DIY SMU: Source Measure Unit Page 5:

Calibration,  More Testing, Applications, Curve Tracer

The Schematics, PCB files, and BOM are here

Page 1: the Analog part
Page 2: the Digital part
 Page 3: Board Bringup
Page 4: Board Bringup 2
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101 uses for an SMU

Having 2 DIY-SMUs on the bench has been very handy.

High resistance (and voltage) tester

To build an SMU, the six power transistors are screwed to the heat sink with insulating thermal pads and nylon shoulder washers. But they must withstand over +/- 300V with very low (or no) leakage. 10nA at 150V is 15 Gohms. An insulation tester (which I don't have) is good for this, but so is a SMU. So after first checking for an open circuit by measuring the heat sink to Amp GND with a DMM (20Meg ohms and a few volts max), I use a DIY-SMU set to 150V and the 1uA range, and look for < 10nA. This works swell.

Christmas light repair

If you have tested an LED with a power supply, you may be familiar with this scenario:
 What happened is that the power supply output capacitor, about 100uF was charged to 5V, and when the LED was connected, the big capacitor dumped all its stored energy at 5V into the 2-3V LED before the current limit could kick in. If you turn the power supply off before connecting the LED, things will go better. It's hot-connecting the LEDs that caused the damage. An SMU, on the other hand, has a minimum of output capacitance and so very little stored energy. So you can hot-connect it to LEDs, and is ideal for testing strings of LEDs. I recently struggled with repairing a string of LEDs and SMU to the rescue!

A string of Christmas LEDs typically operates at 120VAC RMS and 10 to 20mA. When one or more bulbs are out, the entire string goes out. Then your job is to find the bad bulbs. Usually white or blue LEDs use ~4V and have about 33 in series for a total of 120V.

Connect the AC input to a SMU, set to Force current mode, 150V range, 10mA. Strings of ~33 LEDs should light. Change the output to -150V and the other LEDs should light. Once you figure out what string the bad LEDs are on, remove the middle light from that string, and apply  the correct polarity of the 150V current source to the pins. If half the LEDs light, then the problem is on the other half. Then split the remaining half and try again (binary search, divide and conquer...). Repeat until you find the fault(s). I was able to repair the lights in a few minutes this way.

As a supposedly smart EE with a lifetime of experience, I was always frustrated by my inability to solve this problem. Instead of fixing the string, I would sometimes cut out the bad section of lights and bypass it (solder and heat-shrink). Or sometimes I would toss the string and buy a new one, very unsatisfying.

Testing a 4-20mA depth transducer

A buddy wants to monitor lake depth at his remote cottage using a water depth transducer, and I am trying to adapt it to an off-the shelf weather station.  It outputs 4-20mA. Here it is connected to DIY-SMU. 4ma is the output current with 0 depth.

depth

2021-2022 Parts Shortage Hell!

This is a very tough time to develop an advanced electronics project. The industry semiconductor shortages cause a minefield of problems every time you order boards. Parts that were plentiful 6 months ago, now have over 1 year lead times. Some key components in DIY-SMU that are hard to find, super-long lead times, or are unavailable:

Early 2022,  Digikey showed a few thousand ADUM261 isolators in stock. This solved my Isolator problem, but I needed to qualify them, so I put 10 in the shopping cart. A day later when I pushed 'order now',  there were 0 in stock!! There are no 6 channel, 5/1 fast isolators available anywhere.  ADUM261 is showing delivery in January 2022, only 2 months away, so I ordered. A tube of them which arrived late January 2022. I have not tried one yet but plan to try them out. 

Fortunately the parts on the Amp and CPU board are common as dirt and readily available,  as are the Nextion LCD and the Teensy 3.2. For now anyway.

3Q2022 update: I spoke too soon. Teensy 3.2s shipments are way out. Teensy 4.0 is unavailable but should be in stock  12/2022. Should be an easy change. I hope.

