Want to design a Source Measure Unit(SMU), does anyone have any insights into those?

Hello.

While I went through designing a DC-DC converter(not done yet) I became aware of SMUs or Source Measure Units and I immediately realized that I want one of those(minimum 2 channels), but the price tag of such a device is too prohibitive for me to be able to get one.

But I don't see a reason why I wouldn't be able to design my own, and I am somewhat surprised to find that there are pretty much no such design on the web.
The commercial SMUs seems to be in most cases real marvels of engineering with ranges that can span down to some 10's of femtoAmps, which I where told by someone on a blog or something is equivalent to 1 electron per 16uS, which sound insane.

Of course I have no such illusions as to think that I can accomplish anything near that, but then again I don't see why I would need nor do I see any point for me to have a unit with such insane resolution(since I feel it to be insane resolution obviously I have no need for it), if I had such a device I could surely find some cool thing to do with it but sine the use that sparked of my interest in SMUs is to characterize my DC-DC converter design or to analyse it's output response(I am not quite sure if a SMU can do both of those things or only one of them), so given the fact that it wouldn't have to be very fancy I think it is a very cool project to implement.

It would involve:
1. A control unit based on a microcontroller(µC).
2. A Matlab script that establishes communication with the µC through USB(from Matlab it looks like a ordinary COM port but from the µC it will look like an USB host through the µC's USB transceiver)
3. An switch mode power supply to convert a single positive input voltage into both positive and negative outputs(I haven't yet decided on what kind of voltages I what but I think at least ±15 or ±20V).
4. Followed by a linear regulator stage to clean up the supply rails.
5. A analog output stage that interfaces ADCs to measure both channels output voltages/currents and DACs which will steer the analog output power components to output a variety of waveforms(including steady DC voltages, sine-waves, square-waves and PWL derived custom waveforms) while being able to supply high currents(high being at least 1A). This stage would also contain a sort of large number of switches that is managing the output polarity, whether the constant current feedback loop is used or the constant voltage feedback loop is used, the range settings, and probably some more stuff that i can't think of right now.

The analog output stage is the biggest mystery for me at this point, although I have been reading and trying to figure out some patents regarding SMU circuits but that has not revealed anything conclusive yet but I am hoping that I will be able to derive a basic structure for the output stage from those patents.

As to the output capabilities, I haven't decided on what output current limit I want but I suppose I want between 1A and 3A which is really just a random number... and resolution will also have to be decide upon, as to the voltage I think I would like ±20V at the output terminals with a resolution that is the same as for the current.

There are surely a lot of compromises to be made, usually I feel that I get my self into trouble due to a desire for higher resolution even though the desire for that is disconnected from the actual application but in light of this project I feel that I will have no problem in making compromises which is very positive. Although I will not get away with using the µC's internal data converters, actually using an XMEGA A1U µC could work since it have 2 * 12bit 2MSPS ADCs and 2 * 12bit 2MSPS DACs but even though I don't remember why I don't want to use them right now I believe there are a good reason why to use external data converters in a design like this, it might have to do with maintaining isolation between the 2 channels of the SMU.

One concern I have though is the speed of the µC required to realize the sampling frequency needed to be able to properly work with a DC-DC converter which is switching at 100kHz, could someone give me some kind of estimate for how fast a sampling frequency you think I will need to be able to properly make measurements on a DC-DC converters who's internal switching frequency is 100kHz?

I don't know where I got the following idea from but I read some blog about some SMU circuit and the blogger wrote that he needed a µC with a 100MHz clock in order to realize a 100kHz sampling frequency, which sounds very weird to me, if I want a 1MSPS system then surely a µC running with a 32MHz clock must be perfectly sufficient?

Any and all suggestions/opinions/criticisms are very appreciated.

I put this thread in the analog section because I am going to focus quite a lot on achieving a low-noise output and in that aspect this is very much a analog design issue.

Regards

Oh, here is a quick suggestion for how such a device could be constructed, though I have to admit there are some possible flaws and not so elegant solutions. Nevertheless it would be functional and very flexible.

If you look at "big iron" ATE or bench / rack systems
like PXI, LXI, VXI you can get a lot of ideas about
what is available. Designing good test equipment is
not an easy thing, you have to respect the folks
who design the gear that the rest of us use to test
our own designs. Often really leading edge components
at the front end.

All the ATE I ever used had fixed range/resolution and
the test engineers would have to set the range to get
what kind of accuracy we needed.

