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Micropower Circuits for Bidirectional Wireless Telemetry in Neural Recording Applications

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Micropower Circuits for Bidirectional Wireless Telemetry in Neural Recording Applications 1950 IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 52, NO. 11, NOVEMBER 2005 Micropower Circuits for Bidirectional Wireless Telemetry in Neural Recording Applications Nathan M. Neihart, Student Member, IEEE, and Reid R. Harrison*, Member, IEEE Abstract—State-of-the art neural recording systems require One solution to these problems is to replace the cables with a electronics allowing for transcutaneous, bidirectional data wireless transmitter powered through a magnetic transcutaneous transfer. As these circuits will be implanted near the brain, they link. Such a system eliminates any cables protruding through the must be small and low power. We have developed micropower integrated circuits for recovering clock and data signals over a skin, reducing the chance of postoperative infections. A trans­ transcutaneous power link. The data recovery circuit produces mitter implanted near the electrode array also shortens the dis­ a digital data signal from an ac power waveform that has been tance that the neural signal must travel over a wire, thus reducing amplitude modulated. We have also developed an FM transmitter a substantial source of noise. The transmitting system should with the lowest power dissipation reported for biosignal telemetry. also amplify the weak neural signals before transmission fur­ The FM transmitter consists of a low-noise biopotential amplifier and a voltage controlled oscillator used to transmit amplified ther improving the signal to noise ratio. neural signals at a frequency near 433 MHz. All circuits were fab­ A wide variety of transmitters for telemetering biological ricated in a standard 0.5-/urn CMOS VLSI process. The resulting signals have been previously reported [3J—[7]. However, these chip is powered through a wireless inductive link. The power con­ transmitters are not well suited for implantable neural recording sumption of the clock and data recovery circuits is measured to be applications. The circuits reported in [3]-[5] use a large number 129 /xW ; the power consumption of the transmitter is measured to be 465 /jW when using an external surface mount inductor. of discrete components, which makes these transmitters larger Using a parasitic antenna less than 2 mm long, a received power than the integrated solution detailed in this paper. The circuits level was measured to be —59.73 d B m at a distance of one meter. reported in [6] and [7] are optical transmitters, and suffer from Index Terms—Biotelemetry, low-power CMOS circuits, RF limited transmission distances (often in the millimeter range). telemetry, transcutaneous data link, transmitter. Moreover, most of the previously mentioned transmitters use batteries as power supplies. These batteries have a finite lifetime and would require additional surgeries in order to be replaced. I. Introduction Even rechargeable batteries have a limited number of recharge 7TFH advances in integrated circuit technology and the cycles before they must be replaced. V y development of new biocompatible materials, biomed­ In many medical applications, bidirectional wireless data ical implants are becoming more common. Modem MEMS transfer is necessary to transmit configuration data and a refer­ technology has led to the development of large-scale neural ence clock signal to the implant. In a neural recording device, recording devices such as microelectrode arrays containing as the configuration data could be used to select which electrode many as 100 independent electrodes [1], [2]. These arrays have should be actively recording, or which bank of amplifiers could many potential uses in both the scientific and clinical realms. be powered down to save energy. A clock signal is often nec­ A current limitation of this technology is the way in which essary to synchronize the implant’s telemetry with the outside recorded neural signals are transferred from the recording de­ circuitry. vice, which is ideally implanted in the body, to signal processing In this paper, we present a low-power bidirectional telemetry equipment used for scientific or neuroprosthetic applications. In system that is suitable for use in a fully implanted neural most modem neuroscience laboratories, wires are used to con­ recording application. The system is capable of recovering nect the implanted recording device to external amplification a clock and data signal that is transmitted into the body, as and signal processing equipment. There are two main problems well as transmitting amplified neural data out of the body. The with the use of transcutaneous cables: (1) the risk of infection, entire system performs these operations while powered from a and (2) noise pickup due to the high impedance and low signal magnetic transcutaneous link. In Section II, we describe circuits levels associated with neural recording. for a wireless power and data link. In Section III, we present a low-power integrated FM transmitter. A discussion of the test setup and testing results, including a comparison of this work Manuscript received August 2, 2004; revised February 20, 2005. This work with other similar work, is given in Section IV. We summarize was supported in part by the National Science Foundation (NSF) under CA­ and draw conclusions in Section V. REER Grant ECS-0134336. Asterisk indicates corresponding auhtor. N. M. Neihart was with the University of Utah, Salt Lake City, UT 84112 USA. He is now with the Department of Electrical Engineering, University of II. W ir e l e ss P o w e r e d D ata L in k Washington, Seattle, WA 98195 USA. 