Benedetto Vigna Executive VP & GM of Analog, MEMS and Sensors Group of STMicroelectronics
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Executive VP & GM of Analog MEMS and Sensor Groups at STMicroelectronics
How one of the largest semiconductor companies is revolutionizing the MEMS and sensor areas.
This weekâ€™s latest products from EEWeb.
Shaping the Future of MEMS & Sensors
How MEMS microphones deliver breakthrough innovation in sound sensing.
The AXP Logic Family
NXPâ€™s new logic family addresses the industry trends of faster performance and lower power consumption.
Measuring Low Current Applications
How picoammeters offer the economy and ease of use of a digital multimeter with significantly better low current sensitivity.
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STMICROELECTRONICS is one of the world’s largest semiconductor
companies headquartered in Geneva, Switzerland. The company is known to have one of the broadest product portfolios, especially in the MEMS and sensor markets. Their diverse microelectronics products can be found in everything from mobile phones to popular gaming consoles.
The company’s Executive Vice President and General Manager of the MEMS and Sensors Group, Benedetto Vigna, joined the company’s MEMS division back in 1995. Since then, he has steered the company to develop an ever-expanding MEMS product portfolio that has resulted into mass adoption in many consumer products. We spoke with Vigna about the next big wave of MEMS, what types of sensors have been growing in demand, and the ways in which the company is constantly innovating.
PULSE “You will soon see products from ST that allow us to continue to shrink the technology while still maintaining high performance.”
What sensor product lines does ST have? ST is a company with a rainbow of MEMS products. We have seven different kinds of MEMS—one for each color of the rainbow. We have temperature sensors, gyroscopes, magnetic sensors/compasses, microphones, pressure sensors, inkject/microfluidics and multisensor packages. We are also investing heavily in other types of sensors such as environmental MEMS like humidity sensors and other non-MEMS sensors such as Touch Sensors, where we have our FingerTip technology. We are developing Smart Sensors that embed the Microcontroller and we want to make all of these sensors available in wireless options, because in some cases the wires are inconvenient or impractical. I would define the position of ST as a broad range supplier of sensors and we are constantly looking to expand our portfolio.
Are you looking at module solutions to be integrated into your wireless sensors? We offer our customers the choice to integrate both the sensing features and the wireless communication features in the same package. I believe the next big wave of MEMS is going to be wearable technologies and you are going to see sensors distributed in the environment around us. When you wear technology that interacts with distributed sensors, what does it need? It needs the wireless sensor, which needs a battery. If you want to put sensors in your shoes, for example, you don’t want to walk around with a wire that runs down your leg to your shoe to capture the sensor data. Better to put a wireless radio with the sensor to transmit the data.
In terms of a product like this, do you feel that ST is going to provide all of the technologies needed within that one package, or will you be partnering with other people? We will provide the customer with one package because that is what they want. Today, we have all the components—the sensor and we also have Bluetooth and lowenergy Bluetooth for more consumer-oriented applications. We have a radio as well that will address the smart environment applications. For example, we are working with a company to put a wireless radio in streetlights in a couple of cities all over the world—we mentioned a few weeks ago that the lights are rolling out in Paris, the City of Lights. When you put a wireless radio into a streetlight, you can add sensors into the mix and communicate among the streetlights to save power when no one is on the street or adjust the lights as the sun rises or sets. That’s the rationale behind the next big wave—wearable technologies and the smart environment.
What type of radio do you have in these packages? We have two radios. One is Bluetooth and the other one is called the SPIRIT1, which is a lowpower, low-frequency radio that works below one gigahertz. It is a radio that has the longest range—up to several kilometers, depending on the environment--and it’s more robust than Bluetooth. If it’s raining or there is a thick wall, there will be no interference with the SPIRIT1 radio. Also, you would not use Bluetooth for an application where security was important. Instead, you can use this SPIRIT1 radio to add communication to more sensor applications like with metering and automation, where you need to protect data.
INTERVIEW In recent years, what kinds of sensors have been growing the most? Most recently, the product that has been growing the fastest has been the gyroscope. Today, a lot of people are integrating the gyroscope together with an accelerometer. In absolute numbers, these are the things that have been growing most. Along with gyroscopes, the use of pressure sensors and microphones has also been growing very rapidly.
