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3328 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 55, NO. 10, NOVEMBER 2008 An Integrated Full-Wave CMOS Rectifier With Built-In Back Telemetry for RFID and Implantable Biomedical Applications Maysam Ghovanloo, Member, IEEE, and Suresh Atluri, Member, IEEE Abstract—This paper describes the design and implementation of an integrated full-wave standard CMOS rectifier with built-in passive back telemetry mechanism for radio frequency identification (RFID) and implantable biomedical device applications. The new rectifier eliminates the need for additional large switches for load modulation and provides more flexibility in choosing the most appropriate load shift keying (LSK) mechanism through shorting and/or opening the transponder coil for any certain application. The results are a more robust back telemetry link, improved read range, higher back telemetry data rate, reduced rectifier dropout voltage, and saving in chip area compared to the traditional topologies. A prototype version of the new rectifier is implemented in the m n-well 3-metal 2-poly 5 V standard CMOS process, AMI 0 5 occupying 0 25 mm2 of chip area. The prototype rectifier was powered through a wireless inductive link and proved to be fully functional in its three modes of operation: rectification, open coil (OC), and short coil (SC). Index Terms—Back telemetry, CMOS, full-wave rectifier, implantable biomedical devices, inductive coupling, load shift keying, radio frequency identification (RFID), wireless. I. INTRODUCTION R ADIO FREQUENCY identification (RFID) systems and implantable microelectronic devices (IMD) are two major categories of devices that inductively communicate with a reader over a short distance through a pair of loosely coupled coils, which constitute a transformer [1]–[11]. Contactless data transmission from the data-carrying device, also known as transponder, to the reader is one of the key functions, called back telemetry. It is also used in wireless sensing technology, where direct electrical contact to the sensing element is not feasible such as in tire pressure monitoring [12]. What is in common in these applications is the extreme limitation in size, which eliminates the use of a power source within the transponder especially when combined with the need for long term usage. A viable solution is to inductively power up Manuscript received May 7, 2007; revised October 5, 2007. First published April 18, 2008; current version published November 21, 2008. This work was supported in part by the Department of Electrical and Computer Engineering, North California State University, Raleigh, NC 27695 USA. This paper was recommended by Associate Editor P. Heydari. M. Ghovanloo is with the GT-Bionics Lab, School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30308 USA (e-mail: [email protected]). S. Atluri is with Integrated Device Technologies Inc., San Jose, CA 95138 USA (e-mail: [email protected]). Digital Object Identifier 10.1109/TCSI.2008.924877 Fig. 1. Simplified block diagram of an inductively powered RFID or implantable microelectronic device (IMD). the device via the same telemetry link that is setup for data transmission. As a result, the data communication and power handling blocks in RFID/IMD systems are highly intertwined and should be designed and developed together [1]. Fig. 1 shows a simplified block diagram of an RFID/IMD system. Power and data transmission take place across the and secondary inductive link formed by the primary coils on the reader and transponder sides, respectively. by a large sinusoidal signal, A power amplifier (PA) drives known as power carrier, at specific frequencies dedicated to RFID/IMD applications in the Industrial-Scientific-Medical and a series capacitor, , can form an (ISM) band [13]. oscillating tank circuit and become part of a class-E PA, which is a popular topology for RFID/IMD applications since it can theoretically reach power efficiencies up to 100% [14], [15]. It is possible to modulate the amplitude, frequency, or phase of the power carrier by applying a modulating signal (MOD) to the PA and detecting these changes on the transponder side. Therefore, MOD can be used to transmit commands along with power from the reader to transponder, a function that is usually referred to as forward telemetry [16]. Only that part of the