September 2014
TEGAM turns Legacy Products into Niche Market Expertise Interview with Adam Fleder President of TEGAM
Sourcing & Measuring Higher Currents The Logic of Oscilloscopes
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pulse circuits yourself. Pulsed in the device heats, the conduction re often essential for testing current decreases because the carriers vices such as MOSFETs or IGBTs have more collisions with the vibrating DC test currents would skew the lattice (phonon scattering). Therefore, nder-test (DUT) resistance values the measured current will be erroneously ule heating. Although highlow due to self-heating effects. Given How has TEGAM evolved since lse generators are available, that these types of devices typically run the company first started? e no built-in measurement in pulsed mode (i.e., intermittently rather es, so they require synchronizing than continuously), the erroneously low ation of a separate ammeter with DC-measured currents won’t The company was actually started asaccurately a d test signal. reflect their performance. In these contract manufacturer in 1979 by Terry circumstances, pulsed testing must Gambill, the company is named. Sweeps for Higher Power for whom be used.
The most common method of combining SMU instrument channels to achieve higher DC currents is to connect the current sources in parallel across the DUT, as shown in figure 1.
Modern Test & Measure
Since he excelled at mergers and acquisitions, ubstitute a pulsed the sweep for You must take two factors into account company acquired product lines, ep to achieve higher power I-V when making theproduct change from the use especially instrumentation lines. In of with little impact on results. a DC sweep to a pulsed sweep. The pulse the process, TEGAM acquired some Keithley with some DUTs, such as must be wide enough to allow sufficient handhelds. However, by device the time we were s, pulsed sweeps may not time for the transients, cabling, adequately with DC circuitry settle so insweeps negotiationsand toother buy interfacing TEGAM in 2004,tothe large displacement currents, the systemsuch can make stable, repeatable business had changed thata a company n be generated at the sharp measurement. At the same time, however, really had to do its own product development the voltage pulse, may change the pulse cannot be so wide that it instead of acquiring Our vision vices’ electrical properties. On the exceedstechnology. the test instruments’ maximum nd, pulsed IV testingfor is essential pulse width duty cycle limits, which the company was to and bring our product device types, such as high-power would violate instrument’s allowed development expertise tothe bear on TEGAM, r amplifiers and even low-power power duty cycle. Pulses that are too wide which already had good distribution channels e devices, to obtain optimal can also create the same device selfand good manufacturing. uring high-power continuousheating problems that can occur with
“We developed new microwave power-sensor elements, for which we were issued patents last year.” Figure 1. This test setup takes advantage of a wellknown electrical principle (Kirchhoff’s current law), which states that two current Since the acquisition, has been the sources connected to what the same circuit most interesting challenge? node in parallelproduct will have their currents added together. Both SMU instruments source current and measure voltage. All The most interesting project since the of the LO impedance terminals (FORCE acquisition was the development of a and SENSE) of both SMU instruments 2.4 mm,are 50GHz which tied tomicrowave earth ground. standard, Table 1 provides overview microwave of the characteristics this is a veryan accurate powerofsensor particular configuration.
TECH REPORT
The Logic of Oscilloscopes When that’s a Logic Adds Value used inAnalyzer calibrating field sensors. This
to obtain maximum output. Whenever possible, one SMU instrument should be in a fixed source configuration and, the other SMU instrument performs the sweep. This is preferable to having both sweeping simultaneously. If both SMU What are some challenges in the instruments are sweeping, their output industry that TEGAM is uniquely impedances are naturally changing, for situated toasaddress? example, the meter autoranges up and down. The DUT’s output impedance also be changing significantly, such Whatmay interests us right now is continuing to as from a high-resistance off-state to a extend the capability of our RF and microwave low-resistance on-state. With so many power measurement. Recently, for the U. S. of the impedance elements in the circuit Air Force, wethis developed a higher-power RF changing, could increase overall circuit settling time at each point. measurement system thatbias produced low Although this is a transient effect that uncertainty. The product is able to measure damps out, fixing one SMU instrument’s up tosource 100 watts of RFthe power lower and sweeping other with usually uncertainty thanstable anyone else has been results in more and faster-settling measurements, able transient to achieve so far. for higher test throughput.
CONTENTS
New SMU instrument architectures simplify merging the power-enhancement In the case of medium-power RF, it was advantages of the pulse-sweep method necessary to use a well-characterized, waterwith multiple SMU channels operated in cooled RF load and then measure the rise of parallel. For example, some dual-channel SMU instruments allow increasing the the fluid circulating. This determines how number of channels from two to four. much energy is imparted into the water over Using pulse sweep and multichannel time,capabilities which allows theallows ability to calculate, in tandem sourcing far with higher somecurrents basic SI units, power is. than usingwhat a singlethe SMU instrument with DC sweeps. Obviously, Accurate measurement of flow rates and implementing this test method demands temperatures provides a standard to calibrate the exercise of extraordinary caution to the medium-power RF system against. By ensure personnel safety. For safety, it is critical to insulate or install barriers to
Tablean 1. extension of one of TEGAM’s project was existing product lines, but fundamental CHARACTERISTIC DEFINITION ital data logger data: only qualified data is sampled. Interesting data research was needed to differentiate the How didfilters you initiate product new product from the old. WeIDUT gained development within TEGAM? DUT Current = ISMU1 + ISMU2 de enough to allow understanding at the physics level of how device transients, DUT VDUT = VSMU1 =at VSMU2 to develop a Voltage circuit that would operate We had to rebuild the development team er interfacing those frequencies as well as how to develop first, so we brought in people from various settle.” Maximum Source Current IMAX = IMAX SMU1 + IMAX SMU2 new sense elements, for which we were companies. However, it took time for the team issued patents last year. It was interesting, to be a cohesive unit. This was ultimately Smaller of the two SMUs maximum Maximum Voltage voltage capabilities challenging, and we ultimately succeeded in accomplished by working together on what we set out to do. development projects.