What gyroscope options does ST provide? We have gyroscopes that cover the full range of applications that the industry is asking for. We have a gyroscope that is much more full-scale from degrees per second up to 6,000 degrees per second. When you take pictures, and you need adjustment sfor optical stabilization, you need a more fullscale option because the small movements of the hand produce more vibration. When you play with a mobile phone or game console, the movement of your hand is faster around 2,000 degrees per second. ST has a complete portfolio of gyroscopes for all applications that our customers are designing. We have single-axis, dual-axis, and triple-axis gyroscopes, which are the most complex ones we offer. ST is the market leader for threeaxis gyroscopes in the consumer market, and we are also the only manufacturer that has the three-axis gyroscope fully qualified for the automotive industry—and available in million-piece quantities. In addition, ST is the only manufacturer whose gyros can cope with ultrasonic cleaning and the drop test; it is exceptionally rare to be able to combine good performance with strong mechanical resistance.
Is there a lot of innovation still taking place with MEMS processes and capabilities? Beyond the progress we’re making in processing, there is definitely development in the software and technology. You will soon see products from ST that allow us to continue to shrink the technology while still maintaining high performance. Just recently, we announced the development of a new technology for pressure sensors—the first
The motions of ST’s gyroscopes technology that allows you to use a fully molded package incorporating an isolated sensing element for pressure sensors to make the sensor more accurate and more robust. Smartphones and tablets have been the launching pad for gyroscopes and microphones and we believe these applications will boost pressure sensors, too. Like other components, the sensors must be small. We announced that we were able to miniaturize the pressure sensor down to 2 by 2 millimeters. Think of the pressure sensor as an empty room in a house. We found a way to fill a room of the house with plastic in a way that is more robust while we leave a hole for the air pressure to get through only when it’s needed. Today, I believe that sensor technology is just as important as software development in the development of new applications. You can simplify the software, but when you announce an improvement to the sensor, it enables the development of better applications for the customer.
“ST is the only manufacturer whose gyros can cope with ultrasonic cleaning and the drop test; it is exceptionally rare to be able to combine good performance with strong mechanical resistance.”
PULSE What is the e-Compass?
“Today, I believe that sensor technology is just as important as software development in the development of new applications.”
The compass is another product that is going to be used more and more in mobile devices. In addition to the pressure sensor, we recently announced a new accelerometer and compass in a 2x2mm package—the smallest available for a 6-axis compass. This is very important because the accelerometer tells you how many steps you take, but the compass tells you in which direction you took those steps. The compass is meant for navigation, but that isn’t its only use. It is also ideal for the user interface for gaming. Still, I believe the compass will help enable a broad range of applications using indoor navigation.
Could you talk a little more about ST’s upcoming conference? In a mobile phone, pressure sensors can help determine if you are going up stairs or down. The weight of the atmosphere increases and decreases depending on altitude. In Death Valley, the weight of the atmosphere is much heavier than in the Rocky Mountains because there is less oxygen and air at higher elevations. We have a very sensitive device that can sense the pressure variations when you go step by step up the stairs. ST pressure sensors can even detect the pressure difference of 10 centimeters. What could you do with this? This resolution would be very valuable for indoor navigation. Combining a pressure sensor with a gyroscope, accelerometer, and e-Compass can completely replace GPS when you are indoors and there is no signal.
The Shaping the Future of MEMS and Sensors conference has two main benefits. First of all, it serves as an opportunity for us and speakers from across the country to talk about the latest news and to share with the audience our vision for the future. Also, because we’ve tried to make it as interactive as possible, it will offer our customers and partners an opportunity to share their views on the future in terms of using sensors in their products. We’ve invited the Who’s Who of the industry to come and talk about how they are using MEMS and Sensors to Shape the Future and they will help us showcase how Sensors will improve how we work, live, and play. ■
Sensors Will Become Ubiquitous From location-based services to wearable sensors for health and wellness; from gaming and remote controls to sensors for safety and performance; from your smartphone to your tablet; expect to find a sensor--in fact, many sensors--in most all designs of the future, improving the way we work, live and play. You are invited to participate in a FREE full day Shaping the Future of MEMS and Sensors Summit, on September 10, 2013 at the Santa Clara Marriott Hotel, CA, that will feature thought-provoking keynotes and panel discussions, stimulating presentations, live demonstrations and hands-on seminars. The goal? To identify, highlight and help you take advantage of important trends in technology. Top technologists from industry and research organizations will demonstrate how they are adopting sensors to enable applications, augment the user experience, and expand markets. Industry visionaries will describe where the nextgeneration of sensors is creating opportunities to shape the future of electronics.