electromagnetic flux generated by which passes through can participate in inducing current in and powering the transponder. This is characterized by the coils mutual coupling , which is dependant on the coils geometry, relative distance, orientation, and magnetic properties of the medium (air, tissue, or ferrite core) [16]–[18]. A capacis usually added in parallel to in order to form a secitor ondary tank circuit, tuned at , in order to boost the received voltage and improve the efficiency. To convert the received ac 1549-8328/$25.00 © 2008 IEEE Authorized licensed use limited to: Georgia Institute of Technology. Downloaded on December 7, 2008 at 20:50 from IEEE Xplore. Restrictions apply. GHOVANLOO AND ATLURI: INTEGRATED FULL-WAVE CMOS RECTIFIER power carrier to dc, the thank circuit is followed by a rectifier, which is the focus of this paper. Back telemetry can be performed in two ways: active and passive. In active back telemetry, which is used when there is a high volume of data to be sent to the reader at high rate, the transponder is equipped with a transmitter and a dedicated antenna operating at a frequency often much higher than [20]. The high-power consumption of the transmitter and the space needed for its circuitry and antenna, however, disfavor active back telemetry in most applications where lower data rates are acceptable. Passive back telemetry is possible by load modulation, also known as load shift keying (LSK), which is the most common technique for data transmission from RFID/IMD transponders back to the readers by a large margin [1], [10]. In this method, the loading of the transponder coil in Fig. 1) is varied with the outgoing serial data stream, ( due to the coils mutual altering the impedance seen through coupling, . The impedance change can either be resistive or capacitive, resulting in amplitude shift keying (ASK) or phase shift keying (PSK) of the back scattered signal [21]. In the rest of this paper we will focus on the ASK. It is desired to establish a robust back telemetry mechanism that can handle a large enough distance between the reader and transponder, known as the reading range or interrogation zone. Depending on the application, a large reading range also means that the system is robust enough to handle a certain degree of coils misalignments, motion artifacts, nearby metallic objects, and external electromagnetic noise and interference. The dynamic reading range of the RFID/IMD systems depends on detectability of the LSK impedance changes above the noise and interference levels on the reader side. This in turn depends , unloaded quality factors on the coils mutual inductance , and magnitude of the transponder load variations as a result of load modulation. To achieve a large reading range, we would like to compensate the effect of small with the other parameters. In this paper, we assume that and , which are mainly determined by the coil designs, are such already optimized. Thus, our goal is to maximize that it can be easily detected. Load modulation in current designs depends on the nominal loading of the transponder coil. In low power RFID appliis large, a parallel MOS switch shorts cations, where ahead of a rectifier for a short period of time [1]. Alternatively, a parallel switch can inflict impedance changes by altering the configuration of the rectifier from full-wave to half-wave and vice versa, as explained in [10]. In medium and high power tends to be small, a series RFID/IMD applications, where MOS switch is being used after a full-wave rectifier to open the could load [11]. In complex RFID/IMD systems, however, itself be a highly variable parameter depending on the state of the transponder while being interrogated. For example, the idle current consumption of a multichannel implantable microstimulating system, used in auditory or visual prostheses, might be negligible compared to its current consumption when multiple stimulating channels are simultaneously active [22], [23]. alone may not provide a large Therefore, shorting or opening in all conditions, degrading the nominal reading enough range. Further, the load modulation MOS switches, which have 3329 Fig. 2. (a) Basic circuit model for the analysis of the inductive link and its back telemetry mechanism. (b) Simplified model of the transponder circuit. to be large to provide small “on” resistance, consume chip area and in case