Author Topic: DIY-SMU Project  (Read 14039 times)

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Offline djerickson

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DIY-SMU Project
« on: December 29, 2020, 05:14:18 pm »
I'd like to introduce my DIY Source-Measure Unit SMU project. http://www.djerickson.com/diy_smu
Youtube intro: https://youtu.be/B26SW3N2zoA

[attach=1]

It seems fundamental to have an accurate source for a wide range of voltages and currents, with the ability to measure both.  Source and measure are basic requirement for electronics test and measurement tasks and many scientific applications. This functionality should be available at a reasonable cost.

I worked on many DC and AC source / measure instruments for a number of industry leaders (Analogic Test and Measurement group, Teradyne DC Instruments group, Zoll and HP Medical) plus numerous startups and home projects. I always wanted to build a DIY Source Measure Unit: SMU.

Recognizing that this is a challenging project for DIY, now in retirement I finally have the time to develop the hardware, software, controls, and packaging. Fortunately the nerd stars are aligned for such a project. Precision 0.1% and better SMT resistors are readily available at low cost. Precision amplifiers, switches, ADCs and DACs are low cost and easy to apply. Digital isolators and DC-DC power supplies are small and readily available. Hand-built SMT is available and easy to DIY. Single chip CPUs and TFT LCDs with touch are powerful, low cost and DIY friendly.

I pored through old Keithley documents for their 220/240 series sources, and 236 and 237 SMUs and found their general architecture to be flexible and very capable. I suspect that most modern SMUs share the Keithley 236 general architecture.

So after a few years of planning and a year of detailed design, layout, and build, here is DIY-SMU.
   Voltage source and measure from tens of microvolts to +/- 150V in 3 ranges: +/- 1.5V, +/- 15V, +/- 150V
   Current source and measure from nA to .1 Amp in 6 ranges: 1uA to 100mA
   True 4-quadrant operation
   Source accuracy .01%
   Measure accuracy .005%
   Graphic LCD with touch screen
   Small, half-rack 2U package
   DIY friendly and low cost

It is not complete yet, and I plan to continue the project this winter. PC boards are built and working. An initial GUI is working.  It has a basic enclosure. My web page discusses the idea, requirements, design, and implementation. Check it out at www.djerickson.com/diy_smu

Thanks,
Dave Erickson

Offline fcb

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Re: DIY-SMU Project
« Reply #1 on: December 29, 2020, 05:37:20 pm »
Awesome!

The 236/237 architecture is a great place to start. Look forward to following this project.

We built a baby 2ch SMU (known internally as SMUSB), 1uA to 45mA, +/-12V. Went with a PC based front end rather than a display on the device.  My understanding is that some of the newer SMU's use software-in-the-loop to deal with some of the stability issues that crop up from time-to-time. Are you planning on guards?

TTi have a new SMU that's overdue.  So I suspect this will be a growing market.  I think Marcoreps was involved in project called OSMU?
https://electron.plus Power Analysers, VI Signature Testers, Voltage References, Picoammeters, Curve Tracers.
 
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Online helgel

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Re: DIY-SMU Project
« Reply #2 on: December 29, 2020, 06:27:27 pm »
Nice project !  Congratulations ! I have a similar project going myself (https://poormanssmu.wordpress.com). When I started that project I was surprised to find very few attempts on similar projects out there. Sounds like your previous background and now retirement is perfect for such a project. With a full time job as software developer I unfortunately have less time on my own the project than I would like to, but it's still work in progress. I'll be following your project !
 
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Offline prasimix

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Re: DIY-SMU Project
« Reply #3 on: December 30, 2020, 08:37:22 am »
Hi Dave, and thanks for sharing this project with us, looks great and very promising. About half rack enclosure (9.5"): you can check All Metal Parts from UK, e.g.: http://www.allmetalparts.co.uk/814-2u-95-inch-rack-mount-300mm-vented-enclosure-chassis-case-5055726240082.html

I would suggest putting a fan controller high on the TODO list. There is no reason that "tiny" fan that you're using is working all the time. The fact that many manufacturers are still saving on that component has no reason to be the case here :).

Good thing you realized that the Arduino Leonardo is not enough, even supported with extra processor on the Nextion (in charge of display). We had a similar thing: we started with Leonardo, then Mega2560 and Due (EEZ H24005 project) and finally finished on our own design based on STM32F7 with 8 MB (expandable to 32 MB) SDRAM with micro SDcard in the background for data logging, etc.

I would love to see a video in the future that covers the following:
1. What does output enable/disable look like, that there are no over-/under-shoots.
2. How it behaves with a slightly higher capacitive load

Keep up the great work, maybe a combination of your SMU and Helgel Poor man’s SMU could become a new module for our EEZ BB3 in the near future. In that case it would get all the support for SCPI via USB and Ethernet, GUI editor, MicroPython scripting, MQTT, NTP, Node-RED that we already have.
 
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Offline djerickson

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Re: DIY-SMU Project
« Reply #4 on: December 30, 2020, 02:17:25 pm »
Thanks for the feedback. Yay, a half rack 2U enclosure! Will order one right away.

I agree, fan speed control is a must, and will be on the new CPU board.

For the last month I battled a 300KHz noise issue, 50mV sine wave, thought it was oscillation. It drove me crazy! It only occurs when the Out- or Out+ is grounded.  Turns out it's the 3W DC-DC common mode noise. I had a common mode choke, but that made it worse. I tried 3 manufacturers DC-DCs so far. They all have common mode noise: Meanwell is worst, then CUI, Recom is best. More testing.....

The output dynamics are very good when changing the output current, voltage, or resistive loads, and when clamping.

I've done a bit of testing with capacitive loads. With 1000pf to 1uF it rings or overshoots a bit, and settles in a few mS. It's good with large caps. Tuning the loop may help.

I need to thoroughly test the dynamics vs. the output and remote-sense relays and will report. If the OFF/ON transients are ugly I may be able to fix in software with some simple sequencing.

Thanks!
Dave

 
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Offline prasimix

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Re: DIY-SMU Project
« Reply #5 on: December 30, 2020, 02:33:38 pm »
For the last month I battled a 300KHz noise issue, 50mV sine wave, thought it was oscillation. It drove me crazy! It only occurs when the Out- or Out+ is grounded.  Turns out it's the 3W DC-DC common mode noise. I had a common mode choke, but that made it worse. I tried 3 manufacturers DC-DCs so far. They all have common mode noise: Meanwell is worst, then CUI, Recom is best. More testing.....

Perhaps you should consider DC-DC converter built around SN6505 push-pull converter. It depends how much power you need on +/-15 V rails, probably not so much. I'm just start experimenting with it after giving up from Mean Well and TRACO. It looks like this (with 4x multiplier as suggested by Kleinstein):



I derived 5 power rails on the secondary side: +3.3 V for MCU/ADC, +/-5 V and +/-15 V for analog circuits.

 
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Online Kleinstein

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Re: DIY-SMU Project
« Reply #6 on: December 30, 2020, 05:31:17 pm »
The ready made DC/DC converters often have quite a lot of common mode injection. The smaller the transformers the closer the coupling and a higher voltage also makes things worse. 

I don't think the SN6505 is special, it just gives the chance to choose your own transformer. For a one off DIY unit one could use a relatively large core with some extra space to reduce the capacitance. A low voltage to start with also helps. For DIY winding I used 2 twisted wires for both halves of the dual winding. This should give relatively good symmetry and thus little voltage to drive the CM signal even without an extra shield.

CM injected signal is relatively difficult to filter, but usually a CM choke would be the way to go.  It needs a special type with 3 coupled inductors for a +-15 V output. So the CM choke on the primary would be easier.  There may be a resonance with the CM choke, if just at the wrong value / frequency.
 
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Offline djerickson

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Re: DIY-SMU Project
« Reply #7 on: December 31, 2020, 02:02:19 pm »
Thanks, Prasimix, appreciate the design idea.

DC-DCs and even AC transformers with low common-mode noise generally use multiple internal shields. Unfortunately my transformer design and build skills peaked with 1980 Ferroxcube potcores ;).
I suspect I can get it low enough by using enough of the good external stuff (low ESR ceramic and Electrolytic caps, Y-caps, beads, common-mode chokes...) plus finding the quietest off-the shelf DC-DCs, and careful measurements. I feel a test board coming on.

One of my project goals to be DIY-friendly, is to avoid custom transformers. I know of no off-the shelf transformers with shielding, other than medical grade AC ones. 

My personal pet peeve about switching power supplies is that common mode noise is not specified. They are often tested with their output common grounded, so common mode noise is minimized. The way I measure it is with a resistor (10-50 ohms) between input and output commons. Measure the voltage across the R with a wideband scope. This gives a decent indication of common mode noise as a current, good for comparative studies. 

Another quiet DC-DC design method is to use a HF sine wave or resonant DC-DC to eliminate having to clean up nS rise-time edges that switchers generate. But in all cases, shielded windings help.