Ordering New PCBs

I am getting ready to order Rev3 Amp boards and Rev2 main boards. Here are the board changes:

The DAC, AD5686 is not currently available in the high precision version in the SSOP package. It is available in the lower grade. I thought about changing to the QFN (leadless) package, or laying out the board to accept both. The layout in that area is pretty tight, so I decided to suffer with the lower grade part for now.

New Board Bringup

The new rev boards are in. I assembled 2 units each of the Amp and Main boards, plus a new Rev1 CPU board, and one new case. The second new board will go in an existing case. This brings the number of complete units up to 3.

The Amp board came right up. Since it was new artwork I started bringup by powering it from a +/- 40V DC power supply with current limiting. The Main board had a problem, no negative output. -15V to the Amp board was GND by a schematic error. I cut the ground plane around J2 pin 4, and added a wire (of shame!). Board #3 came right up after that. Board #4 had a strange problem: The +2.50V voltage reference worked fine when +12V power was applied, but when installed in a system, the voltage reference raised to +3.6V. I thought it must be a short form Ref+ to some digital signal. Ultimately I replaced the isolator Si8661 which was drawing excess +5V current on the input side, and the ADR421 reference. Then the board worked fine, but sad since Si8661's and ARD421s are both on shortage and I only have a few. Both boards ran voltage calibration nicely.

I wired up a new (third) chassis, and realized that there is no wiring diagram or schematic for the chassis. So I made one (above). Also instead of measuring and drilling the bottom plate, I carefully measured, marked, and drilled a drill template for the bottom. Now making bottom plates can be done quickly and easily. I made one for Unit #3.

I measured the output noise using the new DC-DC and the common-mode choke circuit, and they work well. Same ~5mV noise as Rev1.

I noticed that one new board started up with -150V output voltage for a few seconds until the firmware set up the hardware and set the DACs and ranges, and turned off the output relay. This is not good! It means that some bits of the two 74HC595 control registers were coming up HI. I have a reset circuit, but according to the 74HC595 data sheet, the reset pin only clears the input shift register, not the output register. Also when I went to buy the original NXP devices, there was a shortage, so I bought TI parts instead. I think that the NXP units act differently, which is concerning. I need to determine whether the reset behavior is guaranteed or just a fluke. I'll buy some NXP and find out. Not a very satisfying solution any time you have to select one vendor's part....

After ordering NXP parts and an hour or so of googling "595 reset" and the like, to find out how others deal with this, I finally found one posting that said that the '595 with tri-state outputs controlled by /OE is intended for bus oriented applications. The '594 is a similar pinout part but with an output reset pin RCLR/ and no tri-state. Doh! Then why the heck does everyone use the '595?? I can rework in a '594 by simply lifting pin 13 and connecting it to pin 10. Another change for Rev3.

Next is to install the new Amp board and test the high-low fault current limit settings.

Amplifier Current Limit Tuning

I discussed on-line the need for amplifier current limiting when the instrument is in force current mode and the applied external voltage is outside the voltage clamp range. In this case, the current is limited only by the inherent limits of the amplifier which exceed the DG441 maximum currents. When I first did this test, the 10mA DG441 and current sense resistor (R42, 499 ohms) fried. The design was waiting for the new rev boards to address this. The Rev2 Amplifier only had a single +/-150mA current limit. The Rev3 amplifier adds a FET switch and bias resistors to reduce the current to < 20mA for the lower current ranges. I also added a bias resistors R24 and R25 to adjust the <20mA current. Without these, the drop across the 25 ohm current sense resistors is about 20mA * 25 = 0.5V, not enough to turn on the current limit transistors.

I was nervous to apply external voltages outside the voltage clamp range, but it works well. With the initial components, (R24,R25 = 50K) the current limits for 100mA was 130mA, and for 10mA was 23mA. Not bad, but I want the 10mA current limit to be < 20mA. I found that the simulation model for the circuit didn't completely agree with reality.  In simulation, the 2N3904/3906 transistors bases conduct with about +/- 0.705V, but in reality it is about 0.72V. When I changed the 50Ks to 36Ks, the currents dropped to 18mA / 122mA. This is excellent.