of the series switch [11] result in additional dropout voltage, degrading the overall system power efficiency. We have developed an integrated full-wave rectifier in standard CMOS technology with built-in back telemetry, which can , to produce a large both short and open the transponder coil regardless of the actual load variations. In addition, by combining rectification and back telemetry functions in one circuit block (the gray box in Fig. 1); we have achieved significant saving in chip area and eliminated the need for any additional large switches before or after the rectifier. This can also reduce the dropout voltage and improve the overall power efficiency. In the following section, the theoretical basis for load modulation scheme has been laid out with emphasis on the parameters that have the most significant effects on the ASK back telemetry mechanism. Section III describes the circuit topology of the new rectifier and its modes of operation. The simulation and experimental measurement results of a prototype rectifier with built-in back telemetry are depicted in Section IV followed by a short discussion and concluding remarks in Section V. II. LOAD MODULATION FOR BACK TELEMETRY Fig. 2(a) shows a circuit model of Fig. 1 block diagram that helps analyzing the load modulation mechanism. The only independent power source in this circuit is a sinusoidal voltage operating at , which represents the class-E power source amplifier with an output impedance of . Current induces across that is shown as a separate a voltage current controlled voltage source, which drives the transponder circuitry by creating . To model the rectifier, which is a nonlinear circuit, we have is much larger than assumed that the load time constant . This is usually the case in order to reject supply ripples across the load. We replaced the rectifier with an ideal diode in series with a dc voltage source equal to the rectifier dropout . Therefore, the voltage across would be a dc voltage , where . We can then voltage equal to for the rectifier and by find an ac equivalent resistor considering the amount of power consumed by [5]. Authorized licensed use limited to: Georgia Institute of Technology. Downloaded on December 7, 2008 at 20:50 from IEEE Xplore. Restrictions apply. (1) 3330 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 55, NO. 10, NOVEMBER 2008 (2) , the transponder By replacing the load and rectifier with simplifies to a series/parallel RLC circuit, shown in Fig. 2(b), and loaded quality factor of with a loop impedance of (3) (4) is the parasitic resistance of the transponder coil [1]. where The transponder reflected impedance onto the reader is shown in series with and its numerical value can be in Fig. 2 as found from [10] Fig. 3. Reflected transponder impedance onto the reader side (Z ) versus equivalent resistance seen across the rectifier input (R ). Other inductive link parameters are listed in Table I. (5) where is the parasitic resistance of coupling coefficient, which is defined as and is the coils TABLE I SPECIFICATION OF THE INDUCTIVE LINK (6) In order to increase the interrogation zone, variations as a changes should be maximized. Equations (2)–(6) result of can provide significant insight in understanding which paramand consequently the interrogation eters in Fig. 2(a) affect zone. Some conclusions are following: and dei) is the most significant factor that affects pends on the coils distance, orientation, inductance, geometry, and magnetic properties of the medium [18]. The reading range depends on the minimum for which the variations. transponder can produce detectable and need to be maximized by increasing and ii) and decreasing and . However, these parameters are interrelated and there is usually a physical size conand should straint especially on . In addition, with and , respectively. Therefore, resonate at a compromise should be made to optimize these parameters with proper choice of the coils geometries, number of turns, and wire thicknesses, which have been covered in the literature extensively [5], [11], [16]. It should also be noted that the efficiency of the inductive link, %, is proportional to , which is present in the numerator of (5) [5]. because it iii) It would also be helpful to maximize would increase , as shown in [1], and results in higher efficiency, as well as improved back telemetry signal. By