Dave
 

Offline prasimix

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Re: DIY-SMU Project
« Reply #8 on: December 31, 2020, 02:38:54 pm »
I'm agree with your approach/attitude that a DIY friendly project should have as few custom components as possible (preferably none). I suggested this DC-DC converter because it uses an off-the-shelf transformer from WE: 750315371. A whole converter with LDO's don't use a lot of space: IC6 is SN6505, IC5 is +15 V LDO, IC7 for -15 V and IC4 for +3.3 V.


Offline alm

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Re: DIY-SMU Project
« Reply #9 on: January 01, 2021, 10:53:06 am »
Awesome start! Have you considered replacing the DC-to-DC converter for the analog circuit with a simple linear power supply from a small off-the-shelf 15-0-15V transformer? I realize that this is less neat, more expensive and bigger, but it would remove the concern of common-mode noise.
 

Offline djerickson

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Re: DIY-SMU Project
« Reply #10 on: January 01, 2021, 08:44:35 pm »
Thanks for the advice, Alm. Using an AC transformer might be a last resort.

I dug into the issue and also looked with more detail at the SN6505 and your DC-DC design, Prasimix. I like your design, nice use of a voltage multiplier.

My design with a CM choke feeding a Meanwell 3W DC-DC had 3 volts(!) of 300KHz common mode noise on the DC-DC input. I have a handful of different DC-DCs and tested 6, 8 and 7 pin SIP parts. Meanwell is worst for CM noise and doesn't like the CM choke.  Tracor and Recom are best. CUI is in the middle. I'll buy more to test.

I think I found a fundamental issue: In an SMU design, the Out- is the common of the power supply, driven by the +/- 150V supplies common. When you ground it through a scope, for example, the path back to chassis ground is through the power supply, Amplifier, the DC-DC's Y cap,  plus the AC transformer's inter-winding capacitance, and a Y cap that I add.  The common mode inductor's 1mH plus those caps makes an pretty ugly impedance.  Bypassing the CM choke reduced it to just capacitance, no big inductor. It works much better.

In a 'normal' isolated instrument, grounding the floating ground to chassis should quiet the common mode. With a CM choke in the power path, both sides of the LC resonant circuit formed by the Common mode choke plus any Y caps would be kinda grounded. But you could imagine some weird stuff happening to your circuit-under-test when you apply that big-old LC circuit to your DUT. Not sure how DMMs deal with this. Then add in the SMU's complexity... Yikes!

So for now I'm gonna stick with "C good , LC bad". Will try to simulate this. All this funky "grounding" hurts my brain.

Thanks,
Dave
 

Online Kleinstein

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Re: DIY-SMU Project
« Reply #11 on: January 01, 2021, 09:39:40 pm »
The purpose of the CM choke is to add impedance for common mode voltage. So having some 3 V at the DCDC converter input side is kind of showing that the CM mode choke is working. The only thing to avoid is to get some series resonance from the coupling capacitance and the CM choke. So one may have to use a different type CM choke, possibly even 2 in series (one for the high and one for the lower frequency range). As a simple fix some 10K in parallel to the CM mode choke may help to dampen a resonance and make it a more well behaved impedance.
The DC/DC converter should be without an internal Y cap - it does no work well with a CM choke. If at all the capacitor would be separate from before the CM-choke.

The plan shows some capacitive dropper to provide +-15 V - it can be nice, though unusual.
 
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Offline arlo_g

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Re: DIY-SMU Project
« Reply #12 on: January 04, 2021, 01:22:10 am »
Hi Dave,
Your SMU project looks great, and your progress to date is awesome.  I like your focus on keeping things DIY friendly. 

The three suggestions/comments below are in no way intended as criticism. 

The voltage sense buffer opamp inputs are protected by BAV199 diodes.  The  leakage specified by some manufacturers of these diodes might be on the high side for the 1uA range: had you considered using the base collector junctions of bipolar transistors for protection diodes, the way that Keithley was known to do?  2n3904's (and surface mount versions) are supposed to have quite low leakage, but this performance might not be guaranteed.

The photoMOS switch on the highest current range(s) will suffer from drain resistance that varies with temperature and impacts current measurement accuracy, and to a lesser extent the drain resistance of CMOS switches on lower ranges will impact accuracy too.  Had you considered adding a calibration path such as e.g. a relay from the output net (inboard of output switching relay) to a precision resistor load?   Something in the 100-200 Ohm range would give good readback voltage resolution to estimate the total shunt resistance in the 100mA and 10mA ranges.    I see now that switches U17.1 - U17.4 on your schematic neatly sidestep the current shunt switch resistance by connecting the instrumentation amp inputs across the shunt resistors.

I think that your choice to drop current ranges under 1uA to keep things manageable makes a lot of sense.  Take the following suggestion as an idle thought experiment:  could the leakage of CMOS switches be managed for lower current ranges by using a t-network of three switches with one of them driving the intersection of the three to guard potential when the other two are open?  I think that this would cut channel leakage currents from the switches down by a large factor as the switch to the output net would have ~0V across its channel. Of course this wouldn't do much to reduce leakage in ESD diodes internal to the switches, substrate diodes, package material (and gate leakage?  hopefully the switches are in a process with thick enough oxide that this isn't significant) etc.

« Last Edit: January 04, 2021, 10:53:50 pm by arlo_g »
 
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Online Kleinstein

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Re: DIY-SMU Project
« Reply #13 on: January 04, 2021, 08:32:02 am »
The leakage current for the transistor junctions is not guaranteed - similar to the BAV199: typically very low, but not tested to a very strict level. Chance are the typical leakage is better with the BAV199.  so there is nothing gained from going from one untested part to another. The often best bet for tested low leakage may be 2N4117-4119, but these are not really cheap. The cheap plastic version is usually not tested to a strict value either.
 
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Offline djerickson

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Re: DIY-SMU Project
« Reply #14 on: January 05, 2021, 06:10:43 pm »
BAV199's are pretty good for low leakage, particularly at low voltages. Where the leakage really matters, I try to run them at lower voltages, such as by bootstrapping to the op-amp output.  If I *have to* I can select them. If I remember correctly, Phillips, (now NXP) has slightly better specs. I'd be interested in seeing any data on low leakage alternatives such as bipolar transistors or FETs.

T-network for switches, what a good idea! I remember that trick from the good-old-days as a solution to high frequency leakage, but sure, why not DC leakage?

Dave

 

Offline MLNSLM

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Re: DIY-SMU Project
« Reply #15 on: January 05, 2021, 08:30:12 pm »
Hello David

I have currently been working on a simple SMU myself.
I'm having some problems with overshoot when current-clamping.
I have looked at your schematics, the ones of Keithley etc. and some others but just can't seem to get the grip of it.
Now that I've read that you've done some simulations in LTSpice, I am kindly asking you (as the chinese would say) to share them with me/us? Maybe I can figure something out then.

Cheers and Nice Work!
 

Offline djerickson

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Re: DIY-SMU Project
« Reply #16 on: January 07, 2021, 11:35:19 am »
Sure, I just added the PSPICE simulation .ASC (schematic) file to the .zip file at:
http://www.djerickson.com/diy_smu/files/DIY_SMU_Files.zip
It simulates constant voltage with current clamping. You'll need to change it for Constant current.
The amplifier and crossover models are pretty complete. The voltage - sense amplifier has its own floating power. I'm not sure how to simulate that so I used a behavioral model for it.
Let me know if the simulation doesn't work.
Good luck!
Dave
« Last Edit: January 07, 2021, 11:48:14 am by djerickson »
 

Offline macaba

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Re: DIY-SMU Project
« Reply #17 on: January 08, 2021, 03:59:03 pm »
This is looking fantastic so far, I look forwards to seeing more.

Meanwell is worst for CM noise and doesn't like the CM choke.  Tracor and Recom are best. CUI is in the middle.

I'd be interested to see some numbers alongside part numbers for these, even if it's just relative.

 

Offline djerickson

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Re: DIY-SMU Project
« Reply #18 on: January 09, 2021, 01:18:07 pm »
I ordered a handful of different DC-DCs and will test them when they arrive next week.
Dave
 

Offline jbb

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Re: DIY-SMU Project
« Reply #19 on: January 10, 2021, 11:28:24 pm »
Hi there

Glad you’ve brought this to the EEVblog forum. An SMU is one of those pieces of gear which is very useful but often too expensive. Having a DIY offering is great.

The crossover design is clearly very important. I was really pleased to see your crossover design because it’s really clear how it’s meant to operate and separates the control dynamics from the separate force and clamp circuits.

(I also really liked the output buffet/disconnect for the 2Q power supply on your site. Very nice approach.)