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EM Deliver a
Breakthrough Innovation in Sound Sensing INTRODUCTION TO MEMS MICROPHONES A MEMS microphone is a solid state integrated IC that can sense voice in the same way as an ECM Microphone [Electret Condenser Microphone]. MEMS microphones are widely adopted in modern devices such as mobile phones, tablets, laptops, smart TVs, automotive voice recognition products, gaming and remote controllers.
Vishal Goyal â€“ Technical Marketing Manager, STMicroelectronics India
“MEMS microphones enable dramatic advancements in sound quality in multiplemicrophone applications.”
ccording to IHS iSuppli, the market for MEMS microphones for consumer electronics and mobile handsets is forecast to grow revenue at a CAGR of 23% between 2010 and 2014. By 2015, the shipments of MEMS microphones are expected to reach 2.9 billion units - four times the total in 2010. The increased popularity of MEMS microphones is attributed to their reliable monolithic structure, high tolerance of mechanical vibration, small footprint and height, and optional digital output. In addition, MEMS microphones enable dramatic advancements in sound quality in multiple-microphone applications. Such microphone arrays, facilitated by the small form factor, superior sensitivity matching and frequency response, enable the implementation of active noise and echo cancelling, as well as beam-forming, a soundprocessing technology that helps isolate a sound and its location. These features are invaluable with the increasing use of cell phones and other devices in noisy and uncontrollable environments.
MEMS Microphone Construction There are mainly two types of MEMS microphones – Analog, which convert sound into corresponding voltage output, and Digital, which gives a digital output, typically pulse density modulation [PDM]. Acoustic Transducer A MEMS microphone is also serves as an acoustic transducer. The transduction principle is the coupled capacity change between a fixed plate (back-plate) and a movable plate (membrane). This capacitive change is caused by the sound wave passing through the acoustic holes. This moves the membrane modulating the air gap comprised between the two conductive plates. The back-chamber of the MEMS microphone is the acoustic resonator. The ventilation hole allows the air compressed in the back chamber to flow out and consequently allowing the membrane to move back. Key Parameters of MEMS Microphones Sensitivity The sensitivity is the electrical signal at the microphone output to a given acoustic pressure as input. The reference of acoustic pressure is 1Pa or even 94dBSPL @ 1kHz**. The sensitivity of a MEMS microphone is typically measured as follows:
-Analog microphones in mV/Pa or even dBV = 20 * Log (mV/Pa / 1V/Pa). -Digital microphones in %FS or even dBFS = 20 * Log (%FS / 1FS).
The signals of a couple of microphones are processed** to shape the response along the x direction Directionality
Dynamic Range and AOP
The directionality indicates the variation of the sensitivity response with respect to direction of arrival of the sound. Some MEMS microphones are OMNI-Directional, which means there is no sensitivity change at any sound source position in the space. The directionality can be indicated in a Cartesian axis as sensitivity drift vs. angle or in a polar diagram showing the sensitivity pattern response in the space.
The dynamic range is the difference between the minimum and maximum detectable sound by the microphone without distortion. The maximum signal that the microphone can “listen” without distortion is also called acoustic overload point (AOP). The minimum signal that a microphone can “hear” depends on its SNR. In other words, the minimum signal is equivalent to the residual noise in terms of dBSPL.
Signal to Noise Ratio [SNR] The signal-to-noise ratio specifies the ratio between a given reference signal to the amount of residual noise at the microphone output. The reference signal is the standard signal at the microphone output when the sound pressure is 1Pa @ 1kHz. In other words, this is the microphone sensitivity. The noise signal (residual noise) is the microphone electrical output at the silence. This quantity includes both the noise of the MEMS element and the ASIC. Typically the noise level is measured in an anechoic environment and weighting-A the acquisition. The A-weighted filter corresponds to the human ear frequency response.