'R. R. Harrison is with the Department of Electrical and Computer A. Wireless Power Supply Engineering, University of Utah, Salt Lake City, UT 84112 USA (e-mail: [email protected]). Power is supplied to our circuits by means of a magnetic Digital Object Identifier 10.1109/TBME.2005.856247 transcutaneous power link adapted from [8]. Component values 0018-9294/$20.00 © 2005 IEEE Authorized licensed use limited to: The University of Utah. Downloaded on May 17,2010 at 22:28:54 UTC from IEEE Xplore. Restrictions apply. NEIHART AND HARRISON: MICROPOWER CIRCUITS FOR BIDIRECTIONAL WIRELESS TELEMETRY IN NEURAL RECORDING APPLICATIONS 1951 Fig. 1. Block diagram of the implantable clock and data recovery circuitry. were modified to give an operating frequency of 1 MHz. The choice of operating frequency is a tradeoff between magnetic coupling, which increases with frequency, and power absorp­ tion into the skin, which also increases with frequency. A class E amplifier is used to drive a 10-cm diameter, 22-turn #200/38 Litz wire transmitting coil at 1 MHz (L = 18 ^H, Q > 100 at 1 MHz). The amplifier is capable of amplitude modulating the transmitted power waveform to encode data on the power link. The ac signal is received with a 4-cm diameter coil consisting Fig. 2. Prefilter circuit used to attenuate the received power waveform and set of four turns of 28 AWG wire (L = 1.6 ^H, Q = 100 at 1 the dc level to Vnn/%- MHz). The coils were separated by a distance of approximately 1 cm. Both coils were designed to have a reasonable size for implantable device applications. Little effort was placed into 1) Prefilter: The received ac power waveform cannot be di­ optimizing the magnetic link between transmitter and receiver rectly applied to the input of the clock and data recovery cir­ in this work, but this topic has received considerable attention cuitry for two reasons. First, the magnitude of the ac signal can elsewhere [9]—[11]. be greater than 10 V, and prolonged application of a signal of The received ac signal is converted to a dc signal of the re­ this magnitude could result in damage to, or even failure of, quired magnitude by a bridge rectifier followed by an on-chip the inputs of the clock and data recovery circuitry. Second, an linear voltage regulator [12]. For ease of testing, the rectifier arbitrary dc offset can be present in the received power wave­ is currently an off-chip component, but could easily be fully form. This offset could result in the saturation of the data re­ integrated in the future [13]. The voltage regulator used has covery circuit and must be removed before further processing a dropout voltage of 1.4 V and provided a regulation of 647 can take place. The prefilter is responsible for correcting both of ppm/V. As we will show in later sections, the overall power con­ these problems (see Fig. 2). The circuit consists of a capacitor sumption of the integrated circuit is less than 1 mW. The class voltage divider, which attenuates the received power waveform E power transmitter consumes approximately 2 W, resulting in by a factor of four and removes any dc offset. Two resistors set an overall power transfer efficiency much less than 1%. This the dc bias point to V dd/2, where Vdd is the regulated on-chip low efficiency results in large part from the inefficient coupling power supply voltage. of the transmit and receive coils, and underscores the need for 2) Clock Recovery Circuit: An on-chip clock signal may be ultra-low-power implantable electronics. recovered from the transmitted power waveform. The clock re­ covery circuit consists of a Schmitt trigger followed by a master- slave D flip-flop as shown in Fig. 3. The Schmitt trigger is used B. Circuits for Clock and Data Recovery to make the circuit more resistant to high frequency noise that Traditionally, data is either amplitude modulated or frequency may be present on the received power waveform. The D flip-flop modulated onto the power waveform that is used to power the guarantees a clock signal with a 50% duty cycle, regardless of implant [8], [11]—[15]. For this work, amplitude modulation was the duty cycle of the received power waveform. The frequency chosen over frequency modulation in order to reduce the com­ of the recovered clock signal is one half the frequency of the plexity and power consumption of the implanted demodulation received power waveform.
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