I see the +clamp and -clamp opamps have anti-windup diodes which should help the force -> clamp transition. Do you have any anti-windup on the force opamp to help with the clamp -> force transient? (I had a poke around in LTSpice and found that the basic ‘just add Back to back Zeners’ concept is a no go because their leakage current wrecks the DC accuracy.)
 
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Offline djerickson

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Re: DIY-SMU Project
« Reply #20 on: January 11, 2021, 02:14:52 pm »
Regarding the crossover: The original Keithley 236 crossover design is quite nice for 1990 technology. I discuss it on my web site. For greater detail, read (expired) Keithley Patent 5,039,934, a nice education on crossover design.   A big advantage of it is that it uses one integrator for both current and voltage, good to avoid integrator wind-up.  I talk about it at length. The precision, fast error op-amps before the integrator switch very quickly (microseconds) using a simple precision diode circuit.

I think the disadvantage of using a single integrator is that your voltage and current loops need to have similar loop gains in order to have good dynamic response on both.

While at Teradyne, I learned of a different but similar crossover. It used fast comparators and an R-S flip-flop to switch the error voltages into the integrator, and with hysteresis.  I wonder if they used this more complex approach to avoid the Keithley patent.

My 3 op-amp crossover approach assumes that DACs are cheaper (it uses 3 DACs  vs. 2) and provides fully independent control over the HI and LO clamp levels. I see this as a useful feature. I tested this circuit a few years ago on my PS-Load project (2-4 quadrant power supply) and was happy with its performance.

Thanks,
Dave
 

Online Kleinstein

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Re: DIY-SMU Project
« Reply #21 on: January 11, 2021, 02:53:04 pm »
The same loop gain is not the problem with using a single integrator. One can adjust the gain of the voltage and current channels before one combines them. The difficulty comes if the regulator is more than a simple integrator  - the ratio of an additional proportional would be fixed. Possible differential parts would likely be added before the signals are combined.
 

Offline arlo_g

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Re: DIY-SMU Project
« Reply #22 on: February 11, 2021, 02:18:50 am »
Here are a couple more ideas around the SMU design that might or might not be useful.

1. Guarded cabling: high performance SMU's from the big test equipment players use triaxial connectors to maintain a coaxial guard around output signals and a coaxial shield at safety ground potential. The Trompeter TRB style connectors used by Keithley and HP/Agilent/Keysight are quite expensive. Coaxial guarded connectors and cabling would provide much of the low current performance, but exposed outer conductors on connectors would be a serious safety concern when driven to way beyond the usual 42Vdc safety limit by a SMU.

FAKRA connectors, a plastic shrouded connector family based on SMB coax connectors, might be an inexpensive and safe choice for guarded SMU cabling. These mostly seem to have gold plated center conductors, but nickel or worse plating on the outer conductors so they might only be as good as BNC connectors at best in terms of thermocouples on the guard connection. I have yet to handle a FAKRA connector though, so it could be that the plastic shrouds are really unpleasant in use, and the 100 connection cycles claimed by some manufacturers might be optimistic. Anyways, all of this guarding stuff probably only matters for currents <<1nA.

2. Class G variation on the output stage. Keithley's 237 and more recent SMU's use class G topologies in their output power amplifiers to reduce power dissipation (and also to support higher current at low voltage ranges).  There is a supply connection that is already available in your SMU design, Dave, that allows the maximum amplifier power dissipation to be halved: the output common.  I sketched out an output amp stage in LTspice that demonstrates the idea, see schematic below. Ground symbols in that highly simplified testbench represent the output common of the real SMU. For the simulated case of -100mA constant current load, M4 dissipates the bulk of the power for -150V < Vout < 17V and M3 takes over from 17V < Vout <150V.  M1 and M5 behave similarly for an opposite sign of load current.

Note that I have only run DC sweeps on this circuit, and a lot of component choices are far from careful.  There are no gate protection diodes, no caps for dynamic gate drive, the gate biasing resistor values are suspiciously low and my transistor selection consisted of grabbing the first FET's and BJT's that I saw in the LTSpice library that had roughly the right max Vds/Vceo.

Connecting the amplifier "supply" to output common through D3 and D10 is weird, but those diodes are only conducting in quadrants where the output amp is serving as an active load. All of this class-G business might not be worth the trouble for a single channel SMU, but it could make it easier to support two channels in a box.
 

Offline jbb

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Re: DIY-SMU Project
« Reply #23 on: February 11, 2021, 04:08:58 am »
Isn’t fancy guarded cabling with triax connectors used for really low current measurement? As I recall, Dave decided not to go hunting nA for cost reasons...

On class G: that would cut down dissipation... I think some commercial SMUs use like +-30V rails in there to get a low-V higher-I range too.
 

Offline wizard69

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Re: DIY-SMU Project
« Reply #24 on: February 13, 2021, 03:52:03 am »
I'd like to introduce my DIY Source-Measure Unit SMU project. http://www.djerickson.com/diy_smu
Youtube intro: https://youtu.be/B26SW3N2zoA

[attach=1]

It seems fundamental to have an accurate source for a wide range of voltages and currents, with the ability to measure both.  Source and measure are basic requirement for electronics test and measurement tasks and many scientific applications. This functionality should be available at a reasonable cost.
I'm with you there.    I would have been happy with a voltage only SMU and frankly one that does ±24VDC would have been fine.   In fact I'd love to see a precision voltage SMU that effectively is a USB dongle.   I'm not even sure that sort of functionality can even be squeezed into a dongle.
Quote
I worked on many DC and AC source / measure instruments for a number of industry leaders (Analogic Test and Measurement group, Teradyne DC Instruments group, Zoll and HP Medical) plus numerous startups and home projects. I always wanted to build a DIY Source Measure Unit: SMU.
Looks like you have an incredible work history.
Quote
Recognizing that this is a challenging project for DIY, now in retirement I finally have the time to develop the hardware, software, controls, and packaging. Fortunately the nerd stars are aligned for such a project. Precision 0.1% and better SMT resistors are readily available at low cost. Precision amplifiers, switches, ADCs and DACs are low cost and easy to apply. Digital isolators and DC-DC power supplies are small and readily available. Hand-built SMT is available and easy to DIY. Single chip CPUs and TFT LCDs with touch are powerful, low cost and DIY friendly.
Plus somebody in China will rip off the design, build it with cheap or slave labor and sell it for half what we can buy the parts for.   

By the way I'm not a big fan of Touch screens on instrumentation.   Just a personal thing.
Quote
I pored through old Keithley documents for their 220/240 series sources, and 236 and 237 SMUs and found their general architecture to be flexible and very capable. I suspect that most modern SMUs share the Keithley 236 general architecture.

So after a few years of planning and a year of detailed design, layout, and build, here is DIY-SMU.
   Voltage source and measure from tens of microvolts to +/- 150V in 3 ranges: +/- 1.5V, +/- 15V, +/- 150V
   Current source and measure from nA to .1 Amp in 6 ranges: 1uA to 100mA
   True 4-quadrant operation
   Source accuracy .01%
   Measure accuracy .005%
   Graphic LCD with touch screen
   Small, half-rack 2U package
   DIY friendly and low cost
Half rack is good but I wonder if 1/3 rack is possible.    Bench space is always an issue.   Beyond that I'd prefer good support for computer communications than anything else.   You still would need voltage and current displays on the front panel but a computer connection would be nice.   Bluetooth is probably asking too much.
Quote
It is not complete yet, and I plan to continue the project this winter. PC boards are built and working. An initial GUI is working.  It has a basic enclosure. My web page discusses the idea, requirements, design, and implementation. Check it out at www.djerickson.com/diy_smu

Thanks,
Dave Erickson

Thanks Dave!   I will have to check out your web site in the future.   It is already late and I need to dig out my truck tomorrow.

In any event this sounds like a most interesting project.   It is something that could be very useful.    What is nice is that I learn something new every time I tune into such threads, even if I never build it, I gain.   
 

Online Kleinstein

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Re: DIY-SMU Project
« Reply #25 on: February 13, 2021, 10:36:06 am »
A SMU naturally needs a little more power and also needs to get rid of the heat, especially in sink mode. So like a linear lab supply it can't be build very small and just needs space for heat sinks and so on. So already the 1/2 rack width like the modern Keithley 24xx series is relatively small.

It is more that other instruments could get smaller: some of the simpler bench DMMs and DDS generators are quite some empty space inside and could get even smaller. The 1/2 size got a kind of standard and this helps when stacking instruments - nobody want's a mix of 1/3 , 1/3 and 2/5. It is already bad enough they often have different depth and the feet at the back instead of at a defined depth.

For precision instruments, one wants to avoid any unnecessary EMI - so bluetooth is kind of a no go, and also USB is not really the preferred interface as it is often more like an EMI source than a power source. In this respect I am still expecting some kind of lower end fiber based interface for lab instruments (not 10GBit LAN).  For a SMU the power over USB is too limited - it may be just OK for a bench DMM or maybe small DDS generator.
 

