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Keithley 2450 SMU with I-V Tracer Software - Review

Scoring

Product Performed to Expectations: 8
Specifications were sufficient to design with: 10
Demo Software was of good quality: 8
Product was easy to use: 8
Support materials were available: 9
The price to performance ratio was good: 10
TotalScore: 53 / 60
  • RoadTest: Keithley 2450 SMU with I-V Tracer Software
  • Buy Now
  • Evaluation Type: Test Equipment
  • Was everything in the box required?: Yes

  • Detailed Review:

     

    1 Introduction

     

    Before presenting the instrument, lets begin with the basics: What is a Source Measure Unit (SMU)? An SMU is an instrument that can perform 4 operations: source voltage, source current, measure voltage and measure current. In contrast to common power supplies (which can also do that), SMUs can operate in all 4 quadrants of the I/V plane, as a power source (quadrant I & III) and as a power sink (quadrant II & IV).

     

     

    Another important difference between common power supplies (or electronic loads) and a SMUs is that the latter can source/sink with lower noise and cover a much wider dynamic range (ie: Keithely’s 2450 SMU sourcing/measuring span 5 decades of voltage and 9 decades of current range). Even though SMUs could be used as power supplies, electronic loads, voltmeters (when set as 0 A current sources) or an ammeter (when set as 0 V voltage sources), their real strength lies in their ability to move anywhere (within their limits) in the I-V plane to perform tasks such as voltage or current sweeping. The programmability and tight integration of multiple functions into a single instrument, make SMUs extremely versatile DC instruments.

     

    In terms of measurement capabilities, SMUs tend to be capable of measuring much lower currents than DMMs, and have a lower burden voltage (the voltage drop across the terminals  when measuring current) compared to DMMs. This occurs because DMMs usually use a shunt ammeter, in contrast to SMUs that usually use feedback ammeters. For sake of comparison, The Keithley DMM6500 burden voltage can get > 1 V while the Keithley 2450 is guaranteed to be < 100 μV for any current.

     

     

     

    Of course, SMUs are not a complete replacement for DMMs, SMUs cannot perform AC voltage or current measurements for instance.

     

     

    2 Keithley 2450

     

    The Keithley 2450 was the first instrument introduced by the company that used their modern touch-screen-based “Touch, test, invent” form-factor. But before diving deeper into more details, it might be interesting to quickly look into the history of curve-tracers and SMUs of Tektronix/Keithley.

     

     

    2.1 A bit of history

     

    Curve tracers are roughly the integration of a power supply and an oscilloscope in a single package. They were introduced by Tektronix in 1955, and initially designed to trace I-V curves of vacuum tubes. Later they also supported tracing transistors, diodes, and other solid-state devices. Curve tracers work by sweeping a voltage while measuring the amount of current and plotting the I-V trace, traditionally, on a cathode ray tube (CRT). These instruments could deliver voltages up to Kilovolts and currents up to tens of ampere to the device under test (DUT), but what they had in power they lacked in measurement accuracy.

     

     

     

    Nowadays there are multiple instruments that can perform I-V curve tracing and their names vary widely depending on their target usage. In contrast to classic curve tracers, SMUs are smaller, have a much wider sourcing/measuring dynamic range, and are much simpler to use.

     

    Keithley began producing their first SMUs (series 23x) in 1989. These full-rack instruments, like other Keithley instruments of that era, were brown and used red LED displays. They used triaxial connectors at the back of the instrument and were programmed through a GPIB (IEEE-488) port.

     

     

    In 1995 Keithley introduced their half-rack line of SMUs (2400, 2401, 2410, 2420, 2425, 2430 and 2440). The line switched to vacuum fluorescent displays (VFDs) and banana jacks in the front and back of the instrument, and added an RS-232 programming interface.

     

     

     

    In 2005 Keithley introduced with the 2601 and 2602 their high-throughput automated production testing oriented line of SMUs (series 26xx). They used a similar form factor than previous VFD half-rack SMUs, except that they removed the front banana jacks and put a navigation wheel instead. Later other form factors were added to this series, such as full-rack VFD ones. One of the most important characteristics of this series is that they introduced a script engine that could run TSP scripts (Lua language scripts).