W) testing, the semiconductor
DC sweeps.
TECH REPORT
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HowMerging exactly do Sweeps you address the Pulse measurement uncertainty aspect?
is seen.
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identifying in the syste are able to s of uncertain
When we lo develop, we handle very and microw also measu well. Measu to nano-oh We leverage people who or electrica portable me and oil refin TEGAM has In the instru buy based o making a sp At TEGAM, with micro-
“Mea Conclusions filtering when a Boolean equation from Going Beyond the Spec from Mixed-signal oscilloscopes are extremely n digital lines is true, or even recording a nano Sourcing and Measuring Higher Currents well-suited to most basic and advanced ou fined data quantity each time a trigger testing and debugging tasks on all kinds tion is encountered. of electronic systems. For this reason, the in an additional external logic available memory is important, but What Will Take the Placeinvestment of the Traditional Curve Tracer must be thought out carefully. ent data storage, data qualification, Tegam Turns Legacy Productsanalyzer into?Expertise It’s generally acknowledged that curve tracers, originally introduced Picking a logic analyzer just because ch triggering options bring significant in the late 1950s for characterizing tubes, then later for Adam Fleder, President of vacuum TEGAM transistors, are no longer up to the task of characterizing advanced it ishas 100 channels won’t probably be to different logic analyzer products. power semiconductor devices. The most obvious reason that they are simply no longer available—their production ended in 2007. But even those device design engineers whoworthwhile, still have since alternative and more g relevant data is more important than working curve tracers in their labs face a more significant problem: High Speed Microohm Resistance Meter Digital Thermometer Calibrator Microohmmeter and Bond Tester with Rugged Case M these instruments simply lack the precision and other capabilities cost-effective debugging g all the data. A(Model logic 1740)analyzer capable 840A) (Model R1L-BR) strategies exist. necessary(Model to characterize the latest power semiconductor devices. This is particularly challenging for devices based on materials, gallium nitride (GaN) and silicon carbide (SiC), which have a qualification or data filtering, which is including lower leakage current and lower on-state resistance than devices based on traditional silicon technologies. But a logic analyzer with a multitimes referred to as a data logger, is an and Edges Curves megasample memory, state analysis ent companion tool to an MSO. Replacing the Traditional Curve Tracer capability, and data qualification capability will add value to any common gic analyzers look at signals MSO. A logic analyzer chosen with these e way hardware does. features can view a system the same way as a circuit does and save data e most MSOs, logic analyzers are able to during large windows of time. This kind he reference sampling clock signal of the of logic analyzer is an ideal companion m under test. This means looking at the to a traditional MSO since it provides led signals in sync with the eyes of your the complementary analysis that can ware. This is mode is called “state analysis,” speed up debugging tasks and make any posed to “timing analysis,” where the engineer more efficient in debugging ling clock is generated by the equipment complex digital problems, not only Running equipment in state analysis adding value to an MSO, but adding can be a challenge, because a clean value to your employment! ence clock signal from a system under 3 may not always be available. Nevertheless,
CURVES EDGES
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TECH REPORT
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INDUSTRY INTERVIEW
By David Wyban Applications Engineer at Keithley Instruments
Modern Test & Measure
The Logic of
Oscilloscopes Understanding when a logic analyzer adds value to your mixed-signal scope By Frédéric Leens, CEO of Byte Paradigm, Belgium & Alan Lowne, CEO of Saelig Co. Inc., New York
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TECH REPORT
W
With mixed-signal oscilloscopes (MSOs) being everyone’s “engineering” Swiss Army knife, why would anyone need an additional logic analyzer. MSOs with sampling rates in the GHz range and 8 or more digital lines are now available for well under $3,000 and some even under $1,000. For this reason, many in the electronic industry are announcing the demise of the logic analyzer as a piece of stand-alone equipment.
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Modern Test & Measure
I
t is no surprise that mixedsignal oscilloscopes are to be found in most electronic engineering labs today. They are versatile, affordable, and have become the basic instrument for any engineer who is testing, debugging, or verifying electronic systems. In fact, this may be the only instrument that most electronic engineers will ever have to or perhaps want to use for 90 percent of their lab time. So, it is wise to spend part of an initial engineering or testlab budget on an MSO. But does this mean that there is no longer any need for a logic analyzer (LA)?
Oscilloscopes Versus Logic Analyzers Digital oscilloscopes and logic analyzers are based on sampling techniques. Signal, usually voltage, measurements are transformed into a digital value by a highspeed analog-to-digital converter (ADC) and stored into memory at regular time intervals defined by the instrument’s sampling clock. A logic analyzer can be thought of as a scope with 1-bit vertical resolution on all channels. It displays signals as logic (binary) values, according to whether the measured voltage is above or below a conventional voltage level called the “threshold value.” That is the first fundamental difference between an oscilloscope and a logic analyzer.
Full resolution
1-bit resolution
“A fundamental difference between an oscilloscope and a logic analyzer is how the sampled values are displayed.”