Offline wizard69

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Re: DIY-SMU Project
« Reply #26 on: February 14, 2021, 01:27:12 am »
A SMU naturally needs a little more power and also needs to get rid of the heat, especially in sink mode. So like a linear lab supply it can't be build very small and just needs space for heat sinks and so on. So already the 1/2 rack width like the modern Keithley 24xx series is relatively small.
It sounds like you where responding to my post.   In any event to clarify:
  • The desire for a USB device is for something that can generate and read accurate low voltages.   The goal is to calibrate instrumentation in the common process voltage ranges.   Think field work here where a dongle would be ideal, size matters and big is not good.  We could easily skip current control or high current on the voltage reference.
  • I realize that 1/2 rack is currently a standard but it also wastes a lot of space for many instruments.   I'm a bit of a fan of the old TM500 and 5000 series from Tektronix.   It might not make sense to bring back that concept but narrower instruments would actually work well on many benches.  We just need to rethink how to form the instruments.
In any event I don't see a need to make every meter or bench instrument 1/2 wide.   There are many power supplies on the market setting good examples as far as layout goes.
Quote

It is more that other instruments could get smaller: some of the simpler bench DMMs and DDS generators are quite some empty space inside and could get even smaller. The 1/2 size got a kind of standard and this helps when stacking instruments - nobody want's a mix of 1/3 , 1/3 and 2/5. It is already bad enough they often have different depth and the feet at the back instead of at a defined depth.
My experience is that unless you buy the instruments from the same vendor (not a good idea) stack ability is highly variable, as you note.   So I really don't see that as a big deal.   If we go narrow and higher it becomes easier to justify a shelf for each instrument row.
Quote
For precision instruments, one wants to avoid any unnecessary EMI - so bluetooth is kind of a no go, and also USB is not really the preferred interface as it is often more like an EMI source than a power source.
This may be true but on the other hand instrument makers have been over coming the USB issue for sometime.   There are few instruments these days without USB so the EMI problem can be over come.
Quote
In this respect I am still expecting some kind of lower end fiber based interface for lab instruments (not 10GBit LAN). 
Every time something new comes out with a fiber interface it seems to die rather quickly.   Thunderbolt was suppose to support fiber and for whatever reason it never became a thing.
Quote
For a SMU the power over USB is too limited - it may be just OK for a bench DMM or maybe small DDS generator.

For the USB based device I'm really thinking a more limited SMU type device.   Basically an easily programmed precise voltage source with the ability to measure low voltages.   The ability to source or sink current, less than 100mAs, would be nice but that does lead to an explosion in size.

In the end I have two different instruments in mind the more I think about it.   The other thing is that I'm leaning towards minimal user interfaces on the device itself and expecting the user to connect to a PC for complete control.
 

Offline djerickson

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Re: DIY-SMU Project
« Reply #27 on: February 14, 2021, 05:14:33 pm »
Hi All,

Announcing recent upgrades to the DIY-SMU project:
Project web page: http://www.djerickson.com/diy_smu  The latest stuff is on the bottom of http://www.djerickson.com/diy_smu/smu-bringup.html
Video about the new changes and chassis: 
Study of DC-DC and AC-DC common-mode noise at http://www.djerickson.com/dc-dc/
Gobilda Hardware: https://www.gobilda.com/ Don't miss their https://www.gobilda.com/product-insights/

Changes are:
1) New half-rack 2U enclosure. I came across Gobilda hardware and used 4 of their 8mm square rails to build a frame. The prototype front panel is larger than the original box, so I was able to design...
2) New Teensy 3.2 CPU and front panel board. No more hacked Arduino Leonardo board and front panel controls.
3) Ported the Leonardo code to Teensy. Pretty easy job. Headroom is good......
3) Solved the 300KHz DC-DC noise issue by changing from Meanwell to Recom DC-DC. If you're interested, See my DC-DC and AC-DC common-mode noise study.

Thanks to all for your excellent feedback.

Dave



 
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Offline nikifena

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Re: DIY-SMU Project
« Reply #28 on: February 14, 2021, 07:58:24 pm »
Dear Dave.
I think you have to put a resistor between the U8.2 opamp output and the capacitor in order to prevent oscillations as shown

Also, what about adding a single 100nF next to each opamp between the 15V rails instead of two from positive to ground and the negative to ground. If you want to isolate the supply, you can add two 10ohm resistors in series of the 15V and -15V and each opamp.
 

Offline wizard69

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Re: DIY-SMU Project
« Reply #29 on: February 14, 2021, 08:54:03 pm »
Looks like you are on the fast track here.  By the way the boards appear to be smaller than expected.

As for Teensy is there a reason for going that way over simply soldering in a raw chip?

Hi All,

Announcing recent upgrades to the DIY-SMU project:
Project web page:
...
Study of DC-DC and AC-DC common-mode noise at ...

Changes are:
1) New half-rack 2U enclosure. I came across Gobilda hardware and used 4 of their 8mm square rails to build a frame. The prototype front panel is larger than the original box, so I was able to design...
2) New Teensy 3.2 CPU and front panel board. No more hacked Arduino Leonardo board and front panel controls.
3) Ported the Leonardo code to Teensy. Pretty easy job. Headroom is good......
3) Solved the 300KHz DC-DC noise issue by changing from Meanwell to Recom DC-DC. If you're interested, See my DC-DC and AC-DC common-mode noise study.

Thanks to all for your excellent feedback.

Dave
 

Offline nikifena

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Re: DIY-SMU Project
« Reply #30 on: February 14, 2021, 10:38:20 pm »
^
I think there is a lot of free space on the mainboard. Also, it's good to put the ribbon cable on the opposite side - next to the controller.

 

Online Kleinstein

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Re: DIY-SMU Project
« Reply #31 on: February 15, 2021, 09:18:55 am »
Dear Dave.
I think you have to put a resistor between the U8.2 opamp output and the capacitor in order to prevent oscillations as shown

The 10 Ohms resistor as shown does more harm than good. The resistor only makes sense in combination with an additional capacitor as direct feedback.

The OPA2140 is a nice OP, but not really the right choice as a reference buffer / inverter. In this circuit a BJT based OP, e.g. like OPA2202 (new and relatively cheap) or AD8676 should give better performance - one still needs to check the common mode voltage.
 
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Offline djerickson

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Re: DIY-SMU Project
« Reply #32 on: February 15, 2021, 02:02:16 pm »
Thanks again for the comments and help.

From Wizard69:
"As for Teensy is there a reason for going that way over simply soldering in a raw chip?"

Sure. My motivations are: Design expediency, ease of hand-assembly, Arduino tool and library support, easy Upgrade to the (amazing!) Teensy 4, previous experience, and love for all things Teensy. $20 is one of the high-ticket items on the BOM. Raspberry Pi Pico? Hmmmm.

From Kleinstein via Nikifena:
Dear Dave.
I think you have to put a resistor between the U8.2 opamp output and the capacitor in order to prevent oscillations as shown.

I agree with Kleinstein. A 10ohm resistor will make the circuit less stable, without a feedback cap from an opamp OUT to - IN. This circuit is a bit of a hack, I freely admit. Turns out putting a larger cap with higher ESR (cheap electrolytic or Tantalum) works well on the output of most op-amps, even with an emitter-follower buffer. You get a free ~1-2 Ohm resistor with every cheap cap :-)  Try simulating this circuit stability vs ESR and you'll see that some ESR is your friend, and low ESR causes instability. "But", you say, "There is no guarantee of minimum ESR, Dave". True, but 1) Without some ESR, most 3 terminal regulators aren't stable and 2)  If cap manufacturers could build the same cap with very low ESR, they would sell them for more $$ as low ESR caps, no?
Admittedly, if I were building hundreds of these (by machine) I might add a 1ohm R on series with the cap or the 10 ohm R + feedback C.

I use OPA2140, a fast JFET for a DC reference buffer since I already have a handful of them on the board. It has good enough drift and noise.

From Nikifena:
"Also, it's good to put the ribbon cable on the opposite side, -  next to the controller."
True. The signal flow is SPI->Isolator->DACs->Crossover->Amplifier->Irange->Vmeasure+Imeasure->Output. To put the output near the front, the SPI ended up in back. A long ribbon cable is a minor compromise, particularly since it is isolated.

BTW how do I include quotes from others in this blog thingie? How about in-line images? I'm new at this.

Thanks,
Dave
 
 
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Online Kleinstein

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Re: DIY-SMU Project
« Reply #33 on: February 15, 2021, 04:41:21 pm »
One can get a quote from an older answer, just click on the "inser quote part" to the top right of the answer. If needed delete the part not needed.

 Inline pictures get be inserted similar to other attachments, just chose the right type.

They start to appreciate ESR: there are extra MLCC caps with added well defined ESR available.
 
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Offline jbb

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Re: DIY-SMU Project
« Reply #34 on: February 16, 2021, 09:24:07 pm »
Thanks for the website and video updates. I know it’s a lot of work and I appreciate it.