     

     

     

    In 2013 Keithley introduced a modern line of instruments that they called “Touch, test, invent”. This line of instruments (2450, 2460, 2461 and 2470) uses a touch screen with an a very intuitive graphical interface that feels almost like using a smartphone. Just like the 26xx line of SMUs, this line also supports scripting, but also modern programming interfaces such as USB and Ethernet (which were not initially implemented in the 26xx line).

     

      

     

    2.2 Overview

     

    2.2.1 Package content

     

    The Keithley 2450 comes with:

    • 20A, 1000V CAT III shrouded banana connectors and sprung hook probes.
    • Interlock connector that can be used to protect the user from hazardous voltages.
    • TSP-Link (LAN crossover) cable to daisy-chain connect multiple TSP-Link instruments.
    • USB cable to connect the instrument to a computer.
    • Power cable
    • Quick start manual
    • Software CD
    • Calibration certificate, environment disclosure report, safety precautions and a test lead description

     

     

     

    2.3.2 First impressions

     

    At first sight instrument looks physically well designed. It comes with front and back bumpers that allow easy "Lego-like" stacking of multiple instruments, but can also be mounted on a rack if needed (the instrument is half-rack 2U high). The display is matte and has a decent viewing angle and brightness.

     

    The the output switch lights up very bright when it is on, making it a good warning sign (as the instrument can seriously injure the user if not enough care is taken). Rubber buttons with text on it use a back-light to light the text, making them easy to find in low light conditions. The front/back connection, remote, LAN, IEEE 1588 and Interlock status indicators are lit from behind the front panel (It is worth noting that the 1588 functionality is not supported at the time of writing).

     

     

     

    2.3 Electrical characteristics

     

    2.3.1 Electrical specs

     

    The Keithley 2450 is a 1 channel 20 W SMU that can source from -200 V to 200 V and from -1 A to 1 A. Below 20 V the instrument can source or sink up to 1 A, while over 20 only 100 mA.

     

     

    The sourcing/measuring dynamic range of the instrument is impressive. To highlight the measuring capabilities of the SMU lets compare it to a DMM of a similar price tag: the Keithley DMM7510, which is a 7½ digit DMM. The voltage range of both instruments covers 4 decades, the SMU can go from 20 mV to 200 V, while the DMM from 100 mV to 1 kV. The current range of the SMU covers 8 decades, from 10 nA to 1 A, while the DMM covers only 6 decades, from 10 μA to 10 A.

     

    Here are the key specifications of the SMU:

     

     

     

    2.3.2 Analog connectivity

     

    The instrument uses banana jacks at the front and triaxial terminals at the back.

     

     

    2.3.2.1 Front banana terminals

     

    The frontal banana jacks are useful to quickly perform tests that do not require high precision (ie: measuring/sourcing at 10 or 100 nA). The front panel is easily accessible, and banana cables and connectors are common and inexpensive. There are 5 banana jacks: FORCE HI, FORCE LO, SENSE HI, SENSE LO, and chassis ground. Sourcing/measuring can be done using just the 2 FORCE terminals, but under certain conditions, such as high current or low DUT resistance, the voltage measurement can differ considerably from the voltage that is seen by the DUT. Leads have a low but non-zero resistance, and high currents increase the voltage drop across the leads. The test leads and DUT form a voltage divider, so low DUT resistances reduce the fraction of the sourced voltage seen by the DUT.

     

     

     

    A solution to this problem is to use 4 wires (FORCE and SENSE) instead of 2. The FORCE connection operates just like in the 2 wire configuration, but the voltage is measured with 2 extra leads that connect directly to the DUT leads. So even though the SENSE leads still have a low but non-zero resistance, the current that flows through these leads is negligible causing in this way a negligible voltage drop across the SENSE leads. This makes, for almost all practical purposes, the voltage seen by the SMU SENSE terminals almost identical to the one seen by the DUT.

     

     

     

    2.3.2.2 Back triaxial terminals

     

    To get accurate readings when operating at low currents (<1 uA) or in noisy environments, extra precautions must be taken. There are 2 useful approaches for these situations: shielding and guarding. Shielding requires using shielded cables and an enclosure around the DUT) that are electrically connected to the FORCE LO. What this does not solve though is the current leakage between the FORCE HI, and FORCE LO, which at low currents could add a significant error. To reduce this leakage a guard can be used. A guard is a buffered output that is nearly at the same potential than the lead that it guards (FORCE HI and SENSE HI). By keeping the voltage difference between the guard and the guarded lead close to 0, the current leakage is reduced proportionally to the voltage difference (Ohms law).

