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DIY-SMU Project

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djerickson:
I'd like to introduce my DIY Source-Measure Unit SMU project. http://www.djerickson.com/diy_smu
Youtube intro: https://youtu.be/B26SW3N2zoA



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

fcb:
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?

helgel:
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 !

prasimix:
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.

djerickson:
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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