Here’s a question about the output amp: do the cascade MOSFETs do much to the amplifier dynamic performance? I was thinking that if I built such an instrument I might not need (or indeed want due to safety concerns) a 150V range. Would it be practical to use a lower DC rail (eg +-30V) and only use the TIP41/TIP42 BJTs?
 

Online Kleinstein

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Re: DIY-SMU Project
« Reply #35 on: February 16, 2021, 10:19:34 pm »
The cascaded MOSFETs should only be reponsible for the high voltage part. When used at low voltage they should not contribute.
At lower voltage (like +-40V) a pure BJT based design should work too.  I would still consider faster transistors than TIP41/42, more liky D44H / D45H or BJTs made for audio. These may have a reasonable SOA and speed.
One could still use Juts 1 MOSFET each for a lower voltage version, so essentially without the cascade.
 

Offline djerickson

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Re: DIY-SMU Project
« Reply #36 on: February 17, 2021, 04:54:18 pm »
To see how Keithley does both +/- 200V 100ma and +/- 20V 1A, check out the 2400 service manual.  It has a simplified schematic for the amplifier design. It has one bipolar cascoded with 2 FETs for the low voltage high current, then another bipolar plus 2 more FETs for the 200V. I suspect the 2 bottom FETs are there to share power, otherwise the single bipolar needs to dissipate all the power. When used as a 1A load at 20V: About 20V + 20V + 15V headroom x 1A ~= 55Watts.

With 200V heat-sink isolation, you need a good electrical insulating thermal pad on your transistor, which adds ~1C/W thermal resistance. At 55W, that's 55C rise just for the stupid insulator, never mind the package and heat sink.

I have put dangerous voltage and current on a 5 lb heat sink with 4 FETs and 4 power resistors just to avoid those damn 1C/W thermal pads.

Dave
 
 

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Re: DIY-SMU Project
« Reply #37 on: February 17, 2021, 08:52:14 pm »
Hello Dave.

I have been thinking about your DC-DC converter problem I came with the following transformer design for the transformer:

It's a PCB coil design using a tiny U ferrite core with good isolation between the windings.

The top and bottom layers are used as individual shields in both coils and the board cutout and distance provides little coupling capacitance.

I did not make any calculations in the windings, considering 5V input -+15v output as an example.

This way would be very easy for anyone to assemble the converter.

Regards,
Vini
 

Offline arlo_g

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Re: DIY-SMU Project
« Reply #38 on: February 18, 2021, 01:11:29 pm »
Re heat sink insulation:  I haven't seen them offered at the big distributors yet, but Aluminum Nitride insulators for TO-220 and TO-247 devices are available fairly inexpensively from Aliexpress (with some risk that not all vendors sell quality product). 

Ceramic AlN has very good thermal conductivity and dielectric strength: somewhere from 100 W/mK to 200 W/mK or almost as good as BeO with none of the toxicity.  A 1/16" = 1.6mm thick TO-220 insulator should have at least 6x better thermal resistance than 1C/W of filled silicone thermal pad.  Of course, the AlN insulator would need messy thermal grease at both interfaces. 
 
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Offline djerickson

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Re: DIY-SMU Project
« Reply #39 on: February 21, 2021, 11:04:29 pm »
Re heat sink insulation:  I haven't seen them offered at the big distributors yet, but Aluminum Nitride insulators for TO-220 and TO-247 devices are available fairly inexpensively from Aliexpress (with some risk that not all vendors sell quality product). 
Ceramic AlN has very good thermal conductivity and dielectric strength: somewhere from 100 W/mK to 200 W/mK or almost as good as BeO with none of the toxicity.  A 1/16" = 1.6mm thick TO-220 insulator should have at least 6x better thermal resistance than 1C/W of filled silicone thermal pad.  Of course, the AlN insulator would need messy thermal grease at both interfaces. 

Oops, my mistake, I confused thermal conductance and resistance, Doh! For a TO-220 insulating pad, thermal resistance is about .3-.5 C/W, not 1.0 as I stated. Not a problem when burning 10-20 W,  but something to watch at high power.   Some non-insulating ones are lower but I need insulation to a few hundred volts.
I see TO-220 and TO-247 Al/N pads on Ali Express. Specs are nice, I will buy some.  Having those niceties: you know, like a manufacturer's web site and a data sheet would be lovely.  Thanks for the lead.

Dave
 

Offline jbb

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Re: DIY-SMU Project
« Reply #40 on: February 21, 2021, 11:36:42 pm »
I generally assume 1C/W for a TO-220 insulator too. If I ever do a high volume design I’ll clearly have to do some careful selections and qualification.

The flip side of isolation is capacitance, and that’s where ceramics could be really nice; thicker part = less stray capacitance. I guess a little grease will be needed between the device and the pad and the heat sink. (One good argument in favour of a Sil-Pad or similar.)

I do wonder what shapes are really available. For example, I would prefer to not have a hole in the insulator (which has a very short creepage path) and use a spring clip. Maybe one could source modestly sized rectangles and mount a couple of TO-220s.

The joyous endpoint of this kind of thing is the integrated power module, where one side of the ceramic sheet has circuit traces (ie it’s a single-layer PCB) and power semiconductors (soldered/sintered on, some connections made by wire bond) and the far side bolts straight to the heat sink. Prototyping costs might be high :-)
 

Offline djerickson

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Re: DIY-SMU Project
« Reply #41 on: February 22, 2021, 02:30:02 pm »
Here's the AliExpress page I ordered from. Different sizes, 0.6 or 1mm thickness, hole or no-hole. They specify 350 W/mK, but they are thicker than plastic and other materials. I bought the thicker ones hoping they are more rugged.
https://www.aliexpress.com/item/1005001350971494.html

Dave
 

Offline jbb

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Re: DIY-SMU Project
« Reply #42 on: February 25, 2021, 08:36:19 pm »
Hi Dave

Saw on your website that FIMV mode could cause trouble with voltage clamps.

I think I saw something relevant in a Keithley SMU manual, but didn’t understand it at the time.  It was a couple of MOSFETs that switched the resistor divider ratio of the inner current clamp BJTs of the power amplifier.  You could have a ~120mA amplifier limit for the force 100mA range, and a lower limit for the force 10mA range and below.

Another option would be looking at clamping the maximum voltage drop across the current shunt assembly. If you set up +-7V clamping that would limit the dissipation of the 499R shunt resistor to around 100mW, which might be acceptable.  As usual, such a clamp would need nice low leakage when not conducting and be able to shunt the worst case amplifier output current when required.
 
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Offline djerickson

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Re: DIY-SMU Project
« Reply #43 on: February 26, 2021, 08:11:38 pm »
Hi Dave
Saw on your website that FIMV mode could cause trouble with voltage clamps.
I think I saw something relevant in a Keithley SMU manual, but didn’t understand it at the time.  It was a couple of MOSFETs that switched the resistor divider ratio of the inner current clamp BJTs of the power amplifier.  You could have a ~120mA amplifier limit for the force 100mA range, and a lower limit for the force 10mA range and below.
Another option would be looking at clamping the maximum voltage drop across the current shunt assembly. If you set up +-7V clamping that would limit the dissipation of the 499R shunt resistor to around 100mW, which might be acceptable.  As usual, such a clamp would need nice low leakage when not conducting and be able to shunt the worst case amplifier output current when required.
Thanks much for your inputs, jbb. 7V would be safe, 15V if I beef up the 499R, I'm a little reluctant to add more diodes and leakage, so am leaning towards the 236 solution: lowering the current limit for all but the 100mA range.  Without lowering the current limit, you would get >100mA fault condition on the 10mA range.  Could fry a sensitive DUT.
I considered 2 other solutions. 1) Detect and fix it in software by increasing the clamp V. Is that a hack, or what?  2)  Hardware to detect this fault and drive the integrator to override the clamp. That could be a stability headache.
Dave
 

Offline jbb

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Re: DIY-SMU Project
« Reply #44 on: February 27, 2021, 12:35:42 am »
Yeah, solutions 1 and 2 above sound ... challenging. Adding more control loops to the system will get more and more annoying, and I expect you’re already having some battles choosing acceptable control gains for all the voltage and current range options.

Well what a surprise :-), Keithley have a really good solution in hand. It’s not perfect, though: the main control integrator will wind up when the CV amp (whether high or low) is controlling the loop and the output amp is limiting the current.
 
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Online Kleinstein

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Re: DIY-SMU Project
« Reply #45 on: February 27, 2021, 09:10:41 am »
Well what a surprise :-), Keithley have a really good solution in hand. It’s not perfect, though: the main control integrator will wind up when the CV amp (whether high or low) is controlling the loop and the output amp is limiting the current.
The current limit from the output amp should only be a short time or last resort limit. Normally the fine adjusted current limits (upper and lower limit separate) should set the limit at the integrator input.
 