     

    The following image shows how guarding reduces cable leakage:

     

     

     

    The triaxial connectors look similar to BNC connectors, but they possess 3 conductors instead of 2. The instrument uses 3 lug triaxial connectors, so it is not possible to damage the terminals by mistakenly plugging a BNC connectors (as it often occurs with 2 lug triaxial connectors). The inner conductors are not different to the front panel connectors (FORCE LO, FORCE HI, SENSE LO and SENSE HI). The middle conductors are FORCE LO for the FORCE LO and SENSE LO terminals and GUARD for the FORCE HI and SENSE HI terminals. The outer conductors are connected to the chassis ground.

     

     

     

    At this point it may look like there is no point in making unshielded, unguarded measurements as they are inferior. But there 2 major drawbacks: One is that triaxial connectors, cables and test fixtures are very expensive (in the order of several hundreds of USD), and the other is that it may take more time to setup the DUT in a fixture than to just grab the leads of the DUT with the probe hooks. Moreover, for many measurements it will not make much difference in the measurements to shield and guard.

     

     

    2.3.2.3 Interlock

     

    The interlock is a protection switch that is used as a safety mechanism to protect the user from getting shocked with voltages greater than ±42 V. The connector (3M™ Mini-Clamp Plug) contains 3 pins: ground, interlock and 6 V DC. When the 6 V and interlock pins get shorted, the SMU can source voltages up to ±200 V. The interlock switch may be installed on the lid of a test fixture, so that opening the fixture opens the switch while closing it closes it. I installed just a common switch to manually control its state.

     

     

     

    2.3.3 Digital connectivity

     

    2.3.3.1 Remote control ports

     

    The instrument can be controlled remotely through multiple ports: GPIB, USB, and Ethernet. They are more or less equivalent, but the USB is the fastest, followed by the Ethernet and GPIB. Triggering, required for synchronization, has the highest consistency and lowest latency through the GPIB port, followed by a less consistent and higher latency USB port and the Ethernet port at last. Ethernet is the most flexible one, it supports control through VXI-11, raw sockets, telnet, and even a web server that runs in the instrument.

     

    When the instrument is being controlled remotely a front panel LED lights up. The remote control of the instrument does not disable the front panel display or menu usage, but if one tries to change a parameter, the instrument prompts to switch to local control.

     

     

    2.3.3.2 Digital I/O

     

    The digital I/O port uses a DB-9 connector to provide 6 configurable input/output lines, a 5 V (500 mA max) line, a ground line and a flyback diode clamped line. These lines can be used to read or write data out, as trigger input or output to synchronize multiple instruments or even to drive and control relays.

     

     

     

     

    The TSP-Link is a proprietary high-speed trigger synchronization and communication bus implemented recently in the newest Keithley instruments. It allows to daisy-chain connect multiple TSP-Link instruments in a master/slave configuration with inexpensive LAN Cat 5e (or higher) crossover cables. TSP-Link allows the master to control multiple instruments as if they were one single physical instrument.

     

     

    2.4 Interfaces

     

    The instrument can be controlled through multiple interfaces, lets take a look at some of them.

     

     

    2.4.1 Front panel

    The front panel interface is intuitive and simple to navigate and use. The menu hierarchy is very shallow, making the access to every options very quick. Most options are available immediately after a single click on a menu icon (as opposed tedious navigation of deep menu hierarchies seen on many instruments). The fonts and number are big and easy to read and the color palette is pleasant. The GUI supports different gestures such as dragging and pinch-to-zoom, and it feels almost as intuitive as using a smartphone.

     

     

     

    The home screen shows at the top what is being measured and at the bottom what is being sourced. What is being sourced and measured can be changed by pressing the FUNCTION physical button.

     

     

     

    The instrument can display the statistics, the table, the histogram and the graph of the data to quickly explore it without having to first download it to a computer.