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TECH REPORT The other fundamental difference between an oscilloscope and a logic analyzer is how the sampled values are displayed. In its most common mode of operation, an oscilloscope is essentially a device that repeatedly captures a window of events of a given length (defined by its total memory) and refreshes the display of a portion of it on a screen. Many oscilloscopes simulate “persistence” by superimposing multiple captured windows on the display and by modulating the screen pixel intensity. A logic analyzer is mostly used for single-shot captures (no superimposition of successive captures) and to analyze the sequence of events of sometimes over more than 100 digital signals before and after a trigger event.
It was the appearance of microcontrollerbased systems that required the creation of tools like logic analyzers. First, there was a need to observe digital busses, therefore requiring more than two or four channels. Second, there was a need to see the signals the way logic circuit does, i.e. at the sampling events of the circuit, in the form of binary values. Over time, logic analyzers have evolved into less “pure” instruments, with an ability to perform some analog measurements such as for checking threshold levels, detecting glitches, and verifying the compliance of signals to specific input-output standards.
“Real-Time,” Really? It is very common to hear that the real-time display capability is the main difference between a scope and a logic analyzer. In fact, the automatic display refresh may mislead users into thinking that the user sees data as it occurs. But the scope display is not really refreshed faster than the eye is able to see. In most cases, a logic analyzer (LA) is used by first capturing data and then analyzing it. The logic analyzer’s repetitive trigger capability also enables refreshing the display based on the repetitive occurrence of a trigger event. It is true that data is displayed and presented differently in digital scopes and LAs, but fundamentally, both tools operate by sampling signals and storing the samples into memory.
“Seeing relevant data is more important than seeing all the data.”
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Modern Test & Measure MSO = Oscilloscope + Logic Analyzer? Well, mostly. A mixed-signal oscilloscope features analog channels (usually 2 to 4) and digital channels (usually 8 to 16). On both types of channels, data is sampled at the MSO’s maximum sampling rate, 1GHz, typically. The sampling clock is usually generated internally by the MSO. In other words, the reference time base for sampling is not correlated with the data. This is what is referred to as “timing analysis.” And of course, the logic analyzer’s vertical signal resolution is reduced to 1 bit for the digital channels. MSOs are able to perform some of the functionality traditionally reserved for LAs:
screen is certainly an improvement compared to using a scope and a logic analyzer separately. With electronic systems evolving towards more complexity, debugging often involves finding a mix of analog and logic problems. An MSO’s triggering can be defined by either type of signal. Repetitive (oscilloscope-like) or singleshot (logic analyzer-like) types of display can be used as well. But displaying both analog and digital recording on the same screen as time-correlated data is one of the biggest advantages of MSOs.
MSOs in Need When you think you need a logic analyzer, consider the following:
• Timing analysis on digital signals. • Ability to see more than 2 or 4 channels; on modern MSO, 16 digital inputs are available. • Digital signal integrity check. In this respect, being able to visualize both the analog extension of a digital signal and its digital version on the same
“The FPGA world is so advanced that observing chip behavior through external digital input-outputs may not make much sense any more.”
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1. Logic analyzers have a larger number of digital channels than MSOs. Traditional bench-top logic analyzers can count up to 128 digital channels or more. On MSOs, there are usually a maximum of 16 channels. Applicationspecific integrated circuit (ASIC) and fieldprogrammable gate array (FPGA) design
TECH REPORT engineers, who are heavy LA users, often need 32 to 64 signals and more to decode bus transactions or visualize the internals of a FPGA. But for 80 to 90 percent of the time, however, and even if you are involved in IC design, 16 digital channels are plenty. The rare 1 to 10 percent cases where you’d like to see 100 digital signals in parallel may not justify the investment in a high-end 100-channel logic analyzer which can cost over $20,000. FPGA design is an area where large channel-count logic analyzers used to be basic equipment for debugging. Adding a connector on board and observing the chip through its input-output, or even putting a debug connector on chip-tochip interconnections was very useful for troubleshooting the design. But thanks to its programmable nature, FPGAs allow many internal nodes to be monitored by simply routing them on to the input-outputs connected to the LA. Things have changed; the FPGA world is getting so advanced that observing chip behavior through external digital inputoutputs may not make much sense any more, even if you can see 100 of them in parallel. Today, an FPGA may run at over 300MHz internally, and putting a 100pin high-speed parallel connector on a board brings excessive board constraints (noise, number of layers, crosstalk, etc.) In addition, the chip input-output buffers sometimes are unable to run at the same speed as the internals of the chip. Other approaches, such as using embedded logic analyzers or software-based
debugging, have now become far more efficient and cost effective. The “number of digital channels advantage� of logic analyzers versus MSOs must be considered cautiously. 16 to 32 digital lines will be enough for most engineers. Logic analyzers with 68 to 100 or even more channels can make a difference where there is an absolute need to see more digital signals in parallel. However, this must be carefully balanced with the constraints of adding the required debug connector to a system. In a context where the digital complexity shifts towards inside the chip, it is increasingly difficult to probe highfrequency signals without creating signal or data integrity issues. For this reason, the investment of usually less than $20,000 in a logic analyzer with a large channel count may be worthwhile. 2. Logic analyzers have a larger memory than MSOs. While there may be exceptions since MSO equipment is constantly evolving, one potential strength of a logic analyzer is that it can justify its addition to an existing MSO setup because it helps in seeing a larger window of time before and after a triggering event. Seeing a larger time of execution of a digital system is of great value during debug. It has the potential to speed up an understanding of why a bug occurs and hence speed up bug correction. It can pay for itself rather quickly! Additionally, a large total available memory size in a logic analyzer
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Modern Test & Measure
