Measuring nanoamps and below like a Ninja

[ch_scr]

Amp-porn movie: testing user interface and main functions of new GLIN with 1nF capacitor.
https://ampnuts.com/micron-glin-v2-porn/

Seems like your effort paid out and this one works even better than the last one, good job! Would you share a picture of how the capacitor was hooked up?

[ch_scr]

I use this simple tool. One jack is BNC for connect to GLIN, other is Molex 3-lug triax, connected via 1.5m cable to electrometer (Keysight 16494A-001 cable).

Wow, and for ground return current, just screws / washers biting through aluminium oxide layer are enough? 
That's... mostly an accidental path? Is there actually a DC connection between the two outer connector shells?
Or will return current take long way around device grounding loop? I'm surprised it's this good like this?! 

[ch_scr]

just screws / washers biting through aluminium oxide layer are enough? 

You means resistance of Aluminum oxide layer?
Wall end of aluminum box parts is vanished from paint to provide good connections between outer shield of a triax and shield of GLIN cable.
Aluminum oxide layer is may provide problems, but magic is - you can put some KOhm  resistor between signal ground of a GLIN and ground of a electrometer without any problem. You may add any offset between grounds... Electrometer can contains any input bias.... Ramp through capacitor is eliminate any offset to negligible.

Otherwise - if you generate current with HiMeg resistors, electrometer input bias is important error source.

Thank you, that answers my question.

[branadic]

What's the purpose of magic voodoo slots around OCXO? Is it just for the fancy look of it? 

-branadic-

[SilverSolder]

I guess it is possible to be a "nut" at high frequencies too!  e.g. impossibly accurate frequencies and phases etc.

[Kleinstein]

The frequency part does not really need supper high accuracy.  Even the accuracy of a normal crystal oscillator would be good enough and even an OCXO or similar would be overkill. The accuracy of the capacitors is not even close and in many cases the  I = C du/dt part is used relative to compare currents of different scales (e.g. 1 fA to 100 nA) and not as an absolute from a given capacitance, voltage and frequency.

For the satelite stabilzed clock there are plenty of solutions / plans available. No real need to build everything from scratch here. For the antenna the position (outside with unobstarcted view) may be more important than the actual antenna part.

With the relatively low orbits of the satelites there is not much optimization for regions, as the earth is moving and the orbits are not in sync. It is only a bit the question if they want good coverage also for the polar regions.  GPS currently may have some additional errors over russia, but even this should not effect the frequency part that much, more like some added noise, but not a frequency bias.

[Ole]

I just had a look around ebay and found some reasonably priced triaxial cables.
While I do not know the exact quality of these cables I thought that it might find some interest here.
( https://www.ebay.com/itm/255603797766 )

[Hydron]

I just had a look around ebay and found some reasonably priced triaxial cables.
While I do not know the exact quality of these cables I thought that it might find some interest here.
( https://www.ebay.com/itm/255603797766 )

These are twinax, not triax, despite the auction title. Would stay far away!

[Ole]

Thanks for the warning

[MrYakimovYA]

Dear shodan@micron, you have nice project!

Is USB interface fine to control equipment? GPIB is old, maybe LXI?

Does USB allow to build up of automated system that includes another equipment (DMMs, powers supplys and so on)? I mean almost any equimpent has GPIB/LXI and you GLIN has USB))

[MrYakimovYA]

Ok, USB is good) I'm also developing my highly precision and highly unabtanium device (picoammeter/voltage source). And now I'm thinking about connection interface. Ethernet based on W5500 (Aliexpress have low prices again) looks very very very attractive.
It has:
1. Built-in galvanic isolation.
2. Standard connectors and network structure.
3. Allows to have console (one socket) and up to several data stream (the other sockets).
4. May be the LXI can be added (I don't know about license).
5. W5500 (Wizner) give you hardare TCP/IP stack.

Don't you thing about such solution?)