Finally DIY-SMU is a 4 quadrant SMU with protection against faults on all ranges.

But, but, but... how is 18mA fault current getting through the high resistance current shunts on the low current ranges?  I traced it through the high current shunt (R40 || R41, 50 ohms) and into U17-15, its current sense switch, so the DG441 protection diodes are absorbing it into the +/- 15V power rails. Not good. I have been hoping to avoid the leakage of added protection diodes on the sensitive +OUT, but will bite the bullet and add a low-leakage BAV199 to the next revision Main board when I fix the -15V error. These diodes connect the +OUT to the will nominally have 10-20V back-bias, and if the leakage currents of the 2 diodes in a BAV199 are close, they will cancel each other out a bit. More voltage and therefore more leakage than if they were bootstrapped. I notice that K236 uses low-leakage diodes bootstrapped to Guard. I will look into this approach. 

Weird Problem

The front panel buttons for LEFT and RIGHT work fine, but I had not yet implemented the other 5 buttons and the Encoder Press button since these functions are also available on the touch screen. This should mostly be a no-brainier: copy the working button pin defines, port initialization, interrupt (debounce) code, and point to the commands. But in the process of making these changes, the ADC broke! Readings of voltage and current were all about 1/2 of full-scale off.  DAC and GPIO were fine, only the ADC was busted. I checked the voltage levels, even swapped the main board, same problem, so it's not the hardware. I backtracked to an old code backup, same problem, so it's not the software! Teensy Module? CPU board? Gross errors in the cal. values? Connecting cable? All no. Very weird. 1/2 of the bipolar range error means the data is about 2 bits off, probably shifted down 2 bits. What could cause this? I hadn't touched the ADC code in months.

I started to question the compiler, generally the last refuge of a desperate engineer. I checked the compiler settings, and the optimization was set to 'Fastest with LTO'. I had to look that one up. LTO is Link Time Optimization, an advanced compiler setting that removes redundant code after compiling. I use bit-bang code for the SPI, so I suspect that LTO optimized some of my SPI code away.  Not sure how it got set.  I turned it off and my beloved SMU is all better! So I added comments to the main file about the compiler options and a warning to not use LTO.

Nextion Phase 2

I set forth to improve (fix) the Clamp setting page.  The clamp setting was never really complete: it needs the same spiffy controls as the Force value: Proper units display and the decimal point as a function of the range. Setting the Force value with the encoder and cursor buttons works well, but most instruments allow direct entry of numbers using a keypad. The more values to entry, the more important this capability is.

I was never real comfortable with Nextion, mostly because I am no expert. I have surveyed all the many UI tools available for embedded systems, and considered LVGL, Raspberry Pi on Linux, Java, Python and QT, EVE, and others. There are some expensive proprietary tool kits, and many that are cumbersome and hard to learn. I want something that is inexpensive and easy to learn. I decided Nextion isn't so bad, particularly since its smart displays un-burdens the CPU, but for the next level of UI, I need to learn more about it. I'm planning a whole retirement of projects for instrumentation, home and boat, and little OLEDs just  aren't going to make it. For the next level of Nextion I need to:

After watching a few "Nextion keypad" Youtube videos, I learned enough to build a numeric entry keypad. It was clear from these videos that it it good to  decouple the Nextion pages from the embedded code as much as possible using its rudimentary string editing capability. I should also use the Nextion Editor to do as much UI  development and debug as possible. I figured a keypad would be a good learning project, particularly if I could add my desired features: Numbers, Backspace, Clear, Cancel, Enter, and units selection.  By running this page along with my other pages, I found this a fun and interesting exercise. I modeled it loosely on the Keithley SMU pop-up keypad. Yes, I need to work on the aesthetics.

