DIY SMU: Source Measure Unit Page 1, Analog

The Schematics, PCB files, and BOM are here 
Page 2: the Digital part
 Page 3: Board Bringup
Page 4: Board Bringup II

 YouTube Video Chapter 1: DIY-SMU Intro
YouTube Chapter 2: New CPU and Case
YouTube Chapter 3: New Case and CAD tools
YouTube Video 4: Instrument Control Via SCPI
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Prelim case


Introduction

As a lifelong analog EE, I have built and used many DC power supplies starting with an LM309 5V regulator in the late 60's. Then on to uA723s, various 3 terminal linears, and many switchers.  At Analogic in the late nineties I got to work on Semiconductor ATE equipment. One extreme was the AN/DP8200 VoltBox, a precision 20 bit voltage standard. I was exposed to low power SMUs for semiconductor testing. Then at Teradyne I was an analog EE in their DC instruments group, where we designed serious VIs (voltage-current instrument), DPSs (Device Power Supply) and SMUs for semiconductor and system testing. I have built various DC loads up to 20A, and LED testers up to 50A and 50V. I built a 4 quadrant power-supply and load (PS-Load). Not to mention my many audio amplifiers and Thermo-electric cooler (TEC) controls. I set up the PowerOne hackers blog.

So here it is 2020, and if you want an instrument to do basic semiconductor DC testing, curve tracing or the like, plan to spend $4,000 and up for a new instrument such as a Keithley 2400 Source Meter. You'll need 2 instruments to drive both the base and collector to test a transistor or other 3-terminal device. Need to test a multi-pin device? Add more instruments if they need to operate at the same time, or a multiplexing system if they can operate in a sequence.  Real world applications get very expensive.

I looked at the older Keithley 236/7/8 SMUs, and very much like their capabilities. They can output micro-volts to +/-110V (model 236) or +/- 1100V (model 237). Wide current ranges from 1nA to 100mA full scale, 1A for the Model 238. But these are 30 year old designs and a full 19" rack wide. On Ebay they go for $1,000 or more, depending on condition.

Here is the Keithley 236/237 SMU Service manual with Schematics.  The Keithley model 236/237 has these capabilities:
Here is a great K236 teardown page. Inside these are:
 The 2400 / 2600 Source-Measure units are modern replacement for the 236, with some differences. The 2400 has:
 The newer 2400 design eliminates most of the 236's shielded reed relays and Teflon standoffs since it does not have the lowest current ranges. Instead of a large multi-winding AC transformer, it uses a  smaller, multi-winding, high-frequency transformer. All the analog and power circuitry is on a single board. The unit is 1/2-rack width.  There are other models with higher voltage, higher current, and lower current ranges. There are also newer models with LCD displays. There are 2-channel models.

It is my intention to build a SMU with similar capabilities to the 236 and the 2400, but with important differences:

Mandatory Safety Warning

These instruments can be dangerous to use. Needless to say they are even more dangerous to build or to probe inside. They are capable of putting out voltages in the +/- 100-200V range, and have internal voltages as high as 350V or more, with large high-energy capacitors. These voltages can be lethal to humans.

Another warning: there is no such thing as "GROUND". There are a few circuits that are tied to the chassis which is wired to the AC line safety ground, but the bulk of the circuitry is floating. The + output is near the Analog common, so if you are using a scope or other grounded instrument, DO NOT ALLOW ANY OUTPUT PIN TO BE GROUNDED. Inversely, if you ground one output, assume the Analog ground is at a high voltage.
 
Safety-type output jacks are there for a reason. Your safest bet is to always assume that all outputs are high voltage.

Block Diagram

Here is the overall block diagram. 

Main Block

Here is the block diagram for the analog and power amp boards.

DIY SMU Block
The general architecture is similar to the 236. The entire instrument is floating with a guard to minimize leakage currents and interference from external sources. Major systems are:

Simulation

I built up a simple simulation of the SMU in LT-Spice. I already had a 3-op-amp crossover design left over from the PS-Load project. The clamp circuit, power amplifier and floating +/- 170V power supplies worked well and with good transient response.  I simulated a bunch of low current modes successfully. Time to build some hardware.