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Offline jbb

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Re: DIY-SMU Project
« Reply #46 on: February 27, 2021, 08:42:33 pm »
The current limit from the output amp should only be a short time or last resort limit.

Hmm. Perhaps one could detect the current over range with some comparators and trip the output off? This is, after all, a fault condition.

If pursuing this approach, I would seriously consider implementing the trip logic in the analog control  board rather than trying to catch it with the microcontroller.

Quote
Normally the fine adjusted current limits (upper and lower limit separate) should set the limit at the integrator input.

Yes, but I think Dave’s problem is with the FIMV mode, where the current is uncontrolled once you hit the voltage clamps.

Maybe one could add a set of ‘outer current clamps’ to the crossover circuit which can override the voltage clamps? These could be hardwired to, say, 130% of range setting. That would scale the clamping with the range and avoid integrator windup.
 
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Offline djerickson

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Re: DIY-SMU Project
« Reply #47 on: March 04, 2021, 03:15:02 pm »
Thanks for the current limit feedback. For now I'm going with the Keithley solution: MOSFETs to switch the current limit for the 100mA range: 15mA on the 10mA and lower ranges, 150mA on the 100mA range. Yes, the integrator will wind-up, but this is a serious (but possibly common) fault condition.  I did a simulation of the current limit circuit, and it seems to work well.

I plan to switch the display from Nextion to LVGL. I found what looks like a nice Dev board: Teensy 4.0 + 3.5 TFT with Cap. Touch + LVGL examples.
http://skpang.co.uk/catalog/teensy-40-classic-can-can-fd-board-with-480x320-35-touch-lcd-p-1611.html

Anyone know of other Teensy + LVGL examples out there?

Thanks!
Dave
 

Offline jbb

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Re: DIY-SMU Project
« Reply #48 on: March 04, 2021, 09:10:08 pm »
Doing the emergency current limit in the power amp seems sensible. You want that kind of thing to be as simple as possible. Can you configure it so that it defaults to 15mA limit if the relevant control wire falls off?

You mentioned how to source this switch signal. I would favour using an inverter on the active low 100mA range; that way some confusion in the software can’t leave you in the dangerous configuration.

On integrator windup: I suspect the best approach is to trip/disconnect the SMU in this case; after all, something has gone quite wrong. I assume that a 15mA limit (plus voltage clamping to floating +-15V rails) is enough to protect the delicate parts in the current sense array?  If so, I guess you could detect current over range from the sense ADC. Maybe the trip could be “set voltage clamps to +-160V, set current ref to 0”?

In fact, that makes me think about the general challenge of enabling the SMU when wired to an energised device under test. Your PS-Load project had a nice take on this (a switch to disconnect the output MOSFETs from the preamp and a servo loop to preset the integrator). Is there anything like that in your current system?

Sorry I don’t have much on LCDs.
 

Offline riyadh144

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Re: DIY-SMU Project
« Reply #49 on: March 12, 2021, 02:00:23 am »
My god, that is some great work, I am commenting here to stay uptodate I guess I know what is my next project.

You need to sell that man.
 

Offline wergor

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Re: DIY-SMU Project
« Reply #50 on: March 14, 2021, 06:54:28 pm »
I love and hate such threads... love because I learn a lot, hate because I see how far I still have to go ;D
very interesting project. I'm designing an open source SMU myself (but I'm going in a different direction: +-24V, +-3A, no UI, instead it will be a module for a NI PXI / EEZ BB3 type system), your writeup, schematics and this thread have given me both insight and motivation to go ahead with it. thank you! :D

I was asking about improvements to my current sense circuit a while back, maybe this thread is interesting to you: https://www.eevblog.com/forum/projects/improving-a-current-sense-circuit/msg2986694/#msg2986694 - you may be able to eliminate U17 from your design (at the cost of 1 additional opamp).
« Last Edit: March 14, 2021, 09:52:35 pm by wergor »
 

Offline smgvbest

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Re: DIY-SMU Project
« Reply #51 on: April 11, 2021, 03:51:12 pm »
Hi,

Love the project,   following you on youtube and this thread.   Great work.
sorry to be another one of those with suggestions though

I'm curious why not go for a Teensy 4.1 Board instead of the 3.2?
The 4.1 for almost same price gives more of everything but most importantly adds a SDCARD socket on the board which would really make you ready for datalogging storage and playback.  Also supports Ethernet so you could have USB/Ethernet SCPI support which would likely save you in the future adding those options.

I of course realize what is likely the reason,   you have 3.2 boards on hand ;)
Sandra
(Yes, I am a Woman :p )
 

Offline djerickson

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Re: DIY-SMU Project
« Reply #52 on: April 14, 2021, 04:27:58 pm »
You are right, I'll likely end up with a Teensy 4.x or maybe even an STM32. Currently a cohort is working on a version of the embedded code with Ethernet on a Teensy 4.x. I've designed HW for embedded systems with ethernet and RTOS, but never wrote the code myself. Also looking into changing the UI from Nextion  to LGVL.
Thanks,
Dave
 

Offline RikV

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Re: DIY-SMU Project
« Reply #53 on: May 16, 2021, 10:29:13 pm »
Very tempting project! I would like to build one. Would it be possible to share the software and Nextion configuration of the current version? I do realize this is far from completed but nevertheless would be a nice starting point for som experimentation.
Thanks in advance.
PS: damn nice project!
 

Offline RikV

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Re: DIY-SMU Project
« Reply #54 on: May 24, 2021, 01:54:37 pm »
Dave,
For what reasons would you leave Nextion diaplays?
 

Offline voltsandjolts

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Re: DIY-SMU Project
« Reply #55 on: May 25, 2021, 08:50:59 am »
Thanks to Dave for an interesting and well documented project.

For a lower voltage and simpler SMU, could a power op-amp be used as the output stage?
For example, OPA548 or OPA541, for say +/-30V and few amps SMU.
They have a programmable current limit which could be used as the 'back-stop' current limit but a more accurate current sense/limiter could be wrapped around it.

I'm thinking of an smu add-on for a standard lab bench supply that provides +/-32V @ 3A.
 

Offline jbb

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Re: DIY-SMU Project
« Reply #56 on: May 25, 2021, 09:17:01 pm »
I was thinking about that. A lower voltage version could use reduced supply rails (ie <60V) which are safer to deal with.

But higher current outputs lead to high power dissipation, especially in ‘active load’ mode. Consider using the SMU to apple 2A load a 24V supply. On the face of it, that looks like 2*24=48W, which is substantial. However, that power opamp doesn’t have a current path to ground so the current must return to the -32V rail. So the dissipation is more like 2*(24+32)=112W. Yikes! I expect that a power opamp which could dissipate this energy would be expensive. That suggests a discrete output stage is the way to go.
 

Offline voltsandjolts

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Re: DIY-SMU Project
« Reply #57 on: May 26, 2021, 10:50:22 am »
Yikes indeed! Thanks for pointing that out, I hadn't thought of that.
A couple of power op-amps might be sufficient for a compact dual smu for small transistor characterisation.
But as you say, going discrete helps with heat dissipation, probably the only way to go.
 

Offline prasimix

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Re: DIY-SMU Project
« Reply #58 on: May 26, 2021, 11:06:20 am »
Ok, what's about adding pre-regulators?

Offline voltsandjolts

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Re: DIY-SMU Project
« Reply #59 on: May 26, 2021, 11:17:05 am »
While fine for a DC supply I suspect an smu pre-reg could be a bit trickier, if you need good dynamic response from your smu.
 

Online Kleinstein

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Re: DIY-SMU Project
« Reply #60 on: May 26, 2021, 12:19:19 pm »
The usual SMU circuits use a few supply rails to choose from and this way limit the loss. Switched mode preregulation gets tricky with EMI and also the need to sink and source - so one would need 2 rails. A preregular would still not help when used as a sink / load.  So one needs the relatively high power loss capability anyway.

The SMU power stages look that complicated because they often are made also for high voltage and use series connected transistors. With a more moderate voltage range (like +-45 V) the circuit can be quite a bit simpler, like 4  power transistors from levels like +-10 V and -+50 V.  As a sink it would add the opposite side 10 V and thus have quite some power loss. However just saving the 10 V by sinking towards 0 V would not help that much.
 

Offline jbb

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Re: DIY-SMU Project
« Reply #61 on: May 26, 2021, 07:35:50 pm »
Yes, a pre-regulator gets a bit tricky. Conceptually, it could help a lot with the sourcing modes by limiting main amp voltage drop. You’d probably want one for positive side and one for negative side.

However, when we consider the sinking modes, we have two options.
1) if the pre-regulator goes down close to zero, we still need to dissipate all of the sink power in the linear amplifier
2) if the pre-regulator goes down below zero, the linear amplifier dissipation is reduced. BUT the pre-regulator then runs ‘backwards’ and will push energy back into the main DC link. We then need to get rid of this energy so the DC link doesn’t blow up.

