Repairing a current-to-voltage preamplifier

[LoveLaika]

I need help with this pre-amplifier. Somehow, the op-amp is broken, and I need to find a replacement for it. The problem is that it's enclosed, and I can't figure out what kind of op-amp it is without breaking it open. It's supposed to be part of a special microscope. I was wondering if anyone here had some experience with this kind of stuff? From what I can tell, these are my three options:

• Find a replacement module that's enclosed to swap it out for the old one. Does anyone know where I can find a replacement?
• Break it open and replace the internal op-amp inside.
• Break it open and recreate the module inside.

Options 1 is what I would prefer, but it's hard to find something like this. Options 2 and 3 seem more realistic, but I would have to break it open, and I would prefer not to do that in any case.

[jdragoset]

Please show what surrounding circuit this hybrid works in.
If it's not high frequency/precision, the entire circuit can be fabricated with an off-shelf op amp on a test pcb and plugged in.

[LoveLaika]

Thanks for your reply. Let me see if I can't get back to you on that if I can get the surrounding circuit.

With regards to your suggestion of fabricating an off-the-shelf op-amp, if we fabricate it ourselves, I'd try and break it open to see what's inside first. Going back to option 1 in my original post, can you buy modules like this as standalone parts? If I can buy it as a standalone part, I can determine whether or not it is cost effective to make my own replacement.

Also, are there pre-made off-shelf op-amps assembled on test PCBs like this? Where can I find them?

[duak]

With monolithic techology the way it is, there should be some off the shelf opamps that could deliver equal or better performance.  The problem will be to design and package a replacement.  I designed a PDA in the early 2000s and found Graeme's Photodiode Amplifiers book to be useful.  https://books.google.ca/books/about/Photodiode_Amplifiers_OP_AMP_Solutions.html?id=sHV0c5hBW4QC  I've seen other online material of similar quality here and there.  I think Graeme was with Burr-Brown that was later acquired by Texas Instruments.

I expect from the feedback resistor value that it's a FET input amplifier.  Since it's a hybrid, it might have a matched FET input pair followed by a monolithic op-amp.  The other parts are probably separate and attached to a substrate with conductors screened on.

To get a feeling for the age of the technology, do you know when the preamp and/or instrument was manufactured or when it was first produced?
 

[LoveLaika]

Duak, thanks for your reply. The equipment is part of a vacuum amplifier for a scanning tunneling microscope. Sorry, I don't believe I can show any more pictures of the equipment, but I can give some more info about the circuit as best as I can.

The input going into the pre-amplifier is a current signal which can range from picoamps to nanoamps. In other words, very small. The pre-amp serves to convert that current to a voltage equivalent signal. The device that is reading the output of the preamplifier is reading it at a rate of 500k samples per second. That's about all I can really say about the device.

[LoveLaika]

Duak, going back to my original post, if it was a matter of designing/packaging a replacement, I would probably just open up the broken one and examine it to see what components it used. You mentioned about off-the-shelf opamps. In this case, since we don't know what the op-amp is given the schematic, are there 'pre-made' op-amp modules that can function in the same way, as like a 'drop-in' replacement? We keep on talking about off-the-shelf op-amps, but that would require designing a board and getting the necessary components and whatnot. Eventually, we may have to go that route, but I thought that there were modules like this pre-assembled and can just be bought and used.

[Kleinstein]

There are only very few parts in the circuit. Besides the parts shown, they may be a capacitor at the supplies. The 2 diodes could be one part (e.g. BAV199 or similar).

I don't know of any ready made chips with the function, so it would need  the diodes, 2 resistors and the OP.
For just a single unit one may get away with air wiring or a hand carved PCB.

The 100 M resistor may be slightly tricky to find in a small form factor. The parasitic capacitance in parallel to the resistor may be important too, as it effects the bandwidth: to little and the circuit may oscillate, to much and the circuit gets slow. The values depends on the input capacitance and of the OP and the diodes. It may need a few tries to find the right value / OP with suitable speed and capacitance.

[LoveLaika]

Kleinstein, thank you for your analysis. I kind of figured that it may come to this, building my own, but I wanted to try and at least see if there was a way to find this in the wild somewhere. I found some op-amp evaluation boards, but just looking at them tells me that they are not what I want.


Thank you for your comments about the circuit. I never considered those factors before about the diode and parasitic capacitance, so that's worth looking into. I found some 100 Mohm resistors on Digikey, though they are a bit more pricy than other resistors. Regarding what you said about the parasitic capacitance being parallel to the resistance, are you referring to the 100 Mohm resistor? Perhaps an addition of a feedback capacitor in parallel with 100 Mohms will help?

[Kleinstein]

The critical capacitance is capacitance parallel to the 100 M. Here fractions of a pF can make a difference. So it would not be so much about a discrete part cap, but maybe an isolated wire that may be bend / shorted to right capacity.

It is normal that a 100 M resistor is more expensive and the choice is smaller.

