Picoammeter Design

[Kleinstein]

The protection resistor has one side at the op-amps input and this a high impedance in series. So there is voltage noise, but no extra current noise.

If one uses the alternative version with the protection directly in series with the input, the protection resistor is in series with the signal source / DUT. So there would be high noise if the DUT is low impedance, but in this case the whole TIA amplifier concept does not work - there is plenty of current noise from the source.

[Atomillo]

The protection resistor has one side at the op-amps input and this a high impedance in series. So there is voltage noise, but no extra current noise.

If one uses the alternative version with the protection directly in series with the input, the protection resistor is in series with the signal source / DUT. So there would be high noise if the DUT is low impedance, but in this case the whole TIA amplifier concept does not work - there is plenty of current noise from the source.

Oh I get it, there's no return for the current due to the high impedance of the op amp input so we only have to consider the noise voltage!

Thanks it makes sense now!

[Atomillo]

I've been thinking about this and I must confess I don't fully understand the reasoning regarding the "high impedance" node. After all, the input impedance of the op-amp can never be infinite.
Let's say we connect a resistor from the inverting input of the op-amp to ground (Rtest in the attached schematic). Now the impedance at this node is no longer theoretically infinite. From which value would we be able to neglect the current noise of Rp? And why?

[Atomillo]

Sorry for the doble post but I think I figured it out! Turns out actually drawing the schematic instead of just thinking about it helps a great deal to see this stuff.

Rtest and Rp forms a voltage divider and this sets the voltage at the non-inverting node of the op-amp (Vx = Rtest/Rtest+Rp * en). Since no current flows into the op-amp, the actual input refered current is then Vx/Rtest. As Rtest grows, Vx diminishes. Eventually, we can consider it is zero and thus no contribution to input current.

Also, as Vx approaches zero, we also find that the voltage noise at the input is that of the resistor, also matching what we were expecting.

[magic]

AFAIK you can assume that Johnson noise is an AC current source appearing in parallel with the resistor, resulting from thermal motion of electrons.
If the current has no other path to complete the loop, it flows through the resistor and becomes an AC voltage in series with the resistor.
If there are other circuits in parallel with the resistor, it splits proportionally to their admittance.

[aleruggeri87]

A notice that might be of interest to readers of this thread:

Today I sorted the available opamps on the TI website by their input bias current, and the one with the lowest was no longer the "well-known" LMP7721, but the OPA928:
https://www.ti.com/lit/ds/symlink/opa928.pdf

Still in preview, it has a pinout "à la ADA4530-1", specs seems to be in line or better.
Looking forward to read comments and reviews from the pros 

Best,
Alessandro.

[David Hess]

Today I sorted the available opamps on the TI website by their input bias current, and the one with the lowest was no longer the "well-known" LMP7721, but the OPA928:
https://www.ti.com/lit/ds/symlink/opa928.pdf

I am confused about the absolute maximum ratings on page 4.  Is it a 16 volt part or a 40 volt part?

[SilverSolder]

Today I sorted the available opamps on the TI website by their input bias current, and the one with the lowest was no longer the "well-known" LMP7721, but the OPA928:
https://www.ti.com/lit/ds/symlink/opa928.pdf

I am confused about the absolute maximum ratings on page 4.  Is it a 16 volt part or a 40 volt part?

Recommended 16V, with absolute max 40V.   Maybe some specification or other becomes untenable over 16V,  but the device still survives and kind of works up to 40V?

[Alex Nikitin]

Today I sorted the available opamps on the TI website by their input bias current, and the one with the lowest was no longer the "well-known" LMP7721, but the OPA928:
https://www.ti.com/lit/ds/symlink/opa928.pdf

I am confused about the absolute maximum ratings on page 4.  Is it a 16 volt part or a 40 volt part?

Recommended 16V, with absolute max 40V.   Maybe some specification or other becomes untenable over 16V,  but the device still survives and kind of works up to 40V?

No, in the text it is clearly stated 16V max, 40V in the maximum ratings is an obvious error.

Cheers

Alex

[David Hess]

I know from experience that TI's was not kidding about the 18 volt absolute maximum rating of their old LinCMOS stuff.  At some point I got into the habit of adding 16 volt TVS diodes across the supply to protect against spikes.