So, as usual, Kleinstein is right: it looks like an adjustable preregulator isn’t particularly helpful here. A multi-tap supply (+-40V and +-10V) could deliver quite an improvement without the additional complexity of tracking regulation. All four power rails could be sourced from a single power stage (perhaps LLC for lower switching noise?).

Choosing the ‘outer’ voltage is based on the desired instrument voltage range (plus cable and shunt resistor drop, amplifier headroom and  supply tolerance).

Some spit-balling for a ‘touch safe’ SMU:
- say +-30V output
- amplifier headroom: 9V (note we need headroom for bias circuits)
- shunt R drop: 2V
- cable drop: 2x 1V
- Hence outer DC supply >= 43V
- Allow 10% tolerance: 43 / (100% - 10%) = 48V
- Funny how those numbers worked out :-)

Choosing the ‘inner’ voltage is... probably similar, really. If we just want to improve the power sinking situation, we could just use 0V as previously suggested in this thread.

If we want a ‘high current’ output from the inner voltage rail:
- say +-6V output (enough for ‘USB’ and LiIon testing)
- amplifier headroom: 3.5V (perhaps optimistic, can get bias from outer rails)
- shunt R drop: 2V
- cable drop: 2x 1V
- Hence outer DC supply >= 13.5V
- Allow 10% tolerance: 13.5 / (100% - 10%) = 15V (ish)
 

Offline jbb

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Re: DIY-SMU Project
« Reply #62 on: May 26, 2021, 09:12:48 pm »
I realise that Keithleyhas already produced some nice diagrams which illustrate the point for their https://www.tek.com/datasheet/series-2400-sourcemeter-instruments]2400 series SMUs[/url] (attached).

If we look at the 2440 (bottom row), we see two distinct ranges in play: 10V 5A or 40V 0.5A. Also worth noting are the shaded notches in the sinking quadrants: looks like a power dissipation limit to me (or perhaps power transistor Safe Operating Area limits).
 

Offline dany-boy

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Re: DIY-SMU Project
« Reply #63 on: May 28, 2021, 04:03:00 pm »
Hey Dave,
Thanks for sharing such a complex project! Specially thank you for explaining how it works and the analog design decisions. Reading your blogs along with other materials has been quite a learning experience. I'm finalizing my order for most of the important analog components, and I wanted to ask you directly if its ok for me to convert the boards to kicad and modify the design a little bit. Particularly in the digital isolation department and the ±15v isolated module. I want to change the recom module for the SN6505 controller that @prasimix suggested, and change the digital isolator for a couple of cheaper options, as well as changing the SOIC footprints for SSOP which are also cheaper. Of course these modified files would be public as well.
Cheers for such a great project!
 

Offline jbb

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Re: DIY-SMU Project
« Reply #64 on: May 29, 2021, 02:10:51 am »
... modify the design a little bit. Particularly in the digital isolation department and the ±15v isolated module...

I saw a new part recently that might be handy. The Texas Instruments https://www.ti.com/product/UCC25800-Q1]UCC25800[/url] is a little LLC power driver intended for isolated gate drive. It works with high leakage inductance transformers, which means they can be wound using split bobbins and have very low coupling.

Wurth electronics apparently makes the https://www.we-online.com/icref/en/texas-instruments/UCC25800-LLC-Resonant?sq=750319177&sp=https%3A%2F%2Fwww.we-online.com%2Fweb%2Fen%2Fwuerth_elektronik%2Fsearchpage.php%3Fsearch%3D750319177#750319177]750319177[/url]transformer which has <1pF coupling capacitance.

——

I had a look at the Keithley patent previously mentioned. I see the relationship to the crossover Dave came up with, and I can see it does indeed have lots of current limiting / constant current diodes.

Those parts are hard to get and/or expensive, so I’m now thinking about using diode bridge switches (more often used for RF switching) in their place. The current flow through a diode bridge switch is limited to the control current (I.e. current limiting is available) and the behaviour of the bridge can be changed by adjusting the drive currents (e.g. can open the switch by removing drive currents, or can only supply current to one side to get ‘diode’ behaviour). I can set a low current for the ‘force’ channel and a high current for the ‘clamp’ channels such that the clamps can override the force. Interesting.

It may also be practical to put in another pair of ‘oh no’ outer current clamps which are always active and ready to override voltage clamp limits in the Force Current Clamp Voltage mode (which Dave has had trouble with previously).
 

Offline voltsandjolts

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Re: DIY-SMU Project
« Reply #65 on: May 30, 2021, 09:50:51 am »
I saw a new part recently that might be handy. The Texas Instruments https://www.ti.com/product/UCC25800-Q1 is a little LLC power driver intended for isolated gate drive. It works with high leakage inductance transformers, which means they can be wound using split bobbins and have very low coupling.

Wurth electronics apparently makes the https://www.we-online.com/icref/en/texas-instruments/UCC25800-LLC-Resonant transformer which has <1pF coupling capacitance.

That's an interesting little chip and transformer, nice find.
Links were broken, I fixed them in the quote above.
 
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Offline wergor

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Re: DIY-SMU Project
« Reply #66 on: June 06, 2021, 08:03:19 am »
DIY-SMU and the Keithley 236/237 use integrators as error amplifiers. why don't they use simple inverting amplifiers? how come the integrator's high gain at DC is not an issue?
 

Online Kleinstein

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Re: DIY-SMU Project
« Reply #67 on: June 06, 2021, 08:17:20 am »
A control loop allways needs a limited bandwidth to keep it stable. The simplest case is a kind of integrator.
The usual OPs (dominant pole compensation) are actually working as an integrator with the capacitor inside, setting the GBW. The output of an OP is the integral of the difference between the 2 inputs. It only become a more normal amplifier with feedback, or at very low frequency (e.g. < 1 Hz), where the limited open loop gain gets visible.
Inside the control loop one usually wants a very high DC gain.

An explicite integrator allows to choose a lower speed independent from the OP. It may not be a pure integrator and could have additional tweaks to improve the stability.
 
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Offline djerickson

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Re: DIY-SMU Project
« Reply #68 on: June 29, 2021, 11:43:21 am »
Hi All,
I've updated the DIY-SMU web page and added 2 new project videos on Youtube.
Chapter 3 is the new case, front and rear panels built by PCBway. I show how I used PCB layout tools and FreeCAD to design the enclosure.
Chapter 4 is the SCPI interface and how it was implemented.
Enjoy,
Dave Erickson

http://djerickson.com/diy_smu
https://www.youtube.com/channel/UC6agVsOGqt6E8cMNyzgdOAg
 
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Offline ducreux

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Re: DIY-SMU Project
« Reply #69 on: July 01, 2021, 03:19:36 pm »
Hi,
I don't have license for DipTrace : is it possible to have gerbers files ?
Thanks
 

Offline RikV

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Re: DIY-SMU Project
« Reply #70 on: July 02, 2021, 02:53:21 pm »
Hi Dave,
Thanks for the update of the schematics and for including the software! This is an amazing project.
However, I believe you forgot to include the Nextion file.
I cannot get the soft compiled: DIY-SMU_SCPI\DIY-SMU_SCPI.ino:397:3: error: 'my_instrument' was not declared in this scope

Any suggestion?

There is however 1 remark I would like to make. There is no logic in the naming of your files: SMU_Main becomes SMU1 Cross...
It would be logical that the file name reflects the board  AND the version Number. SMU MainRev.2 10_2020 and SMURev2. which one is most recent at first sight?

Dave, just found out that U26 (74AHC1G14GV,125) is missing in the BOM
« Last Edit: July 03, 2021, 03:10:38 pm by RikV »
 

Offline RikV

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Re: DIY-SMU Project
« Reply #71 on: July 02, 2021, 11:13:38 pm »
Hi, Ducreux,
to save Dave some time I include the Gerbers of the most recent version of the boards. I suppose Dave won't mind.
If you need a free "Gerber viewer", look for FlatCAM.
 

Offline ducreux92

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Re: DIY-SMU Project
« Reply #72 on: July 14, 2021, 02:26:43 pm »
Hi,

I make a prototype with test plate, see photo.

I use only cheap op like TL074, without adc or dac.

in FV mode my two references are variables resistances.

I have a big oscillation problem in out: about 200 Khz with >5V pp
 

Offline ducreux92

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Re: DIY-SMU Project
« Reply #73 on: July 14, 2021, 02:41:06 pm »
The photo.
 

Online Kleinstein

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Re: DIY-SMU Project
« Reply #74 on: July 14, 2021, 04:23:43 pm »
The TL074 may be a bit on the fast side on a bread board. Parts of the circuit may be to high in impedance for the added parasitic capacitance and there may be OPs with intentional different GBW in the design. The compensation may have to be adjusted for slower OPs.

So the simplified circuit / breadboard version may need adjustments / new calculation (simulation) for the compensation.
 