[Kleinstein]

My completely illegal's, sanctioned refs will be delivered soon... So... it's time to start thinking about new Low-Noise, Mirror-free reference concept...
Earlier version use R+R mirror for making -7V reference for DAC, but i want try new mirror-free concept to do not amplified noise and instability.

Using separate references for the positive and negative side is a 2 sided thing. It avoids the inverter, but it also means that at zero output there is noise from both references and not the usually lower noise of the inverter.  For most uses I would consider the inverter the smaller evil.  One could still build the reference in a way to relatively easy switch between the 2 separate references and a more inverter like version:  the inverter like version would have the point between the 2 references not connected to AGND, but driven by an OP-amp to force the average of the 2 outputs to be at AGND. Functionally this is essentially the same as the inverter version, just a little more symetric.

With the extra buffers for the references, one could consider some easy to add RC fitlering for the references.

The reference amplification to provide the reference current should not use AZ OP-amps like the OPA189. There is no real need for them and they can do more harm than good from EMI and possible beating. So I would prefer something like OP07 or a more modern (and lower power) OPA202. In theroy even an LM358/RC4552 could do.

[Kleinstein]

The DAC kind of uses resistors to set the ouput. So one has that kind of resistor related drift anyway.  For the resistors it is not just tempco but there can be other effects like humitidy in the case and mechanical stress through the PCB. The humidity part can lead to rather long time constants to stabilitze.

It is a good question if the resistors are more stable than the references. At least for the noise part the resistors are usually better, but with ovenized references this is not so sure for the temperature drift.
Another possible alternative would be a charge pump to do the inversion, though this also comes with some problems in the form of EMI and switching transients.

[Kleinstein]

U3 and U8 seem to be overkill, but there is no need to remove them alltogether. One can go one step down and use simper OPs for them instead. Even an LM358 should be somewhat more stable than the LM317.
If overkill comes at the price of 2 relatively simple OP amps (LM358, op07,...) this is still moderate.
Depending on the layout there could be a little more additional resistance than the 25 mohms.

I don't see a real need for U6 and U7. The ground currents should compensate quite well and any residual current would be relatively stable. For thermal reasons it would make sense to have the reference close together anyway. So not very much extra resistance for the traces needed.

I would consider to have to option for one OP-amp in combination with 2 resistors to enforce symmetry.  So one could either use a direct link to ground or that one OP-amp and 2 precision resistors to set symmetry. So the same PCB could be used for either case with 1 bridge  or the optional OP-amp+ resistors populated.

It may not be needed, but for peace of mind it may be good to have simple RC fitlering between the references and U2 / U5, to isolate the reference from possible current spikes.

[Kleinstein]

Especially with the LCC8 version it would make sense to have the 2 reference chips rather close together, to minimize thermal loss. It is less important with the to46 version, but it still does not hurt.
So I don't see a large problem with thermal EMF from directly connecting the 2 references together. The thermal situation is a bit asymmetric, but ideally there should be some thermal isolation / cut-out around the references (can be both together). Much of the heat flow would be through the traces and it would be not so difficult to make the ground trace correspondingly thicker than the ones for the output sides.

For the thermal EMF the 95°C are not that relevant. It is still only the variations from the outside temperature that matter. Any constant thermal EMF part can be seen as part of the reference voltages. Thermal EMF from a fixed temperature setting was actually one of the very early stable (and able to provide significant current) voltage sources.
I don't see a problem having the reference close (a few mm) togther, possibly nearly touching at the no 8 pins (no need to solder pins 8 and 4).

The OPA170 is not the best choice, but should still be more stable than the LM317/337. Good symmetry would help with ground current if the center is directly connected to ground.
Having both options on the PCB is probably a good idea.

What is the idea behind the diode D1. If at all I would expect a need for protection with the negative side to make sure that the heater it the most negative. The may warrant a schottkey diode from GND to -15 V to make sure the -15 V does not go much wrong polatity during start up.

[Kleinstein]

With a classical 50 Hz transformer there is little need for a common mode choke at the input. It does not hurt, but no real need. The filtering shown is more like typical for a SMPS.