PCB Design

I designed the Amplifier and the Analog boards in my favorite CAD tool: DipTrace. Here is the early version power amplifier board 3D model.
AmpBoard3D

On the left side of the schematic is an early version of the amplifier. The power NPN/PNP Q3 and Q4 control the currents in the FETs. The R/C voltage dividers on the FET gates control the FET voltage drops.

On the right is the floating +/- 170V power supply with the +/- 15V supply for the Voltage Sense amplifier. It is powered from a 30Watt 120/120VAC (240VCT) medical grade isolation transformer with an electrostatic shield. It uses film capacitors to drop the 120VAC voltage to +/- 15V efficiently.

AmpSch
Here is the main Analog board. On the left is the SPI digital interface, +/- 15V DC-DC, and isolation. Next to the right is the References, ADC and DACs. Then the crossover, then the current ranging, then the current sense, voltage sense, and guard circuits. Top right is the output and sense relays, output and sense connectors on the far right. Click for full res schematic in .PDF

Analog Board

Crossover / Clamps

Clamping is required when the current or voltage limits are exceeded. One good example is when sourcing current. A theoretical current source without a clamp can go to infinite voltage into an open circuit. A real-world current source will eventually clip at some voltage, but that voltage may cause damage or a latch-up condition. Setting clamp voltages to safe values ensures that the voltage will be within safe limits.

FVMI means Force Voltage, Measure Current. FIMV means Force Current, Measure Voltage.  With FVMI the clamp current settings are current limits, similar to a power supply, and preventing the voltage source from outputting dangerous currents. An example would be where a DUT fault causes a short circuit to ground.  It is important to limit the DUT current in this case.

The crossover's job is to cleanly switch from current to voltage control when a clamp occurs.  I have designed and used a few crossover designs in the past. Keithley Patent 5,039,934 is for the crossover used in the 236. It addresses both polarities of both current and voltage. In the old days, 16 bit DACs were very expensive, but nowadays, not so bad. The 236 uses two 14b DACs for voltage and current, and has an additional bit to provide polarity control.  Using a single DAC for both clamp polarities means that both the negative and positive clamp values are always the same but with opposite polarity. This may or may not be a problem.

Another downside to the 236 circuit is that when a force polarity change is made, the output can have a large transient. I don't know for sure but I assume that when a polarity change is made, the output is turned off during the transition. This is an inconvenience when ramping from one polarity to the other.

Another downside is that the crossover uses four constant-current diodes for the force crossover amplifiers. These parts are expensive and hard to get. My design uses one resistor instead. Another downside is the number of switches required to change from FI to FV, and the need for a precision polarity control. Another is the complexity of the compliance circuit . It uses four more CMOS switches plus a dual comparator. Another is that the 236 requires zero setting correction hardware using the polarity signals plus resistor-diode circuits for each crossover. If a small 0 setting correction is needed on the DIY-SMU design, it can be applied in software.

I think the 3 DAC, 3 opamp crossover is a bit simpler and more powerful. I used it on the PS-Load project successfully. A single bipolar DAC is used to set the force value, independent of FI or FV. This has the advantage that if higher precision is needed (better INL or more bits) then only one precision DAC is needed. The clamp DACs accuracy are generally less important than the Force DAC.  The crossover input needs a 2:1 mux to select whether the force error amp receives Measure Current or Measure Voltage. The opposite is true of the clamp error amps.  When changing force modes, the two clamp DACs change values. The compliance detect is simple: 2 comparators to ground on the clamp op-amps. If either clamp error amplifier crosses 0, a clamp (compliance) event is occurring. This signal is passed via a separate isolator to the CPU.

With the 3 opamp design, one risk is that the + Clamp must always be set to a higher value than the - Clamp. Otherwise the two clamp op-amps will fight, and the output will swing wildly to the rails. Don't cross the streams!

