eeweb pulse - volume 18

PULSE EEWeb.comIssue 18

November 1, 2011

David L. JonesWorld’s Preeminent Engineering Video Blogger

Electrical Engineering Community

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TABLE O

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NTEN

TSTABLE OF CONTENTS

David L. Jones 4ELECTRONICS DESIGN ENGINEER; SYDNEY, AUSTRALIAInterview with David L. Jones - Engineering Video Blogger

μWatch 2 Concept BY DAVID L. JONES

Featured Products

Forward Error Correction: Choosing Between Hard-Decision and Soft- Decision CodesBY FRANK CHANG WITH VITESSE

Electrical Measurement Considerations for Nanoscale Devices and MaterialsBY ROBERT GREEN WITH KEITHLEY

RTZ - Return to Zero Comic 19

9

An introduction to Vitesse Semiconductor’s promising new forward error-correction methods.

Jones describes his idea for the construction of a scientific calculator wristwatch.

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Green offers important considerations for nanoscale device and materials testing.


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David L. JonesHow did you get into electronics/engineering and when did you start?The YouTube video telling the story is here.

Very similar to many others in the industry around my time, I started by taking things apart and trying to figure out how they worked. And then I got a Tandy/Radio Shack 50-in-1 Electronics Kit when I was about six or seven, and I never looked back.

What are your favorite hardware tools that you use?The new 3000 series Infiniivision scope is pretty darn nice. Having 1,000,000 waveform updates per second is just phenomenal.

I’ve had the same Phillips screwdriver for about 30 years. It just seems to fit everything. The funny thing is I keep losing it, but it always seems to show up again, like Indiana Jones’s hat. I must be connected to it in some way.

I’m one of those engineers that simply MUST have a REAL calculator within reach at all times. None of this phone or Windows calculator rubbish. There is just something special about hardware dedicated to and designed for a specific task.

World’s Preeminent Engineering Video Blogger

David L. Jones - Electronics Design Engineer; Sydney, Australia

I still have a soft spot for my first Multimeter—an analog Micronta 18 range job I bought with my saved pocket money when I was, well, I don’t really remember, way before 10. I do like to dust it off occasionally and make some measurements for old time’s sake.

What are your favorite software tools that you use?The Web is now the ultimate software tool. The information revolution has completely and permanently changed the landscape of electronics design. If

I could have only one software tool it would be a Web browser.

What is the hardest/trickiest bug you have ever fixed?The next one most likely.

One rather nasty one though was on a third party TMS320 DSP based data logger that ran a Forth based OS written in some German compiler I didn’t understand. We’d get a data glitch about once or twice a week, so setting up to trigger off the event or debug it was completely hit and miss, and no doubt took a while.

http://www.youtube.com/watch?v=XpayYlJdbJk&feature=player_embedded


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The design was spread over five boards in a cube-shaped arrangement, so the ground system was rather nasty. It turned out to be a bizarre combination of some FPGA timing combined with some marginal signal integrity and a silicon bug rolled into one, and throw in an air-conditioning on/off switch thing that ensured the bug mostly only showed up at night. I couldn’t even explain it if I tried.

What is on your bookshelf?• ArtofElectronics• The 20th Anniversary editionof the collective works ofDilbert

• TI and NatSemi analog and digital databooks I kept for old time’s sake, plus some really good app notebook collections.

• And my own book, TheArtofInternetDating.

As much as I love paper, deep down I know that ebook readers are the future, and their time is now. I love my Kindle.

When did you decide to start your Video blog and how did that come about?April 4th 2009. I was talking with someone about electronics blogs, and thought about starting one to complement my existing web page approach, but there were quite a lot of electronics text blogs out there, so I didn’t see much point in it. And text blogs seemed rather boring, as you never got to really know the person behind it. At best you’d get a fuzzy photo

of them on the About page. It all seemed very impersonal.

I saw a few unrelated video blogs around and I liked the idea, and then realized no one had done an electronics video blog. I was nervous at the idea, and had no idea what to do, but decided to just dust off an old 320×240 webcam and record whatever came into my head just for fun.

Always remember that ground ain’t ground. When a

circuit starts acting strange, point your finger at the ground system—guilty until

proven innocent.