Frequency Response The frequency response of a microphone in terms of magnitude indicates the sensitivity variation across the audio band. This parameter also describes the deviation of the output signal from the reference 0dB. Typically, the reference for this measurement is the exact sensitivity of the microphone → 0dB = 94dBSPL @ 1kHz. The typical frequency response of a microphone shows a roll-off at low frequency due to the ventilation hole and a rise up at high frequency due to the Helmholtz effect. The frequency response of a microphone in terms of phase indicates the phase distortion introduced by the microphone. In other words, it indicates the delay between the sound wave moving the microphone membrane and the electrical signal at the microphone output. This parameter includes both the distortion due to the membrane and the ASIC
Directional Acoustic Patterns Using MEMS Microphones An omnidirectional microphone response is generally considered to be a perfect sphere in three dimensions. The smallest diameter microphone gives the best Omni-directional characteristics at high frequencies. But MEMS microphones can also be used in array to modify the response according to desired acoustic patterns The challenging requirements of distant-speech interaction systems require specific physical and acoustic parameters from MEMS microphones. A small form factor allows designers to easily embed entire arrays of microphones in the walls, desks, or speech-enabled appliances of the automated home, while the microphones’ excellent acoustic characteristics, coupled with sophisticated signal-processing technologies, would make it possible to identify and capture an individual speaker from several meters away, in a crowded room with music playing. The distant-speech interaction capability of MEMS microphones will not only dramatically change the way people interact with technology, but can make a real difference for those who can’t easily move around, such as those who are motor-impaired. In addition to home-based scenarios, distant-speech interaction systems can find use in robotics, tele-presense, surveillance and industry automation. Multiple microphones in array are becoming standard in mobile devices, supporting advanced features such as voice command, noise suppression and high-definition video recording. Packaging While there are some MEMS microphone manufacturers that still produce devices with metallic lids, plastic packages can save space and increase durability in consumer and professional voice-input applications, from mobile phones and tablets to noise-level meters and noise-cancelling headphones. To further simplify design in space-constrained consumer devices, MEMS microphones that are suitable for assembly on flat-cable printed-circuit boards streamline manufacturing.
While extremely difficult to do, providing a package technology that allows equipment manufacturers to place the ‘sound hole’ either on the top or the bottom of the package delivers the greatest flexibility to ensure the slimmest possible design and shortest acoustic path from the environment to the microphone. While the microphones with the sound hole on the top (top-port) suit the size and sound-inlet position requirements of laptops and tablets, the bottom-port microphones are mostly used in mobile phones. Conclusion MEMS microphone are entering new application areas such as voice-enabled gaming, automotive voice systems, acoustic sensors for industry and security applications, and medical telemetry. Its unique construction, performance and form factor has made possible what was unthinkable earlier. We hope you will consider joining us on September 10th at the Shaping the Future of MEMS and Sensors Summit at the Marriott Hotel in Santa Clara, CA. Shaping the Future of MEMS and Sensors is sponsored by Berkeley Sensor and Actuator Center (BSAC), EEWeb, EEJournal, MEMS Exchange, MEMS Industry Group, MEPTEC, and Mouser Electronics and hosted by STMicroelectronics. ■
Worldâ€™s lowest power capacitive sensors with auto-calibration NXP is a leader in low power capacitance touch sensors, which work based on the fact that the human body can serve as one of the capacitive plates in parallel to the second plate, connected to the input of the NXP capacitive sensor device. Thanks to a patented auto-calibration technology, the capacitive sensors can detect changes in capacitance and continually adjust to the environment. Things such as dirt, humidity, freezing temperatures, or damage to the electrode do not affect the device function. The rise of touch sensors in modern electronics has become a worldwide phenomenon, and with NXPâ€™s low power capacitive sensors itâ€™s never been easier to create the future.
Learn more at: touch.interfacechips.com
Introduces AXP Logic Family to Address the Need for Lower Power Consumption 22
We talked with NXPâ€™s International Product Manager, Cliff Lloyd, about the new AXP Logic family released in July, and how it takes on the power-consumption problem.