according to (7), which is derived by difchoosing at the resonance freferentiating (4) with respect to can be found for every given quency, the best value for pair of and . iv) Reducing the PA output resistance similar to reducing improves detectability of variations. v) Another important factor in extending the interrogation , which along with are the only paramezone is ters in (5) that can be varied with the telemetry data to af. Fig. 3 shows how the magnitude of changes fect when other inductive link parameters with respect to in (5) are selected from our experimental setup listed in is a monotonic function of in its Table I. Since is maxipractical range, the best way to maximize , which is proportional to according to mizing (2). Therefore, increasing the transponder load variations improves the reading range in passive back telemetry. as a It is instructive to examine the two extreme limits of result of load modulation when the load is short or open cirin Fig. 2(b), the transponder circuit reduces cuit. If we short circuit and the reflected impedance can be found to an from (7) (8) Authorized licensed use limited to: Georgia Institute of Technology. Downloaded on December 7, 2008 at 20:50 from IEEE Xplore. Restrictions apply. GHOVANLOO AND ATLURI: INTEGRATED FULL-WAVE CMOS RECTIFIER 3331 TABLE II RECTIFIER MODES OF OPERATION Fig. 5. Chip micrograph including two rectifiers with built-in back telemetry and a few on-chip ripple rejection capacitors implemented in AMI-0.5 m standard n-well CMOS process (Die size: 1.5 mm 1.5 mm). 2 Fig. 4. (a) Schematic diagram of the full-wave CMOS rectifier with built-in back telemetry. (b) Schematic diagram of a 2:1 multiplexer (MUX) with dc level shifter. On the other hand, if is opened, the transponder circuit in Fig. 2(b) simplifies to an circuit in which and cancel out at resonance frequency and can be found from (9) and for parameters listed The numerical values of in Table I are 1.39 and 7.99 , respectively. III. BACK TELEMETRY RECTIFIER CIRCUIT DETAILS Fig. 4 shows the schematic diagram of the new rectifier with built-in back telemetry mechanism, which is an enhanced version of the CMOS rectifier that we reported in [19]. The recti, and , and fier consists of four main transistors: a few smaller switches that control the operation of these large current carrying transistors based on coil and output voltages. and directly connect to the The rectifier input terminals tank and the rectified dc output is delivered transponder to the load through terminal. and are also connected to a pair of high voltage protection circuits that consist of several nMOS transistors in series which start shunting current to or exceed a certain limit ( V). The ground when rectifier has three modes of operation that are controlled by its Fig. 6. Measured rectifier operation in Mode-0 showing the waveforms at the input terminals (V and V ), output terminal (Vout), and the separated n-well ), which closely follows Max(V ; V ; V ). body voltages (V Open Coil (OC) and Short Coil (SC) digital inputs. These operating modes are also summarized in Table II. Rectifier: In this mode, which is Mode-0 the default rectifier configuration, a pair of 2:1 multiplexers and (MUXp) controlled by OC connect the gates of to turning them into large (4 mm/0.6 m) diode-connected pMOS devices, which direct induced current from Authorized licensed use limited to: Georgia Institute of Technology. Downloaded on December 7, 2008 at 20:50 from IEEE Xplore. Restrictions apply. 3332 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 55, NO. 10, NOVEMBER 2008 = 330 Fig. 7. (a) Measured back telemetry waveforms in Mode-1 using Open Coil (OC) input when R . (b) Measured back telemetry waveforms in Mode-2 : k . From top: serial back telemetry data, secondary coil voltage, rectified output, and primary current through a using Short Coil (SC) input when R current sense transformer. = 11 to the load. Meanwhile, another pair of 2:1 multiplexers and (MUXn) controlled by SC connects the gates of (1.96 mm/0.6 m) to and , respectively, to return the load current back to the coil from the grounded p-type substrate. and which are parasitic diodes between p-sub and and drain terminals also help in returning current back to the coil or V. To eliminate latchup and substrate when and are created on separated n-well regions leakage, (Body and Body ) and a pair