Resources for high frequency measurement may not
be at all the same as ones you'd use for precision
DC. Which is fine because you hardly ever care about
both at once, or even on the same pin.

Your figure looks pretty elaborate. I recommend
you leave the power supplies out of it, consider
them a thing to buy rather than invent. When I
think of this kind of stuff (and we may have read
the same blog) I think more along the lines of a
power op amp with sense resistors inside the loop
that can be measured (for ATE you would probably
have some select-relays to get tolerable range)
for current, presuming your plan is to force voltage
and measure current. ATE SMUs let you do either
FV/MI or FI/MV, so an op-amp-y approach would
need two feedback modes. In either case a DAC
to force (w/ feedback options for range & mode)
and an ADC (probably pair, or a mux) for pin voltage
or current sense.

Another Good Thing that SMUs tend to offer is a
compliance limit for output V, I. Can save the DUT
from mis-insertion, or from debug learn-from-error.

Again in the interest of Getting It Done At All, this
might be left for round 2. But it probably wants
some architectural "hooks", perhaps it acts via the
"power op amp" feedback loop as a limit function.

Even if the converter is chopping at 100kHz,
what about it do you think you need to measure
with a SMU, and to what accuracy? The ramp,
that's kind of a don't-care for output accuracy.
Maybe overshoot and settling time, but that is
decades down from switching frequency. Maybe
switch-node VOH and VOL (for criticizing the
efficiency, conduction losses) but that really
just wants sampled at a good place, and a more
leisurely digitization can be done, and this is not
an attribute that really needs high accuracy.

Proper expectations are key here. And keep in
mind just what is "good enough". Better to make
one that works at all, and then refine piecewise,
than flounder and fail at something too grandiose
to ever get done.

Thank you for your input, I'm quite sure that I will miss to address some of your thoughts/tips in this post but to begin with you made me re-think my approach.

A lower output voltage/current feels easier so perhaps I should tailor the output values to the DC-DC converter I am very interested in using this SMU with, further more I might do well by using the µC's internal data converters and focus on the functionality of the SMU output stage which pretty much is the SMU part.

And because of that my resolution will be set and the sample rate will also be set, the result of which will most probably not meet the goal I have for the finished design but the goal doesn't need to be reached in this version.

Since the DC-DC converter analysis doesn't require isolation between the two channels I suggest that I don't bother with isolation in this first version, this way the point of the hole circuit is firstly to test out a functioning SMU circuit(output stage) in order to then later design a new "better" SMU based on the results of this first version. The second one could include things like isolation and higher voltages/currents and any other improvement which I would want.

If done like that there will be quite a decrease in the challenges presented by the design, that isn't to say that the challenges that this new approach will present is in any way easy, I think this is a project that is quite ambitious for someone like me.

So then to the SMU output stage, I am reading different patents about SMU circuits and I seem to have found two that I need to choose between.
One of them is focused on solving the issue of transients on the output occurring when the ranges is changed, which I can see why one would want to do.
The patents I am studying is:
US 6262670 B1 - Source measure unit having secondary feedback for eliminating transients during range changing.
US 7202676 B2 - Source measure circuit.

Also, I am not at all certain that Dick wrote about it due to anything I had written, but just in case it can appear that I don't respect the work behind commercial devices let me just say that I am in no way doubting the enormous skill, effort and high level of engineering that has gone into the commercial SMU's. And quite obviously mine will hardly be worth calling a SMU compared to the real deal.

I might be wrong but I feel as it would be a good idea to try and implement one of the patent circuits linked to above, although my research into SMU circuits and patents related to that isn't done and I might find others.

But I do wonder if it is misguided of me to focus on the output transient issue right of the bat?
An alternative is to not specifically prioritise that issue and focus more on the other aspects of the output stage and then if I discover down the road that transients is an issue then re-design to deal with that.
What do you think?

In any case I will look up some of the things Dick mentioned as well as keep looking at SMU circuits I can find.

But in order to make things easier to attack here is a list of problems that I know I have yet to solve or know of a way to sole them:

• USB communication
  I know how USB works but I find it really hard to implement it on the PC side(programming isn't my strong suit and although I can program µC I am bad at PC programming but if I am lucky this can be solved by Matlab)
• Range switching
  Will depend on the circuit I choose but one thing I don't seem to be able to avoid is switching the current sense resistor which will mean using a switch which can handle the maximum output current, and I was hoping to avoid mechanical relays.
• How to enable complex waveforms
  I'm not sure this is a problem, as long as I go with a op-amp solution(with external transistors to boost the output current capabilities).
• Power source
  I guess that it will have to be ether a off-line converter or a mains transformer followed by a switching converter. But the simplification of the design makes the power supply easier to solve and perhaps buy complete as Dick suggests.