I didn’t like the result, but I knew enough not to try and perfect things, because then it will never get off the ground. You have to start somewhere, so I just swallowed my pride and uploaded the video to my personal YouTube account and posted it to the aus.electronics Usenet group. To my surprise, most people liked it and subscribed, and I instantly had 50 or so people waiting for another episode. I thought the novelty would wear off, and the audience

would peak pretty quickly, as I didn’t think many would want to watch a weekly video show with some guy just talking off-the-cuff about electronics. But I didn’t count on the popularity of YouTube as a search engine, and continuous non-stop growth in viewer numbers which I still have to this day. Encouragement came in droves, so the enthusiasm to keep it up was there, so I just kept on pumping out content on a regular basis. People seemed to love having a personality in front of the camera instead of just a voice over as was common on many other electronics YouTube videos.

I originally just called it the Electronics Engineering Video Blog, and I asked for name suggestions in the first episode because I didn’t like the name. But everyone else thought different, and EEVblog just eventually stuck. It wasn’t long before the show got its own YouTube channel, domain name, Wordpress blog, iTunes podcast, and forum, almost exactly as you see it today. I’ve tried to change as little as possible in terms of content and appearance over all that time. The rest, as they say, is history.

How did the Amp Hour come about?Before the video blog idea I actually thought it would be a good idea to do an electronics radio show, I don’t remember why. But I had far from a radio voice, and I figured no one would want to listen to me yap on, so I dropped the idea, and the enthusiasm

http://www.eevblog.com/

http://www.theamphour.com/

http://www.theamphour.com/


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morphed into the video blog. Some time after the success of the video blog the thought popped into my head again that some people might prefer a radio version of the show to listen to while at work, in the car, in the lab, out walking, or other times when video isn’t really convenient. I figured a radio show wouldn’t work with just one person like video did, so I started thinking of a co-hosted show.

Not long after, I noticed a post on Reddit from someone asking for audio tutorials on Electronics. Chris Gammell (a fellow text blogger) took this idea on board and produced one, as he had a music background and had the gear. I thought it was okay; Chris sounded good, and of course he was keen, which is the main requirement for something like this. It helped that he sounded very straight-laced and was a Yank, so I thought an Aussie/Yankee combo could work, with a good contrast between personalities. So I drafted an email to Chris but sat on it for a few weeks because I didn’t know if I had the time to devote to two shows at once. But eventually we talked and he liked the idea.

Just like the video blog, I thought the best thing to do was “just do it,” so we grabbed some mics, called on Skype and hit record for an hour. The upload went onto our respective sites, and it took off from there. It started out as the yet unnamed Dave Jones & Chris Gammell show, and paralleled the development of the video blog almost precisely, except that we started out with an existing

audience base this time. It turns out there is a nice niche for such a radio show, and we now have a loyal following.

Do you have any tricks up your sleeve?After you’ve finished your prototype or project, a good way to test it for EMI robustness is to put a transmitting GSM mobile phone all over the board to try and find susceptible points. It’s amazing what you can discover.

Always remember that ground ain’t ground. When a circuit starts acting strange, point your finger at the ground system—guilty until proven innocent.

At work, be creative with names when ordering stuff and you can order almost anything. That fun looking remote control fart machine in the catalog is easily ordered as an “audio transducer.”

And always remember the golden rule of troubleshooting: Thou shall test voltages. In fact, one of the biggest mistakes you can make when troubleshooting is to assume something. Never do that. Measure it or test it, or Murphy will get you every time.

What has been your favorite project?It’s hard to pick a favorite; my current or next project in my mind is always the favorite. But one memorable one is the Australian Navy Barra Sonobuoy that was used for finding a lost ‘round -the-world sailor. The system

was used well outside its noise performance envelope, and they were barely able to detect sounds of him on his boat—or what they thought was him at the time; it could have been some mating shrimp 100 kilometers away or something. I had laid out the front end board which dictated its noise performance, so I couldn’t help but think that if I hadn’t taken care on that layout to lower the noise floor beyond the spec, he might never have been found. Sometimes it does pay to “gild the lily” beyond the design spec.