of transistors to ) is added to each rectifying pMOS complex to dynamically control and ). As a result their isolated n-well potentials ( ) and ) [19]. This mechanism also minimizes the rectifier dropout and by eliminating their body effect, voltage across further improving the overall efficiency. Each MUX in Fig. 4(a) is equipped with a cross coupled dc level shifter, shown in Fig. 4(b), which converts the rectifier input logic levels from to so that they are applicable to the pass gates. Open Coil: In this mode MUXp Mode-1 pair connects the gates of and to their isolated body terand . Therefore, and minals resulting in turn off and effectively detach the load from . and , which stay in the same state as in Mode-0, remain off because Authorized licensed use limited to: Georgia Institute of Technology. Downloaded on December 7, 2008 at 20:50 from IEEE Xplore. Restrictions apply. GHOVANLOO AND ATLURI: INTEGRATED FULL-WAVE CMOS RECTIFIER the flow of current from to has been disrupted and their do not let them to be on out of phase gate voltages faces very large impedance at the same time. As a result, through rectifier input, and and theoretically reach their and , respectively. This would maximum levels, tank and decreased curresult in increased voltage across , which can be easily detected esrent in the primary coil pecially with small to recover back telemetry data from OC [see Fig. 7(a)]. In practice, various leakage currents at the rectito be infinitely large as in (9). fier input do not let Short Coil: In this mode MUXn Mode-2 pair connects the gates of and to resulting in , where is the nMOS threshold and stay on continuously and create voltage. Therefore, tank, resulting in and a small impedance across theoretically reach their minimum values, and , respectively. This would result in decreased voltage across tank and increased current in the primary coil, which to recover back can be easily detected especially with large telemetry data from SC [see Fig. 7(b)]. Practically, and on resistances do not let reach the short circuit level and stay off as long as stated in (8). Diode-connected and , eliminating the storage capacitor which supplies the load in Modes 1 and 2, from being and . discharged into IV. MEASUREMENT RESULTS A prototype rectifier with built-in back telemetry was dem n-well 3M/2P 5 V signed and fabricated in the AMI standard CMOS process. Fig. 5 shows the prototype chip micrograph which includes two rectifiers, enclosed in white boxes, and a few on-chip ripple rejection capacitors. Each rectifier ocmm of chip area in this process. The measurecupies ment setup was similar to the block diagram in Fig. 1 [12]. The rectifier was powered by a class-E amplifier operating at 500 kHz through a pair of planar spiral coils, which specifications are summarized in Table I [20]. Fig. 6 shows the rectifier input, body, and output voltage . waveforms while operating in Mode-0 with and separated n-well regions were not Even though physically connected, their body voltages tend to be very . It can be seen that follows close as a result of operation and therefore eliminates latchup and substrate leakage by keeping all the parasitic components off [19]. Figs. 7(a) and (b) show the rectifier operation in Modes 1 and 2 when there is a dynamic and , respectively. In load condition with this experiment, back telemetry data was applied to the OC and SC inputs as a 15-kHz square wave with 35% duty cycle (first increases trace). The second trace in Fig. 7(a) shows how , which reflects back onto the as a result of current . is measured primary coil by a reduction in wire through a current sense transformer, which by passing output voltage is shown as on the fourth trace. Since the focus of this paper is on the integrated back telemetry rectifier as part of the transponder, and not the reader, the current sense transformer was far from optimal and intended only for showing the primary current variations 3333 on the oscilloscope. In an actual reader, however, the sensing capability of the current sense transformer can be significantly improved by enhancing its coupling with . In addition, the current sense transformer output can be amplified and filtered around the back telemetry subcarrier spectrum to eliminate the much larger main carrier interference [1]. This would result in current variations from an a sensitive reader that can