“We felt we had to address the need for faster performance... and also maintain or improve power savings performance.” - Cliff Lloyd, NXP
“The mobility markets have been exploding in recent years,” NXP’s International Product Manager, Cliff Lloyd told us. “That market is driven by two primary factors: one is the size of a device, and the other is battery life.” In the past, he explained, NXP focused on making smaller and smaller packages. NXP made a breakthrough last year in small package size when it released its Diamond package -- 0.8 by 0.8 in size with a 0.5mm pad pitch between pads -- which made it easier to mount in PCBs. After the Diamond package was released and package size addressed, NXP decided to focus on addressing low power consumption in the mobility market. “We felt we had to address the need for faster performance... and also maintain or improve power savings performance,” said Lloyd.
One of the advantages of the AXP family of devices is its configurable logic, which gives customers a lot of flexibility when selecting parts.
There are two very important parts to power savings performance, explained Lloyd. The first is static power consumption, which is the amount of power a device uses when it is simply connected to a battery. The other important part of power consumption is the power that’s consumed when the device is actually being used, which is called the Dynamic Power Consumption. Keeping these two parts of power consumption low was what NXP has aimed to address with the AXP family. What they have achieved with the AXP family is 15% lower power consumption for unique functions, as well as a top delay of 4 nanoseconds at 1.2V, which is about twice as fast as its previous AUP family at the same voltage node. One of the advantages of the AXP family of devices is its configurable logic, which gives customers a lot of flexibility when selecting parts. For instance, when a customer buys a device -- depending on how he hardwires it on the PCB -- the device could operate as a NAND Gate, an OR Gate, a NOR Gate, a buffer, or an inverter, and the customer doesn’t have to buy a different device to program these individual functions. The customer can get different functions from one device, and use a single qualification to cover multiple functions, which offers tremendous flexibility.
Although many people believe that the devices are programmable, Lloyd explained that that is a misconception. The devices are strictly hardwire configurable on the PCB. For example, if you hooked up pin 5 and pin 3, as in the figure below, you would get a twoinput AND gate. If you connected pin 2 and pin 1 to ground, you would get a two-input NOR gate. It all depends on how the board is laid out -- an inverter, or an exclusive NOR (XNOR) can also be achieved. Because of this flexibility, NXP decided to first release the configurable devices.
Features of AXP Devices • Very low dynamic power dissipation (CPD) • tpd of 2.9 ns at Vcc of 1.8 V • Wide supply voltage range (0.7 V to 2.75 V) • Fully specified at 0.8 V • Schmitt-trigger action on all inputs • 4.5 mA balanced output drive • Over-voltage tolerant I/Os • Fully specified (-40 to +85 °C) • Pb-free, RoHS compliant and Dark Green For more information, visit the NXP Tech Community.
With low static and dynamic power dissipation, a wide voltage range, true Schmitt-Trigger inputs, and configurable logic, NXP has aimed to address the need for low power consumption and reliable logic level switching with the AXP Logic family, and position itself for a mobile market where battery critical applications and battery life conservation is key.
Get the Datasheet and Order Samples http://www.intersil.com
Power Factor Correction Controllers ISL6730A, ISL6730B, ISL6730C, ISL6730D The ISL6730A, ISL6730B, ISL6730C, ISL6730D are active Features power factor correction (PFC) controller ICs that use a boost topology. (ISL6730B, ISL6730C, ISL6730D are Coming Soon.) The controllers are suitable for AC/DC power systems, up to 2kW and over the universal line input.
The ISL6730A, ISL6730B, ISL6730C, ISL6730D are operated in continuous current mode. Accurate input current shaping is achieved with a current error amplifier. A patent pending breakthrough negative capacitance technology minimizes zero crossing distortion and reduces the magnetic components size. The small external components result in a low cost design without sacrificing performance. The internally clamped 12.5V gate driver delivers 1.5A peak current to the external power MOSFET. The ISL6730A, ISL6730B, ISL6730C, ISL6730D provide a highly reliable system that is fully protected. Protection features include cycle-by-cycle overcurrent, over power limit, over-temperature, input brownout, output overvoltage and undervoltage protection.