Do you have any note-worthy engineering experiences?When I was a kid our TV broke down and I naturally decided to take a look at it. I knew all about live chassis TVs, but had a major brain fart that day and decided it was okay to wiggle the inside RF antenna connector coax shield with the TV on. Oops, it blew me across the room ripping my hand in the process. I’ve hated high voltage stuff ever since. I once measured the shock and vibration response of feminine hygiene products. They were actually considered for use to shock dampen an ultra stable crystal oscillator module. By the way, don’t bother rushing out to buy them; their transfer characteristic is terrible, second only to cotton wool. The wings are marketing hype, they don’t help. I could not understand why accounts looked at me strange when I handed in an expense claim for said products.


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What are you currently working on?About half a dozen different projects, which is quite normal for me, and why I never finish most of them. I don’t have the required attention span, and I never seem to learn my lesson. I don’t really like telling people about my projects because I feel rather embarrassed when I never finish them. One of them does involve a quadcopter though.

What challenges do you foresee in our industry?I’d like to come up with something profound and visionary to say here, but I don’t really see any need to look out for and overcome “challenges” that are here or may be coming in the industry. The more things change the more they stay the same. At the end of the day, you have to design and produce stuff with all the myriad of hurdles that have always gone along with doing that. Nothing new there. Head down, bum up. ■

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PROJECTFEA

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μWatch2 Concept Device

By David L. Jones

Back in 2007 I designed the μWatch, the world’s first

scientific calculator watch in 20 years, and the first ever DIY calculator watch. I did that not only because I wanted one, but for a PIC design contest (sadly, I didn’t even place). It was a challenge doing such a design using all off-the-shelf parts, and making it look good without a case, just the PCB and parts.

It generated a huge amount of excitement and I ended up producing a popular kit for it. But it was never robust enough to wear

Figure 1: Tik-Tok Watch Band by Scott Wilson; www.lunatik.com

as an everyday device, it was a prototype. So I’ve always wanted to redesign it. I’ve had a few stabs at it over the years, and have gotten close, but I always think of a new approach to doing a DIY calc watch. One month it’s using off-the-shelf parts again, the next month it’s a fully custom case, then it’s capacitive sense keys, and then back to tact switches again. USB? Rechargeable? Dot matrix? Color? Touch Screen? Micro SD card? The list of possibilities goes on and on!

Just the other week on a live show I said I had decided on touch

Figure 2: μWatch2 Early Prototype with Capacitive Touch Buttons, Off the Shelf Case


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screen, but the very next day I found the Sharp Memory LCDs. Less than15μW consumption for a dot matrix screen—awesome!

This enabled a whole new level of display technology, and so I decided to go “back to the basics” and drop all the fancy stuff. I now wanted a scientific calc watch that had a killer battery life and real tact switches again. None of this rechargeable rubbish and power hungry touch screen, you can buy that on eBay for $50 now, complete with GSM phone.

But a nice looking calc watch needs a custom case, and I’m just an electrical engineer with pretty feeble mechanical CAD skills. This sort of project is easy at work when you have an expert product designer to collaborate with and do the case for you. Someone who knows all the issues with materials and tooling, and has an open budget for custom tooling. But this time it’s just me, with a wife-approved budget in the hundreds.

Is such a watch case possible? I had no idea, but it’s a good thing that tools like eMachineShop do. Even with my feeble skills I can 3D model a nice case in next to no time. But most importantly, the tool tells me if I’m trying to do something stupid that’s not manufacturable. Can you manufacture that radius curve with that size router bit in that material? How much extra does that smaller router bit cost me? —This is knowledge I’d previously only get from wasting a lot of time and money, or having an expert available on tap. 3D models are great, but as with everything in electronics, you

ultimately have to get your idea manufactured to really try it out.

But how about the watch band? That’s always been a real sticking point in terms of looks, case attachment and robustness. Then I remembered the iPod nano is very popular as a watch, and the TikTok watch band for it looked awesome. This was designed by Scott Wilson, founder of MINIMAL and former Global Creative Director for Nike Watches. It was a record breaking crowd sourced Kickstarter project that raised $1million How could I possibly design a better watch band? So I didn’t even try, I knew a winner when I saw one.

So what if I made my watch iPod Nano size compatible that just slipped into the existing watch band? Problems solved! And the calc can be used as a watch or as the world’s smallest scientific pocket calc. Looks good, rugged, unique, high quality, and builds upon other people’s excellent concepts that I know I don’t have the skills, money or resources to match. I like it.