Offline RikV

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Re: DIY-SMU Project update?
« Reply #75 on: July 18, 2021, 09:31:32 pm »
Could someone hint me to where I can find the Nextion .HMI file for this project? According to what Dave says it should be included in his latest update but I don't see it. I would love to proceed on this project.

My fault: just found the Github page: https://github.com/djerickson/diy-smu
Thanks again for the interesting work!
« Last Edit: July 18, 2021, 09:50:03 pm by RikV »
 

Offline RikV

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Re: DIY-SMU Project
« Reply #76 on: July 19, 2021, 03:06:19 pm »
Where can I find those exotic USB-to-panel cables?
 

Offline prasimix

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Re: DIY-SMU Project
« Reply #77 on: July 19, 2021, 03:13:03 pm »
All kind of "impossible" and "exotic" cables could be found on aliexpress

Offline RikV

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Re: DIY-SMU Project
« Reply #78 on: July 19, 2021, 09:29:25 pm »
No doubt about that! All kinds of stuff you can imagine and a lot more! But... You must kno how to ask kindlyk. "Cabinet mounted USB-B to USB-Bmicro cable" doesn't return anything useful. how do you describe such a cable in a way ebay or other search engines understand what you mean? Is there a dictionary somewhere? I have the same problem when seaching componenet in an on-line catalogue. "What is the F** name othis f** thing??". I must be getting too old for this I guess.
 

Offline prasimix

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Re: DIY-SMU Project
« Reply #79 on: July 20, 2021, 05:27:25 am »
 
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Offline poorchava

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Re: DIY-SMU Project
« Reply #80 on: July 20, 2021, 08:31:41 am »
As for GUI I'd say: TouchGFX is the best by a large margin. At least when it comes to performance & capabilities vs cost (which is 0). It used to be horrendously expensive, but ST bought them and provides it for free. Obviously works only with STM32 now. It can provide a modern Android-like experience.

LVGL is nice, but waaaay more rudimentary than TGFX and waaaay lower performance, despite TGFX being written in C++ (and not the reduced kind, full blown with virtual methods and abstract classes). I also found, that LVGL requires much more coding to obtain same basic functionality as compared to TGFX.

But I'm one of those people who can't stand the Arduino crowd (maybe because of number of customers begging their inquiry with 'i've done it with Arduino so 3 weeks is plenty to go into series production for market, right? That's what we told the investors....'). Ohh, and the questions along the lines of 'how to run my AVR at 100Mhz' and 'I CAN HAZ HDMI ON AT MEGA?' </flame>
I love the smell of FR4 in the morning!
 

Offline RikV

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Re: DIY-SMU Project
« Reply #81 on: July 20, 2021, 04:44:29 pm »
@Prasimix: not only creative with chips but also with words.
Thanks for the extra hint. I had already found what I was looking for, only took waaay tooo long. Must be the age.
As lon as I can handsolder a 0603, you won't hear me complain.
 

Offline ducreux92

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Re: DIY-SMU Project
« Reply #82 on: July 26, 2021, 02:24:52 pm »
Hi,
Is this project reproducible ?

How many are working properly in the world ?

Thanks.
 

Offline ducreux92

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Re: DIY-SMU Project
« Reply #83 on: July 28, 2021, 02:20:31 pm »
Hi,

I tested a prototype on breadboard with TL074 with sucess.

I used +-15V powers instead +-170 V and only two powers mosfet ( doesn't works with 4 mosfets at 15V)
At output i have about 3 mVpp noise.

Why not replace the circuits OPA2140  with cheaper circuits like ADA4522-X for example ?
idem for AD5686  ?

 

Online Kleinstein

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Re: DIY-SMU Project
« Reply #84 on: July 28, 2021, 03:46:02 pm »
I had asked about the OPA2140 before - it just was available and cost was not such an issue for just a prototype.
There are different places the chip is used and different places may really want that chip or could use different alternatives (e.g. the ADA4522, MCP6V51, maybe OPA2202 or maybe even TL032 at some places).
 

Offline ducreux92

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Re: DIY-SMU Project
« Reply #85 on: August 09, 2021, 08:47:23 am »
Hi,
I find opa2197 (opa197) which is much cheaper that opa2140 with similar caracteristics, but
architecture is CMOS not JFET amplifier - Is this amp suitable for the project ?
 

Online Kleinstein

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Re: DIY-SMU Project
« Reply #86 on: August 09, 2021, 09:19:29 am »
The OPA2197 has more low frequency noise, but is still  a good OP.  I like the low power consupltion for the speed.
Another cheaper replacement relatively close to the OPA2140 is the OPA1642  (mainly higher offset). It really depends on the positions - the OPA2140 may not even be a good choice in some places.

With hobby projects it is not so uncommon to use the parts you have at hand and know and this may than look odd.
Currently some parts may also not be avialable (or not from the favorite source) - this can also cause strange looking choices.
 

Offline djerickson

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Re: DIY-SMU Project
« Reply #87 on: August 26, 2021, 11:24:25 am »
Thanks for the op amp substitute ideas for the OPA140/2140. I think JFET is the right choice, particularly for low bias current stages. Bipolar is generally higher input current, and CMOS is challenging for +/-15V, generally higher drift and noise. I don't trust the linearity (CMRR) of CMOS: they need circuit tricks to get wide input voltage range that sometimes compromise linearity.

I settled on OPA145/2145 as my go-to precision amp. Similar DC and noise specs to the OPA140/2140, slower (5.5MHz vs 11MHZ), better availability, and ~1/2 the cost.

I have built 2 complete DIY-SMUs so far. I'm getting ready to build the 'final' Rev2 Main boards and Rev3 Amp boards.

Jaromir has designed a very nice take on this project and has built a couple of units. His design uses an off-the shelf enclosure, and a custom AC transformer. And a simpler LED / buttons front panel similar to the original K236 instead of a touch screen.

If there is interest, I can get the new boards machine-assembled and can sell a few. At least the main board with its 175 parts. The Amp board (75 parts) is not bad to hand-build.

There are firmware additions and fixes that I plan:
   EEPROM based calibration. Currently requires recompile. A Python app does the hard part.
   Control of remote sense. Currently SCPI only
   Better Clamp control. Basically works, not accurate.
   Basic plotting page

Thanks,
Dave Erickson

DIY-SMU: www.djerickson.com/diy-smu
Jaromir's DIY-SMU: https://www.eevblog.com/forum/projects/how-i-made-my-smu/new/#new
 
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Offline ducreux92

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Re: DIY-SMU Project
« Reply #88 on: August 28, 2021, 10:37:50 am »
Hi,
Today impossible to find AD5686XX in tssop format.

Do you know an equivalent ?

 

Offline djerickson

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Re: DIY-SMU Project
« Reply #89 on: August 28, 2021, 01:38:07 pm »
Thanks for the heads-up on the AD5686. Last I checked 2 months ago some were available in TSSOP. Just not the most accurate grade.
I guess some options are to change to the harder-to-solder QFN, or add a footprint for the similar 8 channel AD5676.

Dave
 

Offline stephanm

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Re: DIY-SMU Project
« Reply #90 on: September 12, 2021, 10:11:05 am »
Hi,

first of all: awesome project! Using LTspice and looking a bit into Dave's curcuit, I saw that the output voltage can overshoot when the instrument comes out of compliance, depending on the set voltage and compliance. Here's an example: Force voltage is set to 1V, and the current clamp is set to 10mA. The load is just a 1Meg resistor. At 5ms within the simulation I am shorting the 1Meg RLoad resistor with a 1 Milliohms resistor, and keep it shorted until 7.5ms. During the short, the circuit goes into compliance as expected, limiting the output current to the programmed 10mA.

However, there are two transients on the output when applying and, respectively, removing the short:
  • When applying the short, there is a short and large current spike (above 100mA), which is ok because I am basically shorting the 135pF output capacitor of the SMU. There is also some oscillation visible which even drives the output voltage below 0V(!), but this does not concern me so much at the moment.
  • What concerns me more is that when removing the short, the output volage overshoots quite a lot, it goes up to 7V, after which it takes 1.5ms to recover to the programmed 1V.
From my understanding of the DIY-SMU circuit, a voltage overshoot after removing a short on the output is hard or even impossible to avoid due to the fact that the circuit has to deal with the voltage drop on the current sense resistor. As I had set the current sense resistor to 500 Ohms, the voltage drop on that resistor is 5V when the instrument is in compliance, and these 5V plus some additional voltage will end up on the output when the load current suddenly drops from 10mA to a few microamperes due to removing the short.

Looking at the Keithley 236 schematics, I was looking for some circuitry to deal with this situation and reduce the overshoot, but so far I have not found anything that looks like doing so. So, what I am curious of is whether such overshoot is also encountered on other SMUs, such as the Keithley 236?

Thanks and kind regards,

Stephan.
 
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