• Reduce component size requirements - Enables smaller, thinner AC/DC adapters - Choke and cap size can be reduced by 66% - Lower cost of materials • Excellent power factor over line and load regulation - Internal current compensation - CCM Mode with Patent pending IP for smaller EMI filter • Better light load efficiency - Automatic pulse skipping - Programmable or automatic shutdown • High reliable design - Cycle-by-cycle current limit - Input average power limit - OVP and OTP protection - Input brownout protection
The ISL6730A, ISL6730B provide excellent power efficiency and transitions into a power saving skip mode during light load conditions, thus improving efficiency automatically. The ISL6730A, ISL6730B, ISL6730C, ISL6730D can be shut down by pulling the FB pin below 0.5V or grounding the BO pin. The ISL6730C, ISL6730D have no skip mode.
• Small 10 Ld MSOP package
Two switching frequency options are provided. The ISL6730B, ISL6730D switch at 62kHz, and the ISL6730A, ISL6730C switch at 124kHz.
• TV AC/DC power supply
• Desktop computer AC/DC adaptor • Laptop computer AC/DC adaptor • AC/DC brick converters
FIGURE 1. TYPICAL APPLICATION
40 60 OUTPUT POWER (W)
FIGURE 2. PFC EFFICIENCY
TABLE 1. KEY DIFFERENCES IN FAMILY OF ISL6730
February 26, 2013 FN8258.0
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Copyright 2013, Silicon Frameworks, LLC
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Taking the Mea
W Jonathan Tucker Senior Marketer and Product Manager Keithley Instruments
hen measuring electrical current, the goal is to insert an ammeter in series with the circuit so that the current measured by the ammeter is identical to the current flowing through the circuit. In an ideal world, the meter would have absolutely no effect on the circuit; however, in real-world measurements, several error sources may contribute to substantial uncertainty in the measurement result. Digital multimeters (DMMs) are considered sufficiently accurate for most current measurements, but low level DC current measurements often demand greater sensitivity than these instruments can deliver. Generally, they lack the sensitivity required to measure currents less than 100nA. Even at higher current levels, a DMMâ€™s input voltage drop (voltage burden) of hundreds of millivolts can make accurate current measurements impossible. Picoammeters, in contrast, offer the economy and ease of use of a DMM with significantly better low current sensitivity. What makes these two types of instruments so different?
PULSE SHUNT AMMETERS VS. FEEDBACK AMMETERS The shunt method and the feedback ammeter methods are the two basic techniques for making low current measurements. The shunt configuration is used primarily in DMMs; modern picoammeters and electrometers use the feedback ammeter configuration.
Shunt Ammeter Shunting the input of a voltmeter with a resistor forms a shunt ammeter (Figure 1a). The input current (IIN) develops an input voltage (EIN) across the shunt resistance (RSHUNT). Note that the voltage sensitivity of the circuit is controlled both by the value of RSHUNT and the relative values of RA and RB. Although using a larger value for RSHUNT might initially appear advantageous, RSHUNT should be made as small as possible for several reasons. First, low value resistors have greater time and temperature stability and a better voltage coefficient than high value resistors. Second, low resistor values reduce the input time constant and result in faster instrument response times. Finally, for circuit loading considerations, the input resistance RSHUNT of an ammeter should be small to reduce the voltage burden EIN. As a result, the RSHUNT value can have an impact on measurement performance.
Figure 1a: Shunt Ammeter
Feedback Ammeter Figure 1b shows the general configuration of a feedback-type ammeter. In this configuration, the input current (IIN) flows into the input terminal of the amplifier (A) and through the feedback resistor (RF). The low offset current of the amplifier changes the current (IIN) by a negligible amount. Thus, the output voltage is a measure of the input current, and sensitivity is determined by the feedback resistor (RF). The low voltage burden (EIN) and corresponding fast rise time are achieved by the high gain operational amplifier, which forces EIN to be nearly zero. Circuit analysis of Figure 1b shows that: EOUT + IINRF = EIN
EOUT = –AEIN, and EIN = – EOUT / A
Thus, EOUT + IINRF = – EOUT / A Given that A>>1, EOUT = –IINRF and |EIN| = EOUT / A <<EOUT The amplifier gain can be changed as in the shunt ammeter circuit using a combination of resistors RA and RB that are inserted into the feedback loop, forming a multiplier. The gain of the circuit is determined by the feedback resistor and by the relative values of RA and RB and is given as follows: EOUT = –IINRF/(RA + RB)/RB and again, EIN = – EOUT / A
Figure 1b: Feedback Ammeter
Figure 2: Keithley Model 6482 and two-channel ion beam comparison (below) LIFTING THE VOLTAGE BURDEN The level of voltage burden involved─the voltage that appears across the ammeter input terminals when measuring─represents another important difference between a DMM and a picoammeter. As Figure 1a illustrates, the DMM shunt ammeter method requires voltage (typically 200mV) to be developed across a shunt resistor in order to measure current. This voltage burden will reduce the actual current flowing in the circuit, reducing the accuracy of the measurement. In contrast, a picoammeter’s feedback ammeter method reduces this terminal voltage by several orders of magnitude due to the high gain amplifier with negative feedback for the input stage. As a result, the voltage burden is greatly reduced—on the order of 200μV or less. This low voltage burden reduces both Visit: eeweb.com
PULSE measurement errors and the minimum shunt cable resistance that must be maintained to provide a given meter accuracy. Although the use of a picoammeter can substantially minimize errors in low current measurements, additional errors can arise from extraneous currents flowing through various circuit elements.