And there you have it, the completed new μWatch2 calculator watch concept – maybe...

Personal engineering projects can be fickle! ■

Figure 4: μWatch2 Nano 55mm 3D Concept 2



Figure 3: μWatch2 Early Prototype, Custom Case


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expanding the Atmel ARM processor-based device offering into the Digital Signal Controller (DSC) market. For more information, please click here.

Non-Contacting Rotary Position SensorBourns, Inc., a leading manufacturer and supplier of electronic components, announced the availability of a new non-contacting magnetic rotary position sensor which can help increase product lifespan and reliability of applications that operate in harsh and high vibration environments. Designated Bourns® Model AMS22, the sensor utilizes magnetic hall-effect technology that enables high performance operation even in severe temperature or unclean conditions. The new sensor is the latest addition to the company’s sensor product line, and is designed to meet OEM requirements for a cost-effective, long life position sensor

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LDO in Ultra Small PackageNXP Semiconductors N.V. today announced the availability of the LD6806CX4 ultra low-dropout voltage regulator (LDO), featuring ultra-low dropout of only 60 mV at a 200-mA current rating. With an ultra-small 0.76 × 0.76 × 0.47mm wafer-level chip-scale package (WLCSP), the LD6806CX4 uses minimal board space, making it ideal for extremely space-constrained designs in mobile handsets, where battery life is also a critical success factor. Batteries in mobile phones discharge almost linearly over time, but the LDO compensates for this effect by providing a constant regulated output voltage. For example, if the battery voltage of

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Wideband, Low Noise, Low Distortion, Fixed Gain, Differential AmplifierISL55211The ISL55211 is a wideband, differential input to differential output amplifier offering 3 possible internal gain settings. Using fixed 500Ω internal feedback resistors, the amplifier may be configured for a differential gain of 2, 4 or 5V/V depending on which combination of input pins are connected to the signal source. Internal feedback capacitors controls the signal bandwidth to be a constant 1.4GHz in all gain settings.

Ideally suited for AC-coupled data acquisition applications, the output DC common mode voltage is controlled through an external VCM pin or left to default to 1.2V above the negative supply pin. Where the differential signal source is AC-coupled, the input common mode voltage will equal the output common mode voltage.

Intended for very high dynamic range ADC interface applications, the ISL55211 offers 5600V/μs differential slew rate, <12nV/√Hz output noise, and >100dBc SFDR to >100MHz for 2VP-P 2-tone 3rd order intermodulation. Its balanced architecture effectively suppresses even order distortion terms - an important issue for very wide band 1st Nyquist zone ADC interface applications. Minimum gain operation of 2V/V (6dB) with <1dB peaking ensures stable performance over-temperature. It's ultra high differential slew rate of 5600V/μs provides adequate performance margin for large signal application through 500MHz.

The ISL55211 requires only a single 3.3V (max. 4.2V) power supply and 35mA quiescent current, providing a very low power solution (115mW). Further power savings are possible using the optional power shutdown control - where the quiescent current can be reduced to <0.4mA. A companion device, the ISL55210, offers similar performance where the feedback and gain resistors are external. Both are available in a 16 Ld TQFN (Pb-free) package and are specified for operation over the -40°C to +85°C ambient temperature range.

Features• 3 Fixed Gain Options . . . . . . . . . . . . . . . . . . . . . . .2, 4, or 5V/V

• Constant Bandwidth Over Gain. . . . . . . . . . . . . . . . . . . 1.4GHz

• Differential Slew Rate . . . . . . . . . . . . . . . . . . . . . . . 5,600V/μs

• 2VP-P, 2-tone IM3 (200Ω) 100MHz . . . . . . . . . . . . . . -103dBc

• Low Differential Output Noise (Gain 5V/V) . . . . . .<12nV/√Hz

• Supply Voltage Range . . . . . . . . . . . . . . . . . . . . . . 3.0V to 4.2V

• Quiescent Power (3.3V Supply) . . . . . . . . . . . . . . . . . 115mW

Applications• Low Power, High Dynamic Range ADC Interface

• Differential Mixer Output Amplifier

• SAW Filter Pre/Post Driver

• Fixed Gain Coax Receiver

Related Devices• ISL55210 - External Gain Set Version

• ISLA112P50 - 12-bit, 500MSPS ADC (<500mW)