I see little sense for the extra resistors in series to C1 and C2. A small series resistor (could be in the form of a fuse) in sereis with the transformer secondary would be more useful. It could slow down mains transients and reduce the current peaks to the rather large filter capacitors.
The combination of 2200 µF filter caps and tiny case LM317 regulators is a little strange.
Just in case there is a problem with the negative supply, it would help to have a reverse schottky diode at the negative supply to preven the heater negative side to go much more positive than the reference part.

C6 and C16 are with a rather small form factor. Especially small form factor X7R capacitors loose quite a bit of there capacitance under a DC  bias. So I would suggest adding a 2nd footprint in parallel to have the option to add more capacitance. The 1 µF is in my view more on the low side to start with. 2 x 1 µF should be no problem.

The heated references would not really like a large ground area. The more normal use is with a little extra thermal insulation, like cut outs.
With the SMD case the soldering would cause some stress, that may reverse some of the burn in effect. So if possible I would consider doing the burn in with the final reference PCB.
If build as a pure burn in board, it would make sense to also have a socket for the TO46 version.

[Kleinstein]

The copper area around the reference causes lower thermal resistance and thus more required heater power. With the socket for some added thermal resistance this may still work, but is not ideal.

With the currently slighty difficult situation to get chips I would not exclude a 2nd case option. I think there should be enough PCB space.

It may be a reasonable idea to have a conncetor for the power - so the part could also be used as just a +-15 V supply if not in use for burn in.

[dcarolan]

Quick question. Where does one get a triaxe shorting cap

Thanks

Don

[MiDi]

If triax shorting plugs exist, so far I haven't seen any.
You can DIY one with a TRB connector.

[MegaVolt]

Keithley sells them as a set with a connector 7078-TRX-TBC:
https://il.farnell.com/keithley/7078-trx-tbc/3-lug-female-triax-bulkhd-conn/dp/2672714
https://www.ebay.com/itm/323974010369

Only a cap Trompeter TAI 14949:
https://www.ebay.com/itm/265814239428

[alm]

That looks like a dust cap to me, which only contacts the outer shields and shields the inner conductors from both dust and external fields. Not a short that would actually short the inner conductor to the other conductors.

[Kleinstein]

There is not really an advantage from using capacitive feedback, especially not with high gain. In most aspects a more normal, non inverting, low noise amplifier with resistor for the gain settting is preferable: usually less drift. No need for a Gohms resistor and without the 12 K resistor in series there is less noise.

I would consider this more like a curiosity, but not really a useful circuit. The good performance is from the high end OP-amps, not the dominantly capacitive feedback.

[Kleinstein]

The large capacitor is only needed with BJT based OPs. With an OPA140 or similar a relatively moderate 1-10 µF can do the job. If one gives it time for settling a MKS type can do.
A not so well known trick is to have the lower frequency limit set in a separate later stage and not directly at the input. This helps to suppress the noise of the resistor to ground.

The amplifier part is a relatively small project, so why not have one extra version. I currently don't see the advantage of the version with capacitor feedback, but there may be one. The characteristics is a little different. It may be usefull in the more differentiator mode.

For the high ohms feedback resistor switching would likely be a bit tricky due to extra leakage. For just the bandwidth one could have to option to extend the frequency range to down lower with a T circuit. Switching for this would be on the lower impedance part and not critical. I don't think the resistor noise would be that critical, as it is more on the ouput side.

[bobAk]

You are trying to prove your approach to building an amplifier using our words and wishes, under your desires and restrictions that you have imposed on yourself.  You live in Russia and it is easy for you to get first-class parts that will easily fit the amplifier into a 100x100x40 mm box with batteries and a weight of 150-250 grams.

[bobAk]

You do not understand that this is a description of the size and opportunities? Your experience?  Okay, I agree.  But then again, why was it necessary to build a deliberately meaningless justification misleading less experienced people?) think should be more professional.