GETTING SWEPT AWAY BY NOISE CURRENTS Noise current generators represent unwanted currents generated at a particular point in a circuit. These currents may result from triboelectric, piezoelectric, and electrochemical effects or from resistive leakage or dielectric absorption. Triboelectric currents are generated by charges created at the interface between a conductor and an insulator due to friction between them. Here, free electrons rub off the conductor and create a charge imbalance that causes a current flow. A typical example would be electrical currents generated by insulators and conductors rubbing together in a coaxial cable. When minimizing noise is a priority, many system builders employ special low noise coaxial and triaxial cables to combat the effects of triboelectric currents. Piezoelectric currents are generated when mechanical stress is applied to certain crystalline materials are used for insulated terminals and interconnecting hardware. In some plastics, pockets of stored charge cause the material to behave in a manner similar to piezoelectric materials. Noise currents also arise from electrochemical effects. Here, ionic imbalances can create weak batteries between two conductors on a circuit board. For example, commonly used epoxy printed circuit boards can generate currents of several nano-amps when not thoroughly cleaned of etching solution, flux, perspiration, or other material. To prevent these error currents, all interconnecting circuitry should be thoroughly cleaned using a cleaning solvent such as methanol, and then be allowed to dry completely before use. Dielectric absorption can occur when a voltage applied across an insulator causes positive and negative charges within that insulator to polarize. When the voltage is removed, the separated charges generate a decaying current through external circuitry as they recombine. The effects of dielectric absorption can be minimized by avoiding the application of voltages more than a few volts to insulators that are used for sensitive current measurements. If it’s impractical to avoid doing so, it may take minutes or even hours for these currents to dissipate.
PICOAMMETER APPLICATIONS Ion beam detector alignment applications make good use of a picoammeter’s low current sensitivity. Measurements of ratio and delta (difference) between two channels can significantly improve efficiency in beam alignment applications by performing these calculations within the instrument itself. The beam alignment procedure is important in many different fields, from high energy particle beam monitoring to ion beam monitoring and fiber optics. It involves measuring a signal from a detector and adjusting the position of the source to align it to the sensor so that it matches a calibrated source/detector pair. Ratio and delta measurements can provide immediate feedback about how far out of alignment these beams are, which helps the operator adjust the source more quickly. Keithley’s Model 6482 Dual-Channel Picoammeter (Figure 2) incorporates ratio and delta measurement capabilities as well as dual voltage biases, which allow the sensors to be powered without the need for additional equipment. Two-channel picoammeters are also well suited for other powered applications: • Ion Beam Monitoring – Focused ion beam systems can be used in applications involving nanometer-scale imaging, micromachining, and mapping. Monitoring the magnitude of the beam current carefully with an ion detector is critical. These sensors accept ions and put out a small current. Multiple ion beams and sensors are often operated concurrently and require powered sensors. • Multiple Device Testing – Multi-channel devices increase throughput and reduce maintenance burden by increasing the number of channels in the same form factor. •M ulti-pin Device Testing – Multi-pin devices, such as dual diodes, ICs, and other components often need multi-channel, simultaneous current testing. Multiple channels allow one instrument to take measurements on multiple pins, offering new testing options and increasing throughput. ■
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