nextion keypad

keithley keypad

Nextion Global Variable Revelation

Inspired by this minor success, I attacked the nemesis of my Nextion coding. Nextion is intolerant of sending updates to a page that is not open, so If you have an asynchronous operation like collecting data and displaying it, then you can't send data to a widget unless it is on the current page. You get an error message for each value sent, and these error messages interfere with Nextion commands, making a mess of missed commands. I couldn't get this to be reliable until I controlled all page changes in my Arduino code. This required many, many 'if' statements, as well as polling the page variable. I thought I should be able to use global variables to solve this issue. I searched the Nextion Instruction page for 'global' to try this, but there is *no* discussion of the use of global variables on that page. Finally in November 2021, Nextion Forum posted a discussion of the correct use of global variables. It was in the Editor Guide.
Turns out if you use 't0.val=1234' on the wrong page, it doesn't work and throws an error. But 'page0.t0.val=1234' does work as long as the variable is set to 'global'! It only took me 9 months to figure this out. Now I can simplify my UI coding considerably by removing a lot of 'if (nPage == Main)' junk.

Recycle, Recover...

I had 2 complete Rev2's and one Rev1 built up into 3 chassis. Plus one Rev1 spare board. The Rev1's have the issue of over-current on the low current ranges when applied voltage exceeds the clamp voltage. So I am more careful with them, not using them as a load, so only 2 quadrant operation. Because of the time and cost invested in these 2 Rev1 boards, and the industry parts shortages, I'd been thinking of updating them by adding the inverter. Problem is that I am not fond of SMT hand-rework, requiring a dead-bug logic inverter and at least 4 wires; pretty ugly. Then in the middle of the night I remembered something I had previously considered: to use an unused control bit instead of an inverter, and fix it in software! That way only one rework wire is needed. One of the built-up units already has a Rev3 Amp board. I build up a 4th unit, adding another Rev3 Amp board. Amp boards are easy to build, lower cost, and parts are readily available. Plus I can scavenge some of the more expensive (> $1) parts from them.That brings the number of fully functional DIY-SMUs up to 4 with minimal cost and effort. Finally I can get one or two units into the hands of some other users.

More About Temperature Drift...

I dug further into the V force path gain and offset drift, updating the drift tolerance analysis spreadsheet to see what improvements could be made. I found that 0.1% 25ppm resistors had significant drift when 2 different manufacturer's resistors are used. In the case of a unit that drifted the worst, the 10K resistors that offset the Force DAC, R12 and R15, were from different manufacturers. I replaced the 2 offending resistors with two from the same reel. It makes sense that 2 resistors from the same reel would have better tempco matches than 2 arbitrary resistors, but it is not good to depend on this.

Here is the analysis of drift using 10ppm resistors instead of the original 25ppm ones. 10K, .1%, 25ppm 0805  are about $0.15. 10ppm ones are about $0.40. These reduce the force and measure drifts to about 1/2 of the above spreadsheet. Further improvements can be made by replacing the various 10K/10K pairs in the design with matched 2ppm networks such as SOT23 (Vishay MPM and others) for about $3 each. There are 6 of these 10K resistor pairs in the Force and Measure Voltage paths, and a few more in the less critical Current and Clamp paths. Is it worth another $20 parts cost? Probably so if you are looking for the best accuracy or a wide ambient temperature operating range. It would require a PCB artwork change to use the SOT32 networks.  Changing to 0805 10ppm parts is a simple BOM change.

Changing 10K resistor pairs to a 2 resistor network works as long as there are no other resistors in the stage that affect gain. Typically matched resistors match for tolerance and for tempco drift, but not for  absolute tempco. So the drift won't match another resistor. For example, look at The Force DAC buffer, R12, R14 R15. A Vishay MPM 10K/10K could replace R12/R15. These have a 2ppm/C ratio drift match, but only 25ppm/C absolute drift. So if R14, th 2.15K is 10ppm, the worst case drift could be 25 + 10 = 35ppm. Worse than 2 10ppm resistors. In other cases where there are only 2 10Ks, the drift wold be very good.