adds significant value to an MSO. Large memory means seeing more history of observed signals. 3. Logic analyzers allow more complex triggering or data filtering than MSOs. The total amount of available memory is not the only parameter to consider. How the instrument uses the memory to store data is equally important. How much of the signal history is observable? How useful is this information in debugging the system being tested? Consider how logic analyzers and MSOs select data. Most MSOs are able to trigger on simple events such as a voltage level, or a digital value, or transition on specific digital lines. Where they also provide simple serial bus decoding such as I²C or SPI, MSOs can also be useful for utilizing serial triggers, that is, stopping the capture on the occurrence of a predefined serial value on a digital line. On the other hand, all logic analyzers are able to trigger on a parallel value. They are also able to build up complex sequences of conditions to ultimately trigger the capture of data. Many digital system busses are quiet most of the time. Basic logic analyzers, even when they are fully loaded with memory, risk wasting memory resources with samples even if nothing happens. To make the most of the logic analyzer memory, several strategies coexist. Therefore, some logic analyzers only store data transitions,
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thereby potentially compressing the collected data. This is a somewhat uncontrolled way to save on memory and is highly dependent on the data itself. Another strategy for economical storage consists of using logic equations on the recorded data to define the conditions when the logic analyzer should store the data or when it can discard it. Data qualification, or data filtering, is the ability of the logic analyzer to record only the data that matches user-defined criteria. In this case, the logic analyzer is also referred to as a digital data logger. This strategy is based on prior knowledge of the debugged digital system. It can be as simple as detecting a signal level (e.g. an output enable) and recording the values of a bus when this level is seen. Or it can be much more complex,
“With electronic systems evolving towards more complexity, debugging often involves finding a mix of analog and logic problems.”
An example of a logic analyzer with long memory and complex triggering options for data analysis is the LOG Storm - Logic Analyzer - High Speed Data Logger.
• Large 8-Meg Sample memory (20-bit). • State analysis up to 125MHz.
• Advanced data qualification. • SPI and I2C monitoring, zero latency trigger rearm, and more.
TECH REPORT
The digital data logger filters data: only qualified data is sampled.
like a filtering when a Boolean equation from certain digital lines is true, or even recording a predefined data quantity each time a trigger condition is encountered. Total available memory is important, but efficient data storage, data qualification, and rich triggering options bring significant value to different logic analyzer products. Seeing relevant data is more important than seeing all the data. A logic analyzer capable of data qualification or data filtering, which is sometimes referred to as a data logger, is an excellent companion tool to an MSO. 4. Logic analyzers look at signals the way hardware does. Unlike most MSOs, logic analyzers are able to use the reference sampling clock signal of the system under test. This means looking at the sampled signals in sync with the eyes of your hardware. This is mode is called “state analysis,” as opposed to “timing analysis,” where the sampling clock is generated by the equipment itself. Running equipment in state analysis mode can be a challenge, because a clean reference clock signal from a system under test may not always be available. Nevertheless, state analysis will provide close insight into a system’s embedded software, allowing focus to be placed on the information seen by the hardware and speed up debugging.
Interesting data is seen.
Conclusions Mixed-signal oscilloscopes are extremely well-suited to most basic and advanced testing and debugging tasks on all kinds of electronic systems. For this reason, the investment in an additional external logic analyzer must be thought out carefully. Picking a logic analyzer just because it has 100 channels won’t probably be worthwhile, since alternative and more cost-effective debugging strategies exist. But a logic analyzer with a multimegasample memory, state analysis capability, and data qualification capability will add value to any common MSO. A logic analyzer chosen with these features can view a system the same way as a circuit does and save data during large windows of time. This kind of logic analyzer is an ideal companion to a traditional MSO since it provides the complementary analysis that can speed up debugging tasks and make any engineer more efficient in debugging complex digital problems, not only adding value to an MSO, but adding value to your employment!
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Modern Test & Measure
GOING BEYOND the Spec By David Wyban Keithley Instruments
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TECH REPORT
SOURCING AND MEASURING Higher Currents
H
igh-power test applications, such as characterizing solar cells, power management devices, high-brightness
LEDs, and radio-frequency (RF) power transistors, often require high currents, sometimes as much as 40A or even higher for power metal-oxide semiconductor fieldeffect transistors (MOSFETs) and insulatedgate bipolar transistors (IGBTs), which can require currents in excess of 100A. However, the maximum DC current that a single supply is specified to handle is typically limited. This spec limit is typically dependent on the power supply’s design, the safe operating area of the discrete components used in the instrument, the spacing of the metal lines on the internal printed circuit board, and so on. If you need to increase the amount of current sourced, and you use a SMU (source measure unit) instrument, you can employ several test modes and multiple channels.
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Modern Test & Measure
Although DC current sources typically don’t let you pulse their outputs, you can build pulse circuits yourself. Pulsed sources are often essential for testing power devices such as MOSFETs or IGBTs because DC test currents would skew the device-under-test (DUT) resistance values due to Joule heating. Although highpower pulse generators are available, they have no built-in measurement capabilities, so they require synchronizing the operation of a separate ammeter with the pulsed test signal.
Pulsed Sweeps for Higher Power
material itself will start to dissipate the applied power as heat. As the material in the device heats, the conduction current decreases because the carriers have more collisions with the vibrating lattice (phonon scattering). Therefore, the measured current will be erroneously low due to self-heating effects. Given that these types of devices typically run in pulsed mode (i.e., intermittently rather than continuously), the erroneously low DC-measured currents won’t accurately reflect their performance. In these circumstances, pulsed testing must be used.