• ISLA214P50 - 14-bit, 500MSPS ADC (<850mW)

Related Literature• AN1649 - “Designer’s guide to the ISL55210 and ISL55211

Evaluation Boards”

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Forward ErrorCorrection:

Frank ChangPrincipal Engineer – Systems

Choosing BetweenHard-Decision,Soft-Decision Codes

Forward error-correction (FEC) has been integrated

as a necessary adjunct for 10-Gbit Ethernet networks, and is assumed to be a necessity for any systems operating above that speed, particularly in the next-generation Ethernet speeds of 40 and 100 Gbits/sec. FEC relies on a standardized way of sending redundant data in the same channel as a message, providing a means by which the receiver can recover information if the channel is corrupted. Practical FEC work began in the early 1950s with the work of Richard Hamming, whose code became a standard means of correcting errors in solid-state memory.

System designers in the 21st century can choose between two

approaches in FE: a hard-decision method implemented primarily in chip-level hardware, and soft-decision FEC, implemented in a combination of software and ASICs or FPGAs. Vitesse Semiconductor Corp. has explored a promising new hard-decision FEC method that uses Continuous Interleaving (CI), specifically the CI-Bose-Chaudhuri-Hocquenghem or CI-BCH codes. While no single FEC code fits all applications, CI-BCH holds a promise of being widely applicable in line card and switch applications.

The roots of FEC go back to Claude Shannon’s work in 1948 on noise in a communication channel—the same work that led to the concepts of “entropy.” Shannon introduced the concept of feeding forward to

a receiver the error data collected, based on a comparison of data transmitted and data received in a noisy channel. While Shannon’s work required the existence of an external observer (or piece of test equipment) to analyze the channel, high-speed optical networks substitute a parity code, transmitted along with the data, that allows the receiver to evaluate transmission errors. In modern networks operating in excess of 10 Gbits/sec, optical signal-to-noise ratios are significant enough to seriously erode performance if FEC is not implemented.

Service providers now have 20 years’ experience in implementing optical FEC, beginning with ultra-long-haul undersea optical cables in the early 1990s. Both


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convolutional codes and block codes can be used as the basis of FEC parity data. Convolutional codes utilize a convolution, or math operation, which is performed on two separate functions, similar to a cross-correlation. These codes are popular in wireless RF networks, but are rarely used in optical networks due to latency and error floor limitations.

The study of block codes expanded in the late 1990s to look for optimal plans for implementing FEC in VLSI silicon. Among the block codes, such derivatives as concatenated codes, turbo codes, and Low-Density Parity-Check (LDPC) codes all have been tested in hardware-based implementations.

In the last decade, FEC “camps” have settled into those using hard-decision and soft-decision algorithms. Hard-decision methods were the first to be developed. A hard-wired decoder using these methods makes firm decisions for each input and output as to whether the data corresponds to zero or one. There is no indication of the reliability of this binary decision. Soft-decision algorithms bear a certain resemblance to fuzzy logic, in that binary vector regions are created, indicating the probability that the bit of information examined represents zero or one. To be precise, the decoder sets 2N-1 decision thresholds, where N represents the number of quantization bits.

Traditionally, FEC codes have been grouped into different generations, beginning with early

first-generation Reed-Solomon codes. The codes are usually judged for effectiveness based on their net coding gain (NCG), which represents the gross coding gain measured in dB, minus the bit rate increase in dB due to the redundant overhead in the message. The first generation of Reed-Solomon codes were used in submarine transmission systems in the early 1990s, and used only single-quantization-level bit sampling.

The rise of Wave-Division Multiplexing (WDM) during the optical communication boom of the late 1990s led to a search for more effective FEC codes, and the second generation of FEC was based on the use of concatenated codes. Basic classes of codes such as BCH were enhanced with interleaving and iterative decoding techniques. This second generation of hard-decision codes takes NCG to 8 dB and above, though rarely has a second generation of hard-decision code been demonstrated close to 10 dB.