detect extended reading range in presence of noise and interference. deIn contrast with Fig. 7(a), in Fig. 7(b) (Mode-2), and reflects back onto the creases as a result of current. In these experiments, primary coil by an increase in we have intentionally chosen a small ripple rejection capacitor, nF, to demonstrate on the 3rd trace in both figures how exponentially decays when or , during is temwhich period there is no rectification and the load porarily supplied by the stored charge in . V. CONCLUSION We have developed an integrated full-wave standard CMOS rectifier with dual-mode built-in back telemetry for RFID and implantable microelectronic device applications. The new rectifier eliminates the need for additional large switches for load modulation, reducing the rectifier dropout voltage and saving chip area. The new rectifier also provides more flexibility in choosing the most appropriate LSK-ASK back telemetry mechanism through shorting and/or opening the transponder coil for any certain application. In complex systems with multiple operating modes and variable dynamic loading requirements, in order to maximize load variations for back telemetry, OC can be is small) and SC can be used when power demand is high ( used when the system is not fully active and has a small power is large). The ultimate goal in both condiconsumption ( tions is to improve the read range and achieve a more robust . Alternatively, OC can back telemetry link by increasing variations always be followed by SC to produce maximum . Further, since Modes 1 and 2 can be distinregardless of guished on the reader side by a reduction and an increase in the primary coil current, respectively, it is also possible to assign more bits to OC and SC combinations to achieve higher back telemetry data rates. In order to verify and evaluate the new rectifier topology, we developed a prototype version of the new rectifier in the m n-well standard CMOS process, occupying AMI mm of chip area. The prototype rectifier was tested and proved to be fully functional in all three modes of operation: rectifier, open coil, and short coil. ACKNOWLEDGMENT The authors would like to thank the Department of Electrical and Computer Engineering, North Carolina State University, for their support and the MOSIS Educational Program for fabricating the prototype chip. REFERENCES [1] K. Finkenzeller, RFID-Handbook, 2nd ed. Hoboken, NJ: Wiley, 2003. [2] U. Kaiser and W. Steinhaugen, “A low-power transponder IC for highperformance identification systems,” IEEE J. Solid-State Circuits, vol. 30, no. 3, pp. 306–310, Mar. 1995. Authorized licensed use limited to: Georgia Institute of Technology. Downloaded on December 7, 2008 at 20:50 from IEEE Xplore. Restrictions apply. 3334 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 55, NO. 10, NOVEMBER 2008 [3] A. Abrial, J. Bouvier, M. Renaudin, P. Senn, and P. Vivet, “A new contactless smart card IC using an on-chip antenna and an asynchronous microcontroller,” IEEE J. Solid-State Circuits, vol. 36, no. 7, pp. 1101–1107, Jul. 2001. [4] U. Karthaus and M. Fischer, “Fully integrated passive UHF RFID transponder IC with 16:7 W minimum RF input power,” IEEE J. Solid-State Circuits, vol. 38, no. 10, pp. 1602–1608, Oct. 2003. [5] W. H. Ko, S. P. Liang, and C. D. Fung, “Design of radio-frequency powered coils for implant instruments,” Med. Biol. Eng. Comput., vol. 15, pp. 634–640, Nov. 1977. [6] M. 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Sivaprakasam, and G. A. Kendir, “Design and analysis of an adaptive transcutaneous power telemetry for biomedical implants,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 52, no. 10, pp. 2109–2117, Oct. 2005. [12] Microchip Technology Inc., “Tire pressure monitoring system user’s guide,” [Online]. Available: http://ww1.microchip.com/downloads/en/DeviceDoc/51624a.pdf Microchip Technology Inc., “Tire pressure monitoring system user’s guide,” [Online]. Available: http://ww1.microchip.com/downloads/en/DeviceDoc/51624a.pdf [13] American National Standard for limits and methods of measurement of radio disturbance characteristics of Industrial, Scientific, and Medical (ISM) radio-frequency equipment, ANSI C63.011-2000, ANSI, 2000. [14] N. O. Sokal and A. D. Sokal, “Class E, a new class of high efficiency tuned single-ended switching power amplifiers,” IEEE J. Solid-State Circuits, vol. SC-10, pp. 168–176, Jun. 