     

     

     

    Performing a current or voltage sweep is very simple:

    1. Press the sweep icon from the main menu.
    2. Set the sweep settings.
    3. Press "Generate".
    4. Press the TRIGGER physical button to begin the sweep.
    5. Select any visualization method (home, data table, histogram, or graph) and watch how the instrument performs the sweep.

     

     

     

    Setting the instrument as a voltmeter, ammeter, ohmmeter or power supply is also very simple as well, it requires pressing the QUICKSET physical button followed by pressing the desired operating mode.

     

     

     

    All in all the front panel user interface looks very polished and functional.

     

     

    2.4.2 Web remote control

     

    The SMU hosts a web page that can be used to set a few configuration settings (network configuration, time and date), check the instrument status (configuration, system information, event log), remote control the instrument through a virtual panel, download the data buffers (as CSV files) and send SCPI/TSP commands. Probably the most useful feature of the web interface is the virtual front panel, which can be used to control the instrument almost in the same way as from the physical front panel. Some relevant differences between the virtual and physical front panel are that the virtual panel is less responsive (it refreshes at ~3 refreshes/s) and it does not support the pinch-to-zoom gesture. More than trying to replace the physical front-panel interaction I found that the virtual front panel was particularly useful when debugging TSP-code to easily check the status of the instrument when sitting in front of the computer.

     

     

    2.4.3 KickStart

     

    KickStart is an application that gives control to multiple instruments from Tektronix and Keithley such as DMMs, SMUs, power supplies, and oscilloscopes. The application is very simple and intuitive, you select the instrument you want to control from the left side of the screen, set the instrument settings, press the "play" button, and then check the captured data either from the table or the graph view. The application can set the SMU to operate as a voltage or current source, and perform either voltage or current sweeps. Sweeps can be linear, logarithmic, or of arbitrary values of voltages or currents. Most of the settings available from the front panel are available in KickStart making it a good replacement for the front panel usage. Advantages of KickStart over controlling the instrument from the front panel are that the data can be more easily explored through either the graphing capability or the data table, and that it can save the data (as CSV files) directly to the computer. One of the features that I liked the most about KickStart is that I can see the data as it gets measured from the instrument, in this way I can interrupt the measurement if I don't like the data that I'm capturing (ie: if parameters were wrong set wrong).

     

     

     

    2.4.4 Remote control (TSP / SCPI)

     

    When more complex operations need to be performed, programming the SMU is the only alternative. The instrument can be programmed through either SCPI or TSP, but TSP is far more powerful. TSP is the name for Keithley's adaptation of Lua to control their newest instruments, and supports control flow, making it possible to run scripts in a standalone way (without a controlling computer). TSP can be used in two different ways, as a script that is sent from a computer to the instrument, or as a script that is stored in the instrument and runs in a stand-alone way. Scripts can be saved to the instrument either by remotely saving them to the instrument or by copying the script from a USB drive to the instrument.

    Here is an example of code that could be used to measure resistance and power by sourcing 10 V to a resistor and measuring the current.

     

    -- Reset environment variables to the default

    reset()


    -- Set instrument as a voltage source (can be omitted as this is the default value)

    smu.source.func = smu.FUNC_DC_VOLTAGE


    -- Set the current limit to 1 A

    smu.source.ilimit.level = 1


    -- Set the source voltage to 10 V

    smu.source.level = 10


    -- Set measurement to current (can be omitted as this is the default value)

    smu.measure.func = smu.FUNC_DC_CURRENT


    -- Turn the output on

    smu.source.output = smu.ON


    -- Perform a single measurement

    smu.measure.read()


    -- Turn the output off

    smu.source.output = smu.OFF


    -- Read the measured voltage value

    v = defbuffer1.sourcevalues[1]


    -- Read the measured current value

    i = defbuffer1.readings[1]


    -- Print voltage

    print(v)


    -- Print current

    print(i)


    -- Print resistance

    print(v / i)


    -- Print power

    print(v * i)

     

     

    And its output:

     

    10.000008583

    0.037293303758

    268.14488327

    0.37293335767

     

     

    Note that the voltage is also measured because it may not completely match the voltage that the SMU is trying to source, for instance when the SMU hits the current limit (which here was set to the maximum value of 1 A). print prints the value to the controller (ie: computer).