To achieve a third generation of FEC codes, researchers have turned to either soft-decision codes, or complex iterative codes in hard-decision categories. Limited results have been shown for specialized turbo codes and LDPC codes offering NCG in excess of 10 dB, as well. Recent work on Continuously-Interleaving versions of BCH, CI-BCH, indicate NCG of 9.35 dB with a 7 percent Optical Transport Network overhead rate. This coding gain, achieved in non-optimized silicon, appears capable of being improved beyond 10 dB. The

International Telecommunications Union has created a specific CI-BCH standard, G.975.1-I.9.

Vitesse is one of many companies that have studied an evolution from FPGA to IP core implementations of CI-BCH codes. The use of interleaving with standard BCH allows a simple algebraic equation solver to be used, rather than the more complex Chien search or matrix-inversion techniques applied in more DSP-intensive designs. The code architecture used in current CI-BCH devices provides double coverage of all transmitted bits by using a triple error-correcting BCH. Because a single code word operates in a manner similar to both inner and outer codes of high-gain coding systems, the latency of a CI-BCH is, in worst case, no greater than other high-gain codes.

In early 2010, Vitesse took the unusual step of offering its CI-BCH eFEC circuits as licensable cores for ASICs or FPGAs, with 7 and 20 percent overhead ratios. Within months, other companies utilized internal intellectual property to offer similar FEC library elements. Altera Corp., using software developed by its Avalon Microelectronics subsidiary, introduced two hard-decision FEC cores in March 2011 based on Streaming Turbo Product Code BCH. The EFEC7 and EFEC20 offered 7 and 20 percent overhead ratios, respectively, though the company did not offer specs on net coding gain.

Soft-decision codes can demonstrate advantages in some applications, but they had been


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limited to wireless networks in the past, because the banks of A/D converters required for conversion of a signal into quantization bits would be too cost-prohibitive at the higher speeds of optical networks. In recent years, however, new digital coherent receivers that integrate an A/D converter as a front end for demodulating signals, make it reasonable to consider soft-decision algorithms in 40 and 100-Gbit optical networks.

Early soft-decision methods implemented in FPGAs rely on LDPC codes, because they simplify the parallelization of signal processing steps. The most common problem cited with soft-decision LDPC codes is the flattening of the “error floor” at higher speeds, which may make it more difficult to realize NCG in excess of 10 dB in cost-effective implementations.

Mitsubishi and other developers have applied concatenated LDPC and Reed-Solomon approaches to the problem, in order to eliminate

the error floor. One such approach calls for using LDPC as an inner code, then concatenating it with a Reed-Solomon code to reduce the residual errors left after LDPC decoding (Reed-Solomon is deliberately chosen for concatenation over BCH, because of its higher tolerance for the error bursts occasionally seen with LDPC chip implementations). One simulation of triple-concatenated LDPC has shown a 10.8 dB NCG using dual-polarized QPSK, in a 125-Gbit/sec transmission system.

It is likely to expect more near-term implementations of hard-decision FEC in 802.3ba networks at 40 and 100 Gbits/sec—both because of the maturity of the FEC silicon, and because it can interoperate with existing EDC, CDR, and Serdes devices. Nevertheless, the potential advantages in coding gain that can be realized by soft-decision FEC may make the implementation preferable in many network nodes, particularly if problems of higher overhead and greater data converter

complexity can be solved. In such solutions, a FEC and DSP may be integrated in a single silicon device, thus taking a different device partitioning approach from hard-decision FEC.

About the Author

Frank Chang is a principal engineer for Vitesse Semiconductor. He specializes in optical system engineering, IC product specifications, and application issues for telecom, datacom, and PON access markets. He is also knowledgeable in electronic dispersion compensation (EDC), PMD, PHY, and FEC chipsets and their application in optical networking. He has authored or co-authored over 55 peer-reviewed journal and conference articles. ■

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Robert GreenSenior Market Development Manager

Electrical MeasurementConsiderationfor NanoscaleDevices & Materials

Lately, we at Keithley have been seeing a lot of interest in nanotechnology, and research laboratories in universities and semiconductor companies are developing new nanomaterials, such as graphene. Nanomaterials can, however, be difficult to test and characterize because measuring a material’s impedance, conductance, or resistance requires making very low-level measurements.