[iMo]

Guys, let me kindly ask experts here - I've found in my junkbox an LMC662 SMD still soldered into a pcb and a "500000 Megaohm VICTOREEN" resistor in glass, my bet it means 500GOhm
While considering every hint in this thread - it is even feasible to try with it?
Like an desoldered SMD 662, cleaned up in IPA, its input bent off the pcb, that resistor with a couple of pF gimmic capacitor (made of enameled wire, for example) in parallel, all built on teflon standoffs (or dead bug in air).
Asking before spending too much time with elaborating an apparent nonsense

PS: below a scrap device with that resistor on it - the resistor is wired at the input of the HDIG1030 transistor, perhaps an ion detector or something like that..

PPS: I've been trying to read the pcb and it seems it is a TIA amplifier with the resistor in the feedback, the white block looks like 100J capacitor in parallel with the 500G resistor, and together with the uA776 it creates an amplifier with the socketed HDIG1030 at the input, basically the same as the PAM here.. Hmmm, I have to reverse engineer it..

PPS: HDIG1030 - PMOS ENH (3N164 eq. ??)

[iMo]

FYI - here is the schematics of the probe I quickly draw (it had something with probing ions in a chamber many decades back, afaik). I think they adjusted the offset from outside via the OFFSET input..
Interesting the RC constant of the FB (500G || 100pF, if I read the capacitor value properly).

Nope: the capacitor is Vitramon 10pF/500V, Porcelain

[magic]

It should work, I suppose. With such high resistance you may find you don't even need a capacitor for stability, unless signal source capacitance is really high.

This thing probably has a fraction of pF in itself, enough to slow the TIA down to a few Hz.

[zrq]

I have two of such Victoreen glass sealed resistors salvaged from old vacuum instruments, both of them are pretty inaccurate (up to -10% off) today and exhibit quite significant voltage coefficient. For TIA feedback with limited dynamic range it maybe fine, through.

[David Hess]

A t-network can replace the high valued resistor in a transimpedance amplifier at the cost of increased noise, offset, and drift.

[Gyro]

Personally, I think it would be a shame to strip that thing for its feedback resistor. I know it would be easy to remove but that assembly is nicely made, complete with a low leakage relay etc. I don't know what the gate leakage spec is for the HDIG1030, if intact, but it must be very low to make a 500G resistor worthwhile (btw, a quick forum search show that this mosfet is used in the Keithley 155 Null detector / Microvoltmeter). The other problem is that even the 2fA typical leakage of the LMC662 is probably going to cause a significant offset with a 500G feedback resistor. This might take some a fair amout of nulling out. You're going to end up with a fairly long time constant when you take leakage capacitances into acount [Edit: maybe not], which might be inconvenient for measurements - not to mention inconveniently high input sensitivity.

Your choice of course, but 1G resistors aren't hard to purchase these days, so don't pose a particular problem that would require a salvaged part. Likewise an LMC662, it is probably better to start by IPA cleaning an already clean fresh (preferably DIP) package than one that has been desoldered a board.

[iMo]

Before I remove the resistor from that probe - it could be the probe still works, thinking how to wire it such it shows some signs of life.. Unless the input fet is dead already (50/50) it may show something, there is nothing special there except the fet. It is definitely not clean enough anymore so I would not expect miracles.. 
The sim below shows it may work with a 10t 10k pot and a bias resistor.
With the relay ON I set zero (with the pot) at the output and with relay OFF I should get then +/-10V out with +/-20pA in.. Will try tomorrow.. 

[iMo]

I found a doc from Hughes Aerospace on some tests they did for NASA in 1973/74 - the HDIG1030 chip shot there.. (long before Noopy..)

[Gyro]

If it's NASA, that maximum Gate Leakage spec is probably over an extended temperature range. I would expect the typical gate leakage at ambient temperature to be a few orders of magnitude lower.

P.S. Looking at the Keithley 155 threads I mentioned, they used selected parts. I wouldn't be surprised if the part you have there is selected for low leakage too.