You can substitute a pulsed sweep for a DC sweep to achieve higher power I-V sweeps with little impact on results. However, with some DUTs, such as capacitors, pulsed sweeps may not correlate adequately with DC sweeps because large displacement currents, which can be generated at the sharp edges of the voltage pulse, may change these devices’ electrical properties. On the other hand, pulsed IV testing is essential for other device types, such as high-power RF power amplifiers and even low-power nanoscale devices, to obtain optimal results. During high-power continuouswave (CW) testing, the semiconductor
You must take two factors into account when making the change from the use of a DC sweep to a pulsed sweep. The pulse must be wide enough to allow sufficient time for the device transients, cabling, and other interfacing circuitry to settle so the system can make a stable, repeatable measurement. At the same time, however, the pulse cannot be so wide that it exceeds the test instruments’ maximum pulse width and duty cycle limits, which would violate the instrument’s allowed power duty cycle. Pulses that are too wide can also create the same device selfheating problems that can occur with DC sweeps.
“The pulse must be wide enough to allow sufficient time for the device transients, cabling, and other interfacing circuitry to settle.”
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TECH REPORT Combining Multiple SMU Instrument Channels The most common method of combining SMU instrument channels to achieve higher DC currents is to connect the current sources in parallel across the DUT, as shown in figure 1.
Figure 1. This test setup takes advantage of a wellknown electrical principle (Kirchhoff’s current law), which states that two current sources connected to the same circuit node in parallel will have their currents added together. Both SMU instruments source current and measure voltage. All of the LO impedance terminals (FORCE and SENSE) of both SMU instruments are tied to earth ground. Table 1 provides an overview of the characteristics of this particular configuration. Table 1. CHARACTERISTIC
DEFINITION
DUT Current
IDUT = ISMU1 + ISMU2
DUT Voltage
VDUT = VSMU1 = VSMU2
Maximum Source Current
IMAX = IMAX SMU1 + IMAX SMU2
Maximum Voltage
Smaller of the two SMUs maximum voltage capabilities
You should set the output currents for SMU1 and SMU2 to the same polarity to obtain maximum output. Whenever possible, one SMU instrument should be in a fixed source configuration and, the other SMU instrument performs the sweep. This is preferable to having both sweeping simultaneously. If both SMU instruments are sweeping, their output impedances are naturally changing, for example, as the meter autoranges up and down. The DUT’s output impedance may also be changing significantly, such as from a high-resistance off-state to a low-resistance on-state. With so many of the impedance elements in the circuit changing, this could increase overall circuit settling time at each bias point. Although this is a transient effect that damps out, fixing one SMU instrument’s source and sweeping the other usually results in more stable and faster-settling transient measurements, for higher test throughput.
Merging Pulse Sweeps New SMU instrument architectures simplify merging the power-enhancement advantages of the pulse-sweep method with multiple SMU channels operated in parallel. For example, some dual-channel SMU instruments allow increasing the number of channels from two to four. Using pulse sweep and multichannel capabilities in tandem allows sourcing far higher currents than using a single SMU instrument with DC sweeps. Obviously, implementing this test method demands the exercise of extraordinary caution to ensure personnel safety. For safety, it is critical to insulate or install barriers to
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Modern Test & Measure
prevent user contact with live circuits. Additional protection techniques are needed to prevent damage to the test setup or the DUT. The multiple pulses must be tightly synchronized (with nanosecond precision) so that one piece of equipment is not applying power and damaging units that are not yet turned on.
Figure 2.
PLEASE REPLACE THIS IMAGE / BLURRY
Figure 3.
Figure 2 shows the results of an experiment in which Keithley engineers used a single SMU instrument to generate a 10A pulse; the results were observed on an oscilloscope. A high power precision resistor (0.01W, Âą0.25%, KRL R-3274) was used as the test DUT with a pulse width of 300 microseconds. The oscilloscope showed a nearly square waveform of 0.1V (10A Ă— 0.01 ohm) in amplitude and 300 microsecond width. Combining four SMU instruments in parallel to pulse 40A across the same DUT resulted in a waveform of 0.4V magnitude with excellent synchronization (low jitter) between the channels (figure 2). Pulse consistency was verified using the same test setup and pulse waveform. With the pulse performance verified, the engineers programmed a pulse sweep that combined four SMU instruments and took an I-V curve on a P-N diode DUT (figure 3). Note the correlation of the oneSMU instrument DC sweeps up to 3A with the one-SMU instrument pulse sweep up to 10A. Then, the engineers extended the achievable I-V curve up to 40A. This experiment verified the validity of combining four SMU instrument channels and pulsing to achieve 40A on two-
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TECH REPORT resistance, such as 10 milliohms in the test cable, can generate a voltage drop of 0.4V. So if the SMU instrument is forcing 1V at 40A current and the cable resistance is 10 milliohms and there are two test leads, the DUT might only receive a voltage of 0.2V, with 0.8V dropped across the test cables.
terminal devices (resistor and diode). With certain modifications, this technique is equally valid when applied to testing a three-terminal device, such as a highpower MOSFET. Several implementation factors are critical to maximizing the accuracy and precision of the results obtained using this multi-SMU instrument pulsed sweep approach:
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Using source readback: An SMU instrument has both source and measure functions built into the same unit, so it’s capable of reading back the actual value of the applied voltage using its measurement circuitry. The programmed value for the source voltage may not be the same as the voltage actually applied to the DUT; with multiple SMU instruments in parallel, the source offsets may add up to be quite significant, so using source readback provides a clearer picture of the level of voltage actually being sourced, not just the voltage that’s been programmed. Making four-wire measurements: Four-wire (Kelvin) measurements are necessary when doing high-current testing because this technique bypasses the voltage drop in the test leads by bringing two very highimpedance voltage sense leads out to the DUT. With very little current flowing into the SENSE leads, the voltage seen by the SENSE terminals is virtually the same as the voltage developed across the unknown resistance. At 40A levels, even a small
Unlike source readback, which primarily impacts just the source values, making four-wire measurements will result in significantly better accuracy on both the sourced and measured values because they eliminate the voltage drop in the current-carrying wires that would otherwise affect the measurement.