One way to do this is to use a technique called source-measure testing. Basically, you connect a current source to a sample of the material and measure the voltage across it, or connect a voltage source across the sample and measure the current through it. This method also works for devices that have both passive and active properties with linear or non-linear transfer functions.

Several considerations are important in the electrical test of nanoscale devices and materials:

• Nanoscale devices and materials cannot carry as much current as most conventional passive devices

and materials, unless they are superconducting. This means that when using a current stimulus, the test system must accurately control the current and keep it at a low level (mA, μA, or nA) or it will destroy the device under test (DUT).

• Nanoscale devices and materials will not hold off as much voltage from adjacent devices as conventional electronic components or materials. This is because smaller devices are packed closer together. Smaller devices also have less mass and may be affected by the forces associated with large fields. In addition, internal electric fields associated with nanoscopic particles can be very high, requiring careful attention to applied voltages.

• Given that nanoscale devices and materials are so small, they typically have lower parasitic (stray) inductance and capacitance. This means that instrumentation for characterizing their I-V curves must measure low currents while tracking the short reaction time.


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Because nanoscopic test applications often require low-current sourcing and measurement, you must use instruments, such as the Keithley Series 2600A System SourceMeter® instruments, that have the appropriate source and measure resolution and accuracy. In addition to being highly sensitive, the instrumentation must have a short response time (sometimes referred to as high bandwidth), which is related to a DUT’s low capacitance and ability to change state rapidly at low currents.

When characterizing low impedance (less than 1000 ohms) devices and materials, the source current measure voltage technique will generally yield the best results. Current sources are stable when applied to lower impedances, and a good signal-to-noise ratio can be achieved without great difficulty. This allows for accurate low voltage response measurements.

When characterizing high impedance (greater than 10,000 ohms) devices and materials, the source voltage/measure current technique is best. Stable voltage sources to drive high impedances are easily constructed. When a well-designed voltage source is placed across a high impedance, it will quickly charge the stray capacitance of the DUT and test cables and rapidly settle to its final output value. The small current response of the DUT can be accurately measured with an appropriate sensitive ammeter.

Research scientists are investigating the use of graphene, a single-atom-thick crystal of carbon for making nanoscale devices because of its outstanding carrier mobility. Carriers travel through a single atomic

layer crystal such as graphene unimpeded unlike electrons in a metal whose travel is impeded by nuclei. Electrons in graphene exhibit quantum electrodynamic behavior.

Researchers characterizing graphene use Hall effect measurements and study longitudinal resistance to assess carrier mobility and look for evidence of the quantum Hall effect, whereby longitudinal resistivity decreases to near 0 ohm-cm. Amazingly, graphene exhibits the quantum Hall effect at room temperature! These measurements require very low current and precision sourcing on the order of nanoamps. More importantly, accurate control of the source current is required to ensure that the graphene sample is not forced to dissipate so much power that the sample is destroyed.

At nanoamp source current levels, the voltages across the sample are extremely small, on the order of tens to hundreds of nanovolts. These types of nanovolt-level measurements require special instrumentation with sufficient resolution and extremely high sensitivity. As you can see, only by using the appropriate instrumentation will you be able to make the measurements needed to characterize nanoscale materials properly and realize the advantages to be gained by making devices out of these materials.

For Further Exploration

“Ensuring the Accuracy of Nanoscale Electrical Mea-surements” www.keithley.com/data?asset=55802

Keithley Instruments, Inc., Cleveland, OH, 888-534-8453.

About the Author

Robert Green is a Senior Market Development Manager at Keithley Instruments focusing on low level measurement applications. During his 20-year career at Keithley, Mr. Green has been involved in the definition and introduction of a wide range of products including picoammeters, electrometers, digital multimeters, and temperature measurement products. He received a B.S. in Electrical Engineering from Cornell University and an M. S. in Electrical Engineering from Washington University, St. Louis, Missouri. ■

Nanovoltmeter Vxx = Longitudinal Voltage

Vxy = Transverse Voltage, Hall Voltage with applied B

Vxx = Rxx

Nanovoltmeter

DC Current Source

Graphene

Vxx

Vxy

I

Figure 1: Configuration for simultaneous measurement of the Hall Effect voltage and longitudinal resistance of a graphene sample.


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