1975. [15] P. R. Troyk and M. A. K. Schwan, “Closed-loop class E transcutaneous power and data link for microimplants,” IEEE Trans. Biomed. Eng., vol. 39, pp. 589–599, Jun. 1992. [16] M. Ghovanloo and G. Lazzi, “Transcutaneous magnetic coupling of power and data,” in Wiley Encyclopedia of Biomedical Engineering, M. Akay, Ed. Hoboken, NJ: Wiley [Online]. Available: http://www.mrw. interscience.wiley.com/ebe/articles/ebs1372/bibliography-fs.html [17] C. M. Zierhofer and E. S. Hochmair, “Geometric approach for coupling enhancement of magnetically coupled coils,” IEEE Trans. Biomed. Eng., vol. 43, no. 7, pp. 708–714, Jul. 1996. [18] M. Soma, D. G. Galbraith, and R. L. White, “Radio-frequency coils in implantable devices: misalignment analysis and design procedure,” IEEE Trans. Biomed. Eng., vol. 34, pp. 276–282, Apr. 1987. [19] M. Ghovanloo and K. Najafi, “Fully integrated wide-band high-current rectifiers for wireless biomedical implants,” IEEE J. Solid-State Circuits, vol. 39, no. 11, pp. 1976–1984, Nov. 2004. [20] M. Ghovanloo and S. Atluri, “A wideband power-efficient inductive wireless link for implantable microelectronic devices using multiple carriers,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 54, no. 10, pp. 2211–2221, Oct. 2007. 0 [21] M. Suster, J. Guo, N. Chaimanonart, W. H. Ko, and D. J. Young, “A wireless strain sensing microsystem with external RF power source and two-channel data telemetry capability,” in IEEE Dig. Tech. Papers Solid-State Circuits Conf., Feb. 2007, pp. 380–381. [22] M. Ghovanloo and K. Najafi, “A modular 32-site wireless neural stimulation microsystem,” IEEE J. Solid-State Circuits, vol. 39, no. 12, pp. 2457–2466, Dec. 2004. [23] M. Ghovanloo and K. Najafi, “A wireless implantable multichannel microstimulating system-on-a-chip with modular architecture,” IEEE Trans. Neural Syst. Rehab. Eng., vol. 15, no. 3, pp. 449–457, Sep. 2007. Maysam Ghovanloo (S’00–M’04) was born in 1973 in Tehran, Iran. He received the B.S. degree in electrical engineering from the University of Tehran, in 1994 and the M.S. degree in biomedical engineering from the Amirkabir University of Technology, Tehran, in 1997. He also received the M.S. and Ph.D. degrees in electrical engineering from the University of Michigan, Ann Arbor, in 2003 and 2004, respectively. His Ph.D. research was on developing a wireless microsystem for micromachined neural stimulating microprobes. In December 1998, he founded Sabz-Negar Rayaneh Co. Ltd., Tehran, to manufacture physiology and pharmacology research laboratory instruments. In summer 2002, he was with Advanced Bionics Inc., Sylmar, CA, working on spinal-cord stimulators. From 2004 to 2007, he was an Assistant Professor with the Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, where he founded and directed the NC-Bionics Lab. In June 2007, he joined the faculty of Georgia Institute of Technology, Atlanta, where he is currently an Assistant Professor in the School of Electrical and Computer Engineering. He has more than 60 conference and journal publications. Dr. Ghovanloo is an Associate Editor of the IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS. He has been a member of the Technical Program Committee for the IEEE Midwest Circuits and Systems (MWSCAS), International Symposium on Circuits and Systems (ISCAS), and Biomedical Circuits and Systems (BioCAS) conferences. He has received awards in the operational category of the 40th and 41st DAC/ISSCC student design contest in 2003 and 2004, respectively. He is a member of Tau Beta Pi, Sigma Xi, and IEEE Solid-State Circuits, Circuits and Systems, and Engineering in Medicine and Biology societies. Suresh Atluri (S’05–M’07) was born in India in 1983. He received the B.S. degree in electrical and electronics engineering from BITS, Pilani, Rajasthan, in 2004 and the M.S. degree in electrical engineering from the Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, in 2006. He specialized in analog, digital, and microwave circuit design. His research was oriented toward developing inductive power and data transfer circuits for implantable biomedical devices. He joined Integrated Device Technologies Inc., Atlanta, GA, in 2006, where he works as an electronics design engineer. Authorized licensed use limited to: Georgia Institute of Technology. Downloaded on December 7, 2008 at 20:50 from IEEE Xplore. Restrictions apply.