if you've had your meeting yet, but I have one that you might want to throw into the mix. If you're making a device with 'ultimate' specs like that, then why stuff it into a bog standard SO8 package? They seem to be making it harder for people trying to use it, with PCB layout and leakage concerns. Reading their datasheet and app note where it starts talking about advanced PCB materials (possibly laminated into a standard FR4 stack) it sounds a lot more expensive for the customer than their added cost in using a better package. Surely they could at least put it into an an S014 or SO16 with part populated leadframe? It's been done before for creepage on offline switchers etc. Is it just lack of imagination? 

Thanks,

Chris

Hi Chris,

The meeting is on Friday. I personally see no big problem with the package if the results are as good as the datasheet shows (and my own experience confirms). I have achieved good results even with the SO-8 version of the LMC662 with it's standard pin-out, if a good board material is used, and with the ADA4530-1 clearances are much better due to the non-standard pin-out.

Cheers

Alex

[Gyro]

Hi Alex,

Thanks. I'm sure you're right in terms of a perfectly executed and cleaned design (which I'm certain yours are) 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. It would also help with the 'odd spec of dust' during the operating life of the equipment. The guard pins must help a lot though, an improvement on the two unconnected pins in the same position on the 7721 (I know they said they had redesigned pinout to improve performance).  Just a though anyway.

Thanks,

Chris

P.S. A useful side effect would be to give me more space to air-wire it! 

[krivx]

P.S. A useful side effect would be to give me more space to air-wire it! 

If someone manages a PCB on specialty materials this could be a good candidate for a group-buy.