Current Ranges

DIY-SMU will have 6 decade current ranges from 1uA to 100mA full scale. Future versions may have a 1A current range at lower voltage, like the 2400. The 236 applies +/-10V drop across the current resistors. The 2400 uses +/-2.0V. DIY-SMU uses +/-5.0V to limit resistor power and to still achieve high accuracy. Precision resistors with 5xx and 4.99 values are readily available.

The lowest current range, 1uA uses a 5 Meg ohm shunt resistor. The highest 100mA current range uses 50 ohms. CMOS switches select the resistors on all but the highest range. On that range a 1 ohm photomos relay provides the switch. Sense for the resistors are switched as well. All of the highest value resistors have a common sense line since the switch resistance is negligible compared to the shunt resistance. The higher current shunts have individual sense lines selected by switches along with the range switch.

To achieve 1 part in 1000 resolution or better / accuracy on the 1uA range, the leakage current of the output stage must be less than 1nA. Since there are many passive and active components and CMOS switches connected to this node, the leakage of individual switches and semiconductors current needs to be about 0.1nA. Leakage current also increases with temperature, so keeping the temperature down is good practice. I use low-leakage VIshay DG441 CMOS switches.

If it is necessary to have even lower than 1nA performance or < 1uA ranges, then relays or J-FETs will be needed. But with reed-relays, it is important to watch the relay activation times. CMOS switches change state in ~100 nanoseconds and so switch control is simpler.  

High Voltage Power Amp and Buffer

The high voltage amplifier is similar to the 236 design. It uses bipolar emitter-follower power transistors whose outputs drive GND via emitter resistors. The collectors of these transistors are (indirectly) tied to the +/- 170V power supply. In reality, the +/- 170V power supply common terminal is the output. It's a bit of a mind-twist to think about this. 15:1 voltage feedback is provided back the input of the buffer to set this amplifier gain at -15. +/- 10V on the amplifier input (integrator output) causes the output to swing +/- 150V. The gain resistors stabilize the circuit with all types of loads from 0 to infinity ohms.

To achieve an amplifier with +/- 150V of output swing (300V p-p), 400V or higher transistors wold be needed. The maximum power in these transistors is about 2 x 170V x 100mA or 34W. This is do-able in single devices, but unnecessarily challenging. Instead, the transistor voltages and thus power dissipations are distributed in three devices, wired in "cascode": one bipolar transistor plus two Power FETs in series. A resistor divider string drives the FET gates and so controls the voltage sharing between the three devices. Lower voltage devices can be used in this manner.

Then the whole three-transistor plus resistor string is mirrored with complementary devices on the - side, for a total of 6 power transistors. Local current limiting is provided in case of output short circuits or faults that are too fast for the control loop to respond.

Heat sinking and forced-air cooling are required for these devices. To use a single grounded (and safe) heat sink, high voltage isolation of about 400V is needed on each transistor. Unfortunately high voltage insulators also have relatively high thermal resistance of about 1°C/W. So the 34W maximum is divided by three devices, so each device temperature rise is reasonable.

The power amp needs a high input resistance and to drive the output transistors with about 5mA to output 100mA (beta 20 min.). I use a basic op-amp plus complementary follower driver transistors.The drivers Vbe compensate for the output transistor Vbe drops and so reduce crossover distortion.  The 236 and 2400 use a high-current buffer amplifier and diode bias tricks instead. 

Voltage Sense and Ranging

The requirements of the voltage sense circuit are: stable performance from mV to +/- 150V, and pA bias current. This low bias current is needed because of the low current ranges. In order for the 1uA range to have >1000:1 dynamic range, the leakage currents must be <1uA / 1000 or <1nA.  The SENSE+ amp sees only the +/- 5V across the shunt resistors plus a few volts for the sense lead drop, so can be a 'normal' +/- 15V op-amp. The SENSE- amp needs to buffer the output - voltage which can be +/- 150V. Fortunately the +/- 150V floating power supply can be tapped to obtain -VOUT +/- 15V to power this buffer. This amplifier's low impedance output then drives a high resistance 30:1 divider, to reduce the measure voltage from +/- 150V to +/- 5V.