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“Keithley engineers programmed a pulse sweep that combined four SMU instruments.”
Putting no more than one voltage source at each DUT node: It is common in many test sequences to perform voltage sweeps (force voltage) and measure current (FVMI). In the case where more than one SMU instrument is connected in parallel to a single terminal of the device, the obvious implementation would be to have all of the SMU instruments in voltagesource mode and measure current. However, three factors must be considered. First, SMU instruments when sourcing voltage are in a very low-impedance state. Second, DUTs can have impedances higher than an SMU instrument that’s in voltagesource mode. The DUT’s impedance can be static or dynamic, changing during the test sequence. Third, even when all SMU instruments in parallel are programmed to output the same voltage, small variations between
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Modern Test & Measure
SMUs related to the instruments’ voltage source accuracy mean that one of the SMU channels will be at a slightly lower voltage (millivolt order of magnitude) than the others. So, if three SMU instruments are connected in parallel to one terminal of a DUT, and each SMU instrument is forcing voltage and outputting nearmaximum currents, and the DUT is in a high-impedance state, then all current will go to the SMU instrument that is sourcing the slightly lower voltage, which will most likely damage it. Therefore, when connecting SMU instruments in parallel to a single terminal of a DUT, only one SMU instrument should be sourcing voltage. Refer to figure 4 for both correct and incorrect approaches to connecting SMU instruments in parallel.
•
Mitigating excessive energy dissipation due to contact failure: When you connect two SMU instruments with the same output capacity in parallel to a single node in the circuit, one SMU instrument must be able to sink all of the current being output by the other SMU instrument. This scenario can occur, for example, when one of the leads breaks contact with the device (such as when the lead is accidentally disconnected or a contact isn’t made properly). That means there is a short period during which one SMU instrument must sink all the current from the other instrument. However, when there are more than two SMU instruments connected in parallel at a single circuit node, a single SMU instrument cannot sink all of the current coming from the other SMU instruments. The SMU instrument that
Figure 4. Incorrect and correct configurations for connecting SMU instruments in parallel. a) This incorrect configuration could allow high currents to damage the SMU instrument that is sourcing a slightly lower voltage than the other one. b)This quasi-Kelvin configuration, although it doesn’t run the same risk of instrument damage as the first configuration, introduces additional error into the measurement that must be accounted for in the system’s error budget and limits the maximum output of the SMU instrument. c) This “hybrid” approach also prevents SMU instrument damage and allows adding SMU current sources as needed to reach the application’s current sourcing requirements.
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TECH REPORT will be forced to sink current if there’s a break in contact with the device is the one at the lowest voltage or lowest impedance, most likely the one sourcing voltage. In order to protect the signal input of the SMU instrument forcing voltage, a diode such as the 1N5820 can be used. A diode is preferable because a fuse would react too slowly to provide protection and a resistor will cause too large of a voltage drop across it. A diode offers a much faster response than a fuse and has a much smaller maximum voltage drop across it (typically around 1V) than a resistor. However, to be truly safe when using this method, a diode should be used to protect all the SMUs in the configuration. If the DUT goes into a high-impedance state, the current sources will try to force their current into the voltage-sourcing SMU instrument, but that would not be possible because the voltagesourcing SMU instrument is protected by a diode. That would cause the current-sourcing SMU instruments to increase their output voltage until they reached their voltage limit. Once this occurred, the current sources would go into compliance and become voltage sources themselves. That would mean there were now multiple voltage sources in parallel. Even if their voltage limits were set to exactly the same value, their outputs would still likely be very slightly different, and they would damage each other. It’s important to be aware that putting a diode on each and every SMU instrument in the configuration has some
“A diode offers a much faster response than a fuse and has a much smaller maximum voltage drop across it than a resistor.” consequences. First, the inclusion of any diodes in the configuration means this method can only be used to source power but not to sink it because the diodes will not allow current to pass into the SMU instrument. The second consequence is that, in order to obtain maximum output voltage, you will need to use four-wire connections on the current sources around the diode because the voltage drop across diode may cause the current sources to reach compliance prematurely. At these current levels, the typical voltage drop across a diode is about 1V.
Summary SMU instruments offer a simple, highly integrated approach to designing test and measurement systems for a wide range of electronic devices. For the growing number of test applications that demand the ability to source or measure higher currents, the techniques outlined in this article offer useful, cost-effective alternatives to combining separate sources and measurement instruments. For more information on techniques for implementing high-current test configurations, download Keithley’s Application Note #3047, “Methods to Achieve Higher Currents from I-V Measurement Equipment,” available at www.keithley.com/data?asset=52630.