[LoveLaika]

If it comes down to it, I'll have to break open the old broken pre-amp and see what I can work with. The schematics that I posted earlier, it seems to not be set in stone/fixed value, but it seems to 'allow' for a wider range of values. Perhaps the manufacturer intended for this to allow different varieties of op-amps to be used?

[Kleinstein]

The choice of OP will effect the noise and bandwidth and required capacity parallel to the 100 M. The 10 K resistor at the input should not be critical in value. One could try a simulation including estimated parasitic effects.

[PartialDischarge]

It's relatively easy to build such a transimpedance amplifier. I would use a OPA145 with no input diodes, two 0805 50Mohm resistors in series and no capacitance added next to them. The parasitic capacitance from the resistors to that tight metallic enclosure is enough and will give more than 30KHz in BW.

Total BOM cost is less than $5

[Kleinstein]

The OPA145 may be a good choice, at least it is low noise and moderately fast.
The capacitance to the tight case has a different effect than capacitance parallel to the 100 M, so it can not replace it. It depends on the details (e.g. input capacitance, BW of the OP) if one can get away with the parasitic capacitance, or may need a little (e.g. 0.1 - 1 pF range) more for stability.

[KeepItSimpleStupid]

So, with the spec of dust info, your probably dealing with an STM.

I put an EBIC system together.  Building the stage was the hard part.  I built a faraday cup, so that the beam current could be measured.

The 100 M resistor is the real pain.

Ib is a critical parameter and Ib varies with temperature which affects Vos.

See: https://www.analog.com/en/products/ada4530-1.html#product-quality

I used the OP41 for one design.  That one is obsolete.  https://www.analog.com/media/en/technical-documentation/obsolete-data-sheets/75599329OP41.pdf

The AD549 is a suggested replacement.

Usually there is a capacitor across the 100 M resistor .   Put the (-) input on a PTFE post.

[exe]

Why? I checked a few distributors, they are stocked... They are a bit more pricey ($0.1-3), but if you need only one or two, then it's not a problem.

[TimFox]

If the linearity of the current-to-voltage amplifier is critical, you should use physically long resistors to reduce the voltage co-efficient of resistance.  In my work, with output voltage up to 10V and linearity requirements of 0.1%, we found that the VCR of 50 megohm SMT resistors was important, and that 1206 parts were measurably better than 0805 parts.  We also had to choose vendors carefully, and many do not specify VCR.  At the physical level, for a given resistive material, the deviation from Ohm’s law depends on the voltage gradient along the resistance path (over any thermal effects due to self-heating).

[LoveLaika]

Thanks for your reply. If I'm dealing with input currents of picoamps to nanoamps, how does the input bias current affect the results? Ideal op-amps have 0 bias current, but in reality, the op-amp will need some input current. Essentially, the smaller the value, the better, but if the value and the input current coming in are on the same scale (like here, picoamps vs. picoamps), how much difference of a voltage will that make?

Looking at the comments and feedback I got from this post (and through Google searching other STM papers), assuming I'm going the route to build my own, the op-amps I'm considering are the following:

OPA196: https://www.ti.com/lit/ds/symlink/opa196.pdf?ts=1593537201092&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FOPA196
OPA145: https://www.ti.com/lit/ds/symlink/opa145.pdf?ts=1593540646198&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FOPA145
AD549: https://www.analog.com/media/en/technical-documentation/data-sheets/AD549.pdf

OPA196 and OPA145 have input bias currents of 20 pA and 2 pA respectively, but the AD549 has an input bias current of 60 to 250 femtoamps. Such a difference makes me think that this is more suitable for STM applications (not to mention the price tag).

[Kleinstein]

The bias current will give some offset to the measured current. Often the Bias current is highly temperature dependent. It depends on the application if this really is a problem. The other problem is that input current noise also gets higher with more bias, though very low bias does not guarantee low noise. The bias current usually has the corresponding shot noise, but there can be compensation in the bias - so the noise level can be higher, but hardly lower.

Quite often the Johnson noise of the 100 M resistor will dominate anyway. So besides noise and bias the input capacitance can be parameter to look at: less capacitance gives higher speed / less capacitive FB needed.

[TimFox]

The Johnson noise current of the feedback resistor decreases with increasing resistance, so one should use as high a value as possible with low input currents.  The resistor's noise current adds (in quadrature) with the amplifier's input noise current and the amplifier's input noise voltage divided by the total resistance seen at the inverting input.

[LoveLaika]

@KeepItSimpleStupid Thank you for your reply. Yes, I'm indeed working with an STM. Specifically, it's the Scienta Omicron VT STM. I don't know much about it other than the fact that this I-to-V op-amp works with the STM tip in vacuum and converts the current (It) to a voltage where it goes to another pre-amplifier that's outside of the vacuum.