Since 10K/10K/2.2K network is what's needed, a Quad 10K network such as the Vishay ORN series could be used. These are 5ppm ratio drift and 25ppm absolute in a SOIC8 package for a reasonable price: $3. I simulated them with a 2K 10K, 10ppm fixed resistor to make up the 22K, and they work well. This could be used in all 3 DACS plus the two ADCs.

I checked the layout, and as suspected, all the 10K/10K pairs have both resistors close to each other. So the layout change to SOT-23 networks would be pretty easy. I thought of making a clever (to me anyway) small, hybrid footprint that could take either two 0805 resistors or a SOT-23. Too cutesy, and would confuse the BOM. Besides I see exactly 5 10K/10K SOT23 networks in stock at Digikey right now.

For now, I purchased some 10K 5ppm and 10ppm resistors plus a few 10K/10K SOT23s. I'll modify my worst drifting unit to add these parts.

drift2

12/2022 Update to Component Shortages: Good News!

Finally, parts are becoming available through distribution. The problem children have returned! I can build more units. My beloved Si8661, AD5686, AD7190, ADR421, OPA145, OPA2145, DG441, OPA2340, RS3-1215D are all back in quantity stock at either Digikey, Mouser, or both.

Teensy 3.2s are still out to March 2023, but Teensy 4.0 can be substituted. I haven't tried this yet. Nextion LCD, 12V power supply, toroid, AC in, all no problem.

This is great news for anyone planning to build a DIY-SMU.

My plan is to work on the firmware (ADC speed, SCPI updates...)  and to see if a small production run of boards would interest people. 

2/2023 PS-Load Update

I've been working on updating PS-Load, implementing a new CPU and new large OLED display. See the PS-Load page. I plan to use PS-Load to test batteries and solar cells, as well as using it for a high-resolution power supply. It can augment the 100mA range of DIY-SMU up to 3A supply and 5A load, and for low cost.  Great for testing high current transistors (below).

3/2023 DIY-SMU Curve Tracer Working!

Here is an example of two SMUs and some python code testing an old 2N2222 NPN transistor's characteristic curves. One SMU drives the collector-emitter voltage and the second one drives the base  current. Base current steps are 10uA each, so 0 to 90uA.  "Out" means Collector voltage / Collector current. You can see how the Beta increases with collector voltage. And how the collector current varies with collector voltage. I'm real pleased with the results. Next is to adapt the code to test NPN transistors, power FETs, high voltage up to 150V transistors, and vacuum tubes, again, up to 150V. Cool! I plan to have the unit

This is a 2N2222, 10uA per base step
curveTracer
2n3904
mpsa42
The instrument is doing 10 scans of 100 samples or 1000 samples total, and takes about 2 minutes to collect the data. Smooth curves like tubes can use fewer samples.

Here is DIY-SMU testing a vacuum tube, 12AX7 dual triode, parts 1 and 2, showing the matching of the 2 parts.
12AX7-1
12AX7-2

3/2023 DIY-SMU covers

I have 4 units with no covers,  got tired of cleaning the dust off the innards, and fabricated a 3-sided cover for DIY-SMU. Bending sheet metal with a Harbor Freight bender isn't so easy. I learned that 6061 aluminum is hard to bend and can crack, and that 5052 is easier. Good to know.  I built some simple flat-metal top covers for the other units. Covers are good.

Teensy 4.0 Processor

As of March 2023, Teensy 3.2's are unobtainium until 2034! Time to change to Teensy 4.0. I bought and programmed a T4.0, setting the clock to 160MHz, close to the T3.2 speed. This is to keep the current draw down. The processor, LCD, and fan at slow-speed operate from +5V through a linear regulator from +12V, and this regulator gets warm. 

Good news, Teensy 4.0 just works!

As usual, don't forget to cut the power jumper on the back of the module.

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Page 2: the Digital part
 Page 3: Board Bringup
Page 4: Board Bringup 2
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Last Updated: 3/18/2023