[iMo]

Yep, the transistor sits in a socket, thus they hopefully made a preselection, only the gate is wired to the resistor in free air. I've seen somewhere it got 20fA typically, but I cannot find the info anymore.
Btw - here is the Hughes' report for NASA..

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwi3h4TEqcj_AhVtxQIHHcCJCyAQFnoECA8QAQ&url=https%3A%2F%2Fntrs.nasa.gov%2Fapi%2Fcitations%2F19750002239%2Fdownloads%2F19750002239.pdf&usg=AOvVaw1uB3o3vRhWZqYLJ27LeBRi&opi=89978449

[iMo]

OK, I've invested some effort and wired the unknown probe as below..
As usual the most troubles made me the reed relay.. I almost resigned with my attempt to run this 40+ years old device, until I recognized the relay's contacts are switched "on" (shorted) when the coil is not powered. Sure, this was the great idea - as thanks the input shorted with the output (actually via 39k to opamp's output) helped the HDIG1030 survive rough handling and the transistor still WORKS!

I added inside the module an 0.47u capacitor (tantalum drop) across the opamp's power and a diode at the relay coil.

With the relay not powered (the contacts shorted) I can set zero (50k 10t) at the opamp's output (OUT1), the zero then fluctuates slowly some 500uV up and down in time. The voltage at the HDIG1030's source (IN-) is +2.855V with the 13k source resistor and 15V Vcc, when zeroed.

With the relay powered (the contacts disconnected) the OUT1 voltage settles from initial jump after about 20secs and fluctuates then some 80mV peak-peak from +300mV to +380mV in "quiet periods" (from 1.5m distance, while watching it I am interacting with the electrons at the input - the wave functions collapse, when not watching it could be much less  ), and much more based how you move yourself around. While messing in vicinity of the input it sometimes jumps into volts level, thus it seems the transistor works.

I have to investigate the role of the R3=39.2k, on the pcb there is written an old remark "R75=39.19" and the board is populated with the "39.2k" one.. Any idea?? 

PS: ok, the shorted part of the FB is outside the 39k, thus the 39k resistor creates the +300mV offset, it seems.

PPS: Correction - the capacitor is 10pF/500V Porcelain Vitramon

[Gyro]

Looks very promising. That n/c relay has clearly saved the day (and the gate). 

From experience with my Picoammeter (1G feedback resistor), you won't be able to get a true stable zero unless the outer case is fitted and the input needle is fully screened (not shorted), say, with some foil formed as a 'bulb' and held with an elastic band around the end of the case. It would be interesting to see how stable the zero is under those conditions.

Regarding the 39k2 resistor, I think that is probably a fine gain trim to compensate the actual, as opposed to marked, value of the 500G resistor - 39k2 is close enough to 39k19 (maybe its measured value is 39k19). It will be affected by (any) loading on the offset pin anyway.

[mawyatt]

Wow, a uA776

Haven't seen those used since 70s when we used them for a number of special applications.

Best,

[iMo]

..
Regarding the 39k2 resistor, I think that is probably a fine gain trim to compensate the actual, as opposed to marked, value of the 500G resistor - 39k2 is close enough to 39k19 (maybe its measured value is 39k19). It will be affected by (any) loading on the offset pin anyway.

My feeling is the in/out marked "OFFSET" in my schematics is an input for compensating the zero, like with a DAC. The value 39k was somehow important for them to get that default offset (after setting the "zero" with the voltage at IN+ input) into a certain DAC range.
The 500G resistor's and fet/opamp's TC may then fit into the "DAC offset range", like from 0.0 to 1.0V DAC's output (after the initial coarse setting by the voltage at IN+).
The actual "500G value" spread and long term drift could be by many orders of magnitude higher than the 39k value, my guess.. (ie with TC=10ppm/C, what is an absolutely unrealistic value, the 500G will change by 5Mohm with dT=1C).

[magic]

You will see nothing but noise if this thing isn't shielded from static fields. Get a strip of sheet metal and wrap it around the probe, or try to find a matching pipe that would fit around it. It all should be grounded and I'm pretty sure that the metal caps at the ends are grounded, so just connect with them.

If the capacitor is really 100pF, the time constant is ridiculously long, almost a minute.