Needless to say these delicate FET pA op-amps need protection from Bad Things like high voltage transients. Series resistors and low-leakage BAV199 diodes, plus their on-chip ESD diodes should protect them.

These two unity-gain buffer op-amps feed a differential amplifier, similar to a normal 3-amplifier Instrumentation Amplifier.

To change the voltage range to +/-15V or lower, this divider ratio is changed to 3:1.  A Photomos switch can handle the 150V need for this.

The 1.5V range uses the same 3:1 divider as +/- 15V, but reduces the Force DAC voltage range from +/- 5.0V to +/- 0.5V. 

ADC

Keithley, HP/Agilent and others generally use custom designed, multi-ramp, integrating ADCs for their DMMs and other precision DC products. I will use a modern delta-sigma ADC instead. These can achieve 20 or more bits of resolution and stability and can trade off resolution and noise level vs. speed. The requirements are:
The AD7190 meets these needs nicely. Like most modern ADCs, the input voltage range is limited to 0 to Vdd. They perform bipolar conversions by wiring the - input pin to a mid voltage. such as the +2.5V reference voltage. Then low voltage op-amps are used to convert bipolar voltages to the 0 to Vdd range. By using precision rail-to-rail output op-amps, the input range of the ADC can be safely met.

Calibration

With all precision instruments, some type of calibration is required. In the old days trim-pots provided calibration. Since the '80s however most DMMs use closed-box (no trim-pots) calibration. Calibration correction factors are stored in non-volatile memory. I intend to use EEPROM to store offset and gain calibrations for each input and output range.

A standard issue with DIY equipment is that professional cal labs may not be able to calibrate it, and even if they do, DIY's will probably want to, well, DIY. My goal is to allow this unit to be calibrated in a reasonable time with a 6.5 digit or better DMM such as a 34401A. Maybe with some low-cost additional HW such as precision resistors for the low-current ranges.

Power supplies

Several floating (isolated) power supplies are required. On the 236, a custom AC transformer with 5 isolated output windings, and multiple shields provides all of the required voltages.  On the 2400, a high-frequency transformer provides similar functionality.
One goal of this project is to avoid an expensive, custom transformer.  The grounded circuitry is powered by a small, low power (10W) +12V switching power supply.  This supply powers the front panel and CPU as well. The analog control circuitry uses isolated +/- 15V and +5V. These are provided by a small 3 Watt DC-DC converter on the analog board, powered by the +12V grounded supply. This DC-DC uses additional filtering and a common-mode choke to minimize the common-mode output noise on the analog ground. 

The main output power source is from a floating +/- 170V supply which powers the SMU output +/- 150V.  A small 30-50W, troroidal medical-grade isolation transformer is used. Medical-grade transformers have an electrostatic shield between the primary and the secondary. The shield helps to minimize both leakage current on the secondary as well as power line common-mode noise induced to the secondary.  Ideally there would be two or more electrostatic shields, one connected to the chassis ground and one connected to the output common. I know of no off-the-shelf transformers that have this. We will see if using a single shield is adequate. Isolation transformers typically provide 120/240V to 120/240V. By rectifying a 2x120VAC (240VCT) secondary, +/- 170V DC, unregulated is provided at up to 100mA.

This power supply common is the -OUT of the instrument. The high voltage amplifier moves the ground around to vary the output voltage and current.

The -SENSE voltage amplifier  needs to measure the +/- 150V output voltage with very low (pA) current. One way to do this is to power a low-bias opamp from +/- 15V power with respect to the -OUT terminal. To generate the +/- 15V for this amplifier, I use film capacitors and Zener Diodes to drop the 120VAC to +/- 15VDC. Capacitors are more efficient than resistors for dropping large voltages at low current.

I recently learned that the original 236 circuit for +/-15V causes a reliability problem seen on several old units. These units use 15K ohm power resistors to drop the +/-150V to +/-15V. These burn 1.5 watts and get quite hot, so require forced-air cooling from the fan. But if the fan fails, the resistors can overheat and fail, often shorted. Next to fail is the zener diodes which fortunately fail shorted, saving the expensive downstream precision circuitry. My capacitor dropper burns very little power. 