Looking at the schematic I posted, TIP is the current from the STM tip (tunneling current), and {It} is the resulting voltage, but what I don't understand is Io. That is supposed to be a reference voltage, but I don't know what the usual values of it are, as it serves to reduce noise. I'm trying to run a basic SPICE simulation against an ideal op-amp to see how it is so I can get an idea of what the behavior will be, but what would the non-inverting terminal be referenced at?

[Kleinstein]

The non inverting terminal would be ground for the simulation. With the STM one may have the tip voltage (and thus a constant voltage) there and have the sample grounded.

For the simulation the capacitance at the input (tip + cable) and the OPs input can be important.

A larger resistor in feedback can give less noise current, but it would also slow down the amplifier.

[duak]

This is for an STM?  I used Google to look for "STM transresistance amplifier".  Here's some links off the first page:

https://aip.scitation.org/doi/full/10.1063/5.0011097

https://www.researchgate.net/profile/Emiliano_Pallecchi/publication/234889355_Development_of_an_ultralow_current_amplifier_for_scanning_tunneling_microscopy/links/0046351a838873e8f3000000/Development-of-an-ultralow-current-amplifier-for-scanning-tunneling-microscopy.pdf?origin=publication_detail

https://arxiv.org/pdf/physics/0610280.pdf

Choosing the parts shouldn't be that difficult.  I haven't worked with pico and femto Amp currents, however I expect the bigger problem will be to design and fabricate an assembly that will work.  I would look into design and construction techniques for electrometers.  One thing  done is to lift the summing node components off the circuit board and support them with the minimum number of PTFE standoffs.  Looking at the module in the image above, I see the input is separated from the other pins.  Distance is one way to reduce leakage currents.

I wonder if the original design was done as a hermetically sealed module not only to control leakage currents, but also to control outgassing under vacuum?  I worked on various projects that used multi-watt laser diodes and as we gained experience with building these things found that adhesive outgassing affected long term reliability because it deposited a contaminating layer on the optics.  This was at atmospheric pressure and just above the typical room temperature.  We went to low outgassing products and increased the curing and outgassing times.

Do you know if there are any organic based items in the microscope's vacuum chamber?

[magic]

Now that sounds like fun

30kHz means 33µs period. For such bandwidth, your feedback network's time constant must be no more than 5µs. With a 100MΩ resistor this is at most 0.05pF capacitance. That's 1/20th of a picofarad, less than some resistors have between their ends.

You probably don't want to increase that resistor to 1GΩ.

Maybe a different question for a change
How did you test the original amplifier? What really is wrong with it?
How will you test the replacement? You may want to verify its frequency response to avoid surprises.

[KeepItSimpleStupid]

The label looks like Io.  The output of the I-V converter is Generally Vbias-Irf.    The Vbias is your Io pin. It can be used as a zero adjust.

With I-V converters, Vos and Ib are probably the most important parameters.  Guess what, Ib likes to vary a lot with temperature.  Io would normally go to ground.

I think the reason why there isn't a capacitor across the 100M resistor is the speed that's required in your case.  I think you have to worry about stray capacitance.

Like I said earlier, I put the (-) pin on a Teflon standoff on the latest design I did for in-house use.  I primarily cared about AC performance (trapezoid, low frequency generated from a mechanical chopper) and I needed +-100 mA, +-10, +-1 and +-0.1mA full scale at 10V.  I added selectable 2 Terminal/4 terminal measurement capability.  I added guard.  I could do +-50 mA of suppression.

If suppression was used I was limited to biasing to +-5V, otherwise +-10V.  I added a >10V or <-10V RED/Green led that pulsed for 1s.

I was going to make that Io pin and drive it from an D/A converter,  but the plug was pulled (you spent enough time).  I ended up with a 40pA offset and I had no way of adjusting the zero.    The application was a front end to a lock-in amplifier used to do quantum efficiency (wavelength vs efficiency) measurements of solar cells. 

The idea was to add a resistor that would change the I-V converter to amplify the offset voltage and then use two equations to figure out what voltage to output on the offset terminal.

The gotcha was the IEEE-488 D/A converter I used did not output 0V for Zero.  I heard that IOTtech's version of the product used a relay for 0V.

There was a big gotcha for me.  The converter would not work for our calibration cells.  I panicked.  it turned out I had to isolate the capacitance with an LT1010 and OP-amp.

I did use 400 M-ohm resistors to ground for input loads for the differential amplifier across the device.  for low currents, you don't measure the voltage.

Since it was for low-current measurements, I used mercury wetted relays to switch the ranges.

keithley's low level measurement handbook is useful:  https://download.tek.com/document/LowLevelHandbook_7Ed.pdf  is useful.

[magic]

Like I said earlier, I put the (-) pin on a Teflon standoff on the latest design I did for in-house use.

For a one-off it would be sufficient to lift the IN- pin of the opamp and solder the feedback resistor vertically to contact the IN- node in the air. Similar trick was used in the picoammeter thread and it gives sub-pA level leakage essentially for free.