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Modern Test & Measure
Cabling and Test Fixture
SAFETY
In general, test cabling and test connections must all be designed to minimize resistance (R), capacitance (C), and inductance (L) between the DUT and SMU instrument. To minimize resistance, use heavy-gauge wire wherever possible and definitely within the test fixture itself. The gauge required will depend on the level of current being carried; for example, for cabling that must carry 40A, 12-gauge cable is probably necessary. For guidance on choosing cabling for higher current levels, refer to construction industry wiregauge tables, such as the one available at: www.powerstream.com/Wire_Size. htm. Check the “Maximum Amps for Chassis Wiring” column to find the wire gauge needed to carry the level of current involved. Low-resistance cabling is critical to preventing instrument damage. Choose cables with resistances of less than 30 milliohms per meter or lower for 10A pulses. Keep cable lengths as short as possible and always use low-inductance cables (such as twisted-pair or lowimpedance coax types), heavy-gauge cable in order to limit the voltage drop across the leads. Ensure the voltage drop won’t be excessive by checking the SMU instrument’s voltage output headroom spec. For example, if you were using a Keithley Model 2602A SMU instrument to
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output 20V, the test leads should have no more than 3V of voltage drop across them to avoid inaccurate results or instrument damage. It is specified for a maximum voltage of 3V between the HI and SENSE HI terminals and a maximum voltage of 3V between LO and SENSE LO. Although many believe guarding can minimize the effects of cable charging, this is typically more of a concern for high-voltage testing than for high-current testing. Four-wire Kelvin connections must be kept as close to the DUT as possible; every millimeter makes a difference. Also, it should be noted that voltage readback should be done with the SMU instrument that’s forcing voltage, because the current-sourcing instruments’ voltage readings will all vary quite a bit due to the connections, and will differ from what is actually seen at the DUT. The jacks used on the test fixture should be of known high quality. For example, some red jacks use high amounts of ferrous content to produce the red coloring, which can lead to unacceptably high levels of leakage due to conduction. The resistance between the plugs to the case should be as high as possible and in all cases >1010 ohms.
TECH REPORT
Many published test setups recommend adding a resistor between the SMU instrument and the device’s gate when testing a field-effect transistor (FET) or IGBT. When pulsing large amounts of current through these kinds of devices, they tend to oscillate. Inserting a resistor on the gate will dampen these oscillations, thereby stabilizing the measurements; because the gate does not draw much current, the resistor does not cause a significant voltage drop. If voltages in excess of 40V will be used during the test sequence, the test fixture and SMU instruments must have the proper interlock installed and be operated in accordance with normal safety procedures. Many electrical test systems or instruments are capable of measuring or sourcing hazardous voltage and power levels. It’s also possible, under single fault conditions (e.g., a programming error or an instrument failure), to output hazardous levels even when the system indicates no hazard is present. These high levels make it essential to protect operators from any of these hazards at all times.
Protection methods include:
• • • •
• • •
Verify the operation of the test setup carefully before it is put into service. Design test fixtures to prevent operator contact with any hazardous circuit. Make sure the device under test is fully enclosed to protect the operator from any flying debris. Double insulate all electrical connections that an operator could touch. Double insulation ensures the operator is still protected even if one insulation layer fails. Use high reliability, fail-safe interlock switches to disconnect power sources when a test fixture cover is opened. Where possible, use automated handlers so operators do not require access to the inside of the test fixture or have a need to open guards. Provide proper training to all users of the system so they understand all potential hazards and know how to protect themselves from injury. It’s the responsibility of the test system designers, integrators, and installers to make sure operator and maintenance personnel protection is in place and effective.
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Modern Test & Measure
TEGAM Turns Legacy Products
into Niche Market Expertise Interview with Adam Fleder, President of TEGAM
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INDUSTRY INTERVIEW
TEGAM specializes in test, measurement, and calibration instruments, which they design, manufacture, and support. The company is internationally recognized for its offering of radio frequency (RF) power sensor calibration systems, RF attenuation measurement systems, ratio standards, and ohm meters. In addition, the company supplies inductance-capacitance-resistance (LCR) meters and impedance bridges, arbitrary waveform generators, high-voltage amplifiers, microelectromechanical systems (MEMS) engine drivers, handheld thermometers, humidity meters, and other test and measurement solutions. In an interview with EEWeb, Adam Fleder, president of TEGAM, discussed the company’s initiation of product development, their research and development for power sensors, and extending their capability in microwave power measurement. Fleder also discussed measurement uncertainty as well as how the company leverages their expertise with micro-ohms.
RF Thermistor Power Meter (Model 1830A)
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Modern Test & Measure
How has TEGAM evolved since the company first started? The company was actually started as a contract manufacturer in 1979 by Terry Gambill, for whom the company is named. Since he excelled at mergers and acquisitions, the company acquired product lines, especially instrumentation product lines. In the process, TEGAM acquired some Keithley handhelds. However, by the time we were in negotiations to buy TEGAM in 2004, the business had changed such that a company really had to do its own product development instead of acquiring technology. Our vision for the company was to bring our product development expertise to bear on TEGAM, which already had good distribution channels and good manufacturing.
How did you initiate product development within TEGAM? We had to rebuild the development team first, so we brought in people from various companies. However, it took time for the team to be a cohesive unit. This was ultimately accomplished by working together on development projects.
High Speed Microohm Resistance Meter (Model 1740)
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“We developed new microwave power-sensor elements, for which we were issued patents last year.”