I never thought this project would be easy, and I anticipated a large challenge and I was aware of the potential problem that I in this post is going to try to ask for help to solve.

This project have many stages that are based upon techniques derived from subjects that has nothing to do with each other aside from how they interact to accomplish the SMU task. Like there are a digital part that includes management of a USB 2.0 bus which I need to figure out, and there are also partially purely analog part which has nothing at all to do with digital circuits that I also need to figure out. I might appear to make claims that I am not sure is really true but I think my point will show anyway.

I think that post #2 in this thread posted by dick_freebird manages to portray the complexity and difficult of a project like this.
Though I think that this is a large challenge I don't think that I can really be aware of what challenges I have set for my self with this project, and I don't believe that I have the necessary knowledges needed to put this together. But that might be one of the main things that makes me to feel so motivated for doing this project since I at the same time feel that I am surely capable of getting through this project.

But I have begun focusing on the SMU output stage which is the actual SMU circuit and don't want to start working on other things before I have a proven concept for that circuit, but since there are no "how to build an SMU" information online I was wondering if there are other circuits that someone could point me towards that is sort of relevant for something lie an SMU circuit?

I am thinking that an SMU circuit has to have quite a lot in common with some other circuits that are more readily known about, for example:
Do you know about any other circuits that involves the sort of current or voltage range switching that could be applied to an SMU circuit that I can research for ideas?

I am just taking a stab in the dark here asking this, I don't know if it is possible to answer these questions or help me with this at all.

Hi David, your effort is intruiguing, did you find progresses in your project?
Look at this: https://www.youtube.com/watch?v=DJ2nzvX2gBc&t=440s
He has opened the Keithley 2400....

GGTX

Hello.

Yeah right, I begun watching that video but I stopped for some reason and forgot about it, I will continue watching after writing this post though.

And generally I haven't really gotten anywhere, I have found one single project online for a sort of SMU circuit but that project is to professional for me.
It is a really serious project for designing a voltage reference/voltage calibration/SMU device and it uses a LZT1000 as the basis for it's internal voltage reference and the person designing it is really professional and he has written that the project is meant to ... this and that... but also to show how a professional design is done.
But my project all in all may cost less than the guy doing that project spends on the reference voltage circuit, which shows how different projects these are.

I can't seem to find it but it has a thread on and the person behind it is a regular on the EEVblog forum.

I have thought about this quite a bit though and my next step is to start testing out different output stage circuit which contains the range switching circuits.

I have read a few patents that present SMU related circuits and I think I will try to design a circuit from one of those but it might take some time though.

But I will for sure return here with any updates when they occur, I have another thread about a high current booster which is meant to be used in this SMU to buffer a signal from an opamp which in turn are buffering a DAC which is the source for whatever signal is required.

The following is something rather similar and if you find this interesting you might be interested in a ADALM1000, which is a education tool from Analog Devices.
It features 2 SMU channels and it is almost what I would want except that the maximum output current are too low and the output voltage range is a little smaller than what I want as well. But I am tryingto study it's schematic to see if I can implement the circuits in my design or some of the circuits.

I hadn't realized that Dave showed and talked about schematics for the thing in that video, great stuff and thank you for mentioning it.
I will also take a look at some manuals for that device and others as Dave says that he found the schematic in there, I had planned to do so but it slipped my mind.

- - - Updated - - -

giustb, did you link that video in order to start playing at that particular time in the video?

Because I have started watching a tear-down of a SMU by Dave but did not get far, and since the video started in the middle I thought that it was the video I started looking at. But viewing the video you linked to again I realized that it isn't the same SMU tear-down that I had started looking at before.

Another lesson pointing out not to assume things I guess, I notice the link URL ending in 440s, I tested it now and I had no idea you could not only link to a video but link to a particular part of that video. That's very cool.

And I think that you pointed me in a very promising direction as the video in your link informed me about cascode amplifiers, I didn't know about cascode before. cascade I had read about but never cascode and I will be simulating and building some test circuit but from what I have read online cascode seems as a promising solution for my problem with the output stage.

Embarrassing that I almost missed what you where pointing at completely.