     

    To use the code in a stand-alone way, some minor modifications need to be made in order to output the values to the instrument screen.

     

    -- Reset environment variables to the default

    reset()


    -- Set instrument as a voltage source (can be omitted as this is the default value)

    smu.source.func = smu.FUNC_DC_VOLTAGE


    -- Set the current limit to 1 A

    smu.source.ilimit.level = 1


    -- Set the source voltage to 10 V

    smu.source.level = 10


    -- Set measurement to current (can be omitted as this is the default value)

    smu.measure.func = smu.FUNC_DC_CURRENT


    -- Turn the output on

    smu.source.output = smu.ON


    -- Perform a single measurement

    smu.measure.read()


    -- Turn the output off

    smu.source.output = smu.OFF


    -- Read the measured voltage value

    v = defbuffer1.sourcevalues[1]


    -- Read the measured current value

    i = defbuffer1.readings[1]


    -- Print voltage to the first line of the user tab

    display.settext(display.TEXT1, string.format("Voltage: %.4f V", v))


    -- Print resistance and power to the second line of the user tab

    display.settext(display.TEXT2, string.format("R: %.3f \018, P: %.3f W", v / i, v * i))

     

    And its output:

     

     

     

    As it can be seen, the basic functionality is straightforward. To perform more complex operations more advanced concepts need to be learned, such as trigger models, configuration lists and timers.

     

     

    3 Experiments

     

    To show the capabilities of the instruments I performed a few experiments to highlight the instrument features. All the code and data used can be found in the GitHub repository.

    3.1 Solar cell characterization (using different workflows)

     

    I connected a small solar panel to the instrument to show how the different user interfaces (front panel, Web, KickStart, Keithley’s I-V tracer APP and TSP script) are used to perform the same task of measuring the Pmax, Vmax, Imax, Voc and Isc values.

     

    Keithley 2450: Solar cell characterization

     

     

    3.2 Battery characterization

     

    I discharged a battery using a TSP script made by Keithley to characterize the Voc and ESR curves of a 18650 lithium battery. The output of  the script are 2 files, one of them can be used with the Keithley 2281S Battery simulator. Instead of using the model with a battery simulator I used Python to plot the curves to characterize the battery.

     

    Keithley 2450: Battery characterization

     

     

    3.3 DC-DC converter efficiency measurement

     

    I used the SMU and the Keithley DMM6500 to measure the efficiency of a DC-DC converter at varying input voltages. I used KickStart to synchronize the instruments and then Python to integrate the measurements and generate an efficiency plot.

     

    Keithley 2450: DC-DC Converter

     

     

    3.4 LED characterization

     

    I generated I-V curves of 6 groups of 8 LEDs to see how much spread there is on each type of LED, and performed a simple step-stress test to measure how the the stress shifts the I-V curve of a LED.

     

    Keithley 2450: LED characterization

     

     

    3.5 Capacitor characterization

     

    I programmed the SMU using configuration lists, a timer and a trigger model to measure the dielectric absorption of different type of capacitors.

     

    Keithley 2450: Capacitor characterization

     

     

    4 Conclusions

     

    The 2450 is an impressive instrument. Its electrical specifications are excellent, which probably is not surprising if we consider that Keithley's major focus appears to be SMUs, and they build models for almost any need, focusing as much on laboratory as in production testing usage. The support of all kind of interfaces such as GPIB, Ethernet, USB, TSP-Link and digitial I/O make the instrument extremely flexible. The variety of workflows that the instrument supports gives the user freedom to chose whatever suits him/her better for the application

     

    Even though many tasks can be performed using the front panel, any non-trivial task will require programming. One of the greatest features of the instrument is the support of the TSP scripting language which makes the instrument extremely flexible and not dependent of a computer. The ability to run scripts in standalone also increases the speed at which the instrument can react and perform tasks, as there is no latency caused by sending data back and forth to a computer. This may not be important in a laboratory environment, but it can be of prime importance in production testing.

     

    All in all my experience with the instrument was great, except for one issues.

    The unit that I received appears to be defective, it overheats and shuts down the output when sourcing high power. The reason this occurs appears to be that the fan apparently does not always turn on. I've contacted Keithley and they are trying to help me, but so far it is still not clear what the cause of the issue is. I will provide an update once the issue gets solved.


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