Since the acquisition, what has been the most interesting product challenge? The most interesting project since the acquisition was the development of a 2.4 mm, 50GHz microwave standard, which is a very accurate microwave power sensor that’s used in calibrating field sensors. This project was an extension of one of TEGAM’s existing product lines, but fundamental research was needed to differentiate the new product from the old. We gained understanding at the physics level of how to develop a circuit that would operate at those frequencies as well as how to develop new sense elements, for which we were issued patents last year. It was interesting, challenging, and we ultimately succeeded in what we set out to do.
Digital Thermometer Calibrator (Model 840A)
INDUSTRY INTERVIEW What are some challenges in the industry that TEGAM is uniquely situated to address? What interests us right now is continuing to extend the capability of our RF and microwave power measurement. Recently, for the U. S. Air Force, we developed a higher-power RF measurement system that produced low uncertainty. The product is able to measure up to 100 watts of RF power with lower uncertainty than anyone else has been able to achieve so far.
How exactly do you address the measurement uncertainty aspect? In the case of medium-power RF, it was necessary to use a well-characterized, watercooled RF load and then measure the rise of the fluid circulating. This determines how much energy is imparted into the water over time, which allows the ability to calculate, with some basic SI units, what the power is. Accurate measurement of flow rates and temperatures provides a standard to calibrate the medium-power RF system against. By
Microohmmeter and Bond Tester with Rugged Case (Model R1L-BR)
identifying all of the sources of loss and error in the system and then addressing them, we are able to significantly reduce the level of uncertainty. When we look at what products we should develop, we have a couple of areas that we handle very successfully. Besides the RF and microwave power measurement, we also measure very low resistances quite well. Measuring anywhere from 1 ohm down to nano-ohms is an area of specialty for us. We leverage that to produce equipment for people who are testing electrical assemblies or electrical components. We also make portable measurements used on aircraft and oil refineries. These are areas in which TEGAM has a reputation and brand equity. In the instrumentation business, customers buy based on a company’s experience making a specific type of measurement. At TEGAM, we are known for our expertise with micro-ohms.
“Measuring anywhere from 1 ohm down to nano-ohms is one of our specialties.”
Microwave Calibration Standard (Model 2510A)
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Modern Test & Measure
AND
CURVES EDGES
30
TECH REPORT
What Will Take the Place of the Traditional Curve Tracer ? It’s generally acknowledged that curve tracers, originally introduced in the late 1950s for characterizing vacuum tubes, then later for transistors, are no longer up to the task of characterizing advanced power semiconductor devices. The most obvious reason is that they are simply no longer available—their production ended in 2007. But even those device design engineers who still have working curve tracers in their labs face a more significant problem: these instruments simply lack the precision and other capabilities necessary to characterize the latest power semiconductor devices. This is particularly challenging for devices based on materials, including gallium nitride (GaN) and silicon carbide (SiC), which have lower leakage current and lower on-state resistance than devices based on traditional silicon technologies.
By David Wyban Applications Engineer at Keithley Instruments
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Modern Test & Measure
DAWN OF THE PARAMETRIC CURVE TRACER In response to these characterization challenges, device designers are turning to a new concept known as a parametric curve tracer, which combines the realtime, interactive simplicity of a curve tracer with the high-precision and parametric extraction capabilities of a modern parametric analyzer. In addition to one or more source measure unit (SMU) instruments and semiconductor characterization instruments, they include cables, a test fixture, software, and test libraries to provide measurements at up to 3000 volts and up to 100 amps. As new test needs evolve, their modular architecture can be
“Trace test mode is very handy for quickly determining whether a device is good or bad.�
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reconfigured easily in the field. These parametric configurations offer the power required for the vast majority of high-power device design and development applications, and are optimized to address the characterization and test needs of research, reliability, failure analysis, and power-device applications engineers as well as power device designers, incoming inspection technicians, and many others.
TEST MODES Parametric curve tracers offer two modes of operation: trace test mode and parametric test mode. Trace test mode presents an interface similar to the controls and display found on a traditional curve tracer. It allows for rapid generation of device characteristics and for interactive operation based on viewing the results in the graph. It incorporates knowledge of many device types and tests, which speeds and simplifies test setup. An on-screen slider provides real-time control and acts like the knob found on the traditional curve tracer. Trace test mode is very handy for quickly determining whether a device is good or bad. This mode is often used during device development or failure analysis.
TECH REPORT
Watch Keithley’s short demo on Trace Test Mode
Watch Keithley’s short demo on Parametric Test Mode
“A parametric curve tracer combines the real-time, interactive simplicity of a curve tracer with the high-precision capabilities of a modern parametric analyzer.”
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Modern Test & Measure
Parametric test mode offers access to all of the advanced capabilities of the SMU instruments within the PCT, allowing users to specify exactly how a test is to be performed. Built-in test libraries provide support for the most common device and test types; a vector math formulator supports accurate parameter extraction on these devices. Once configured, an entire suite of tests can run autonomously without operator intervention. This mode is often used in device qualification, process monitoring and datasheet generation applications.
CONCLUSION Lack of availability and emerging high-power test challenges have made traditional curve tracers untenable solutions for power device characterization. Fortunately, the extended capabilities that parametric curve tracers offer mean that engineers can be confident solutions for their emerging characterization challenges are available.
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“Once configured, an entire suite of tests can run autonomously without operator intervention.”
David Wyban is an applications engineer with Keithley Instruments, Inc., Cleveland, Ohio, which is part of the Tektronix test and measurement portfolio. He joined the company in 2006, working on the team that developed Keithley’s line of System SourceMeter® instruments. He holds a bachelor’s degree in electrical and computer engineering from Ohio State University. He can be reached at dwyban@keithley.com.
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