An advantage of the '236 design is that the 15K resistors are a constant load on the +/- 150V supplies. So when the AC power is removed, the high voltage capacitors discharge to 37% with a 7 sec. time constant (470uF * 15K). So the voltages are human-safe within about 15 seconds. The capacitor dropper doesn't provide any DC load, and the amplifier only provides only a 5Meg load, so the high voltage can stay up for many minutes, creating a shock hazard. To address this, I use a 100K "bleeder" resistor on each +/- 170V supply to draw 1.7mA and consume 0.3W. This is still a long time constant but much shorter than without the bleeder.  These resistors feed two red LEDs that light when the power supply is dangerous. I may add lower value resistors to burn a bit more current and make the LEDs brighter.

Update 3/28/21: Detailed Explanation of the DIY-SMU and 236 Amplifiers

Here is a simplified block diagram showing the overall DIY-SMU amplifier design. The BUF stage is PNP/NPN transistors.  See the schematics above for details.
amp block

There is a lot of subtlety in the Keithley 236 / DIY-SMU amplifier. The design uses a bipolar transistor and FET stack to convert a low voltage input to a high voltage output. It does this in a clever and non-intuitive method: Instead of the amplifier driving the +Output directly, the +/- 170V floating power supplies of the amplifier are driven indirectly by the current outputs of the +170V and -170V amplifiers. This has the effect of causing the +/- 170V common terminal to change. This terminal is the -Output of the instrument.

There are other circuits required to make this work:

1) The amplifier has a current output (high impedance) as opposed to most amplifiers which are voltage output, low impedance. The gain and frequency response of a current output amplifier is highly load dependent. To drive and stabilize it, there is local feedback for a voltage gain of x-20. This is provided by resistors R35 (200K) and R33 (10K) via buffer opamp U12.3.  The buffer and the gain setting resistors for the amplifier are on the Main board on both the 236 and the DIY-SMU. The overall amplifier circuit is basically a big opamp, wired for an inverting gain of x-20. But, unlike most opamps, the loop gain is high only for high impedance loads. The current output nature of the amplifier causes a fall-off of the closed-loop gain with lower impedance loads.

2) To get a low bias current -Sense input for the -Output, the -Sense op-amp is powered from +/-15VOut supplies, referenced to the -Output. This allows it to buffer the -Sense input over a range of +/- 150V safely and still maintain pA input (bias) current. Because of the potential high voltage input range, diode input protection and output current limit are required for this amplifier.

3) Local, fast, output current limit is required. This consists of transistors that measure and limit the current in the main NPN and PNP transistors in the case of excess current. To protect other circuitry such as the current ranging, 2 limit settings are required: ~ 130mA for the 100mA range, and ~15mA for all the other ranges. MOSFETs are used to switch this current limit as a function of the current range setting.

4) Bias and buffering. The 236 amplifier uses an old LH4001 Hybrid buffer IC to drive the output transistors. It uses bias diodes and resistors to minimize crossover distortion. The 2400 uses a AD847 fast opamp in the place of the LH4001. It is the only DIP IC on the board. I believe they used a DIP to increase the power dissipation.

DIY-SMU uses complementary transistors driven by an op-amp to drive the amplifier, minimize crossover distortion, and to replace the expensive buffer IC.

To really understand the amplifier, study the 236 service manual in detail, and build a Spice model for the amplifier. Spice will easily simulate the floating +/- 170V and +/- 15V power supplies, but I haven't figured out how to model the -Sense buffer's floating power supplies with an op-amp. I use a behavioral model instead, an ideal voltage-to-voltage source. My LTSpice model is the .ASC file is in the .ZIP file above.

BTW the 236 has a closed-loop gain of x-11.0 on the 110V and lower ranges, and x-110.0 on the 237 1100V range. These assume +/- 10V input from the integrator. DIY-SMU's x-20 gain may be higher than needed for its +/- 150V output.
 
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Last Updated: 10/14/2022