A 4-Channel Wearable Wireless Neural Recording System Moosung Chae, Kuanfu Chen, Wentai Liu, Jungsuk Kim, Mohanasankar Sivaprakasam Integrated Bioelectronics Research Group University of California at Santa Cruz, Santa Cruz, USA Abstract— This paper presents a 4-channel wearable wireless neural recording system that consists of wireless head stage, receiver board, and user interface software. The wireless head stage is implemented using a custom designed font-end IC for bio potential amplification, filtering, and channel selection and a commercial analog FM transmitter chip. Adjustable gain and bandwidth of the channel allows the system to be used in recording various neural signals. The size of the head stage is 1.55Ǝ × 1.725Ǝ × 0.5Ǝ and the weight is 20g. The power consumption of the head stage is 28mW from two 1.55V button cell batteries that allows continuous operation of at least 15 hours. The system was tested for ex-vivo and in-vivo extracellular recordings from a dissected snail brain and a live rat respectively. FM for data link [4-6]. [4], [5] use custom-designed FM transmitter and [6] uses commercial off-the-shelf (COTS) FM IC. A recording system that meets all those requirements mentioned is possible through a hybrid design method of employing both COTS components and custom designed subsystems [1]. In this paper, we present a 4-channel wearable wireless neural recording system implemented using COTS FM transceiver and custom front-end chip with adjustable channel gain and bandwidth. The system architecture is described in section II. Section III presents the design of the custom designed front-end chip; Section IV presents the testing result of the system; Section V presents the conclusion. I. INTRODUCTION Neuroscientists employ neural recording systems to monitor the behavior of live animals such as birds, reptiles, rats, and primates. In those experiments, animals under testing are usually anesthetized or bound by tethered wires, which imposes a great limitation on the behavior being observed and the information quality of the signals. Therefore a neural recording system with wireless telemetry is preferred to the one with wired outputs. Also, this particular system should be as small as possible so that it is wearable by the animals. It is also preferred to have the gain and bandwidth of the recording channels variable to accommodate the dynamic range of the signals. Because low power consumption is one of the most critical issues in this applications and wireless transmitter is usually the most power consuming component in the system, the choice of a suitable data transmitter is crucial. Digital transmission, such as FSK and QFSK, are used in several previous works [2-3], but under the same signal bandwidth, a much higher carrier frequency is required to transmit digitized data compared to analog modulations. Higher carrier frequency tends to consume more power, which is not desirable in battery powered neural recording system. For example, the single channel neural recording system in [2] consumes 130mW. Therefore, the analog transceiver has an advantage over the digital one at the power consumption point of view. Several previous neural recording systems choose 978-1-4244-1684-4/08/$25.00 ©2008 IEEE 1760 Figure 1. System architecture of the wireless neural recording system. II. SYSTEM ARCHITECTURE The Fig. 1 depicts the block diagram of proposed neural recording system. The entire system can be separated into three major parts: wireless head stage, receiver board, software user interface. Neural signals from the subjects are amplified and filtered by the custom IC. Then one selected channel is connected to the FM transmitter through the control signal supplied by the on-board switches. The 433MHz frequency modulation (FM) signal is transmitted via an antenna. At the receiver side, after demodulation, a low pass filter is used to remove high frequency noise. Since the AC output level of FM demodulator is only 400mV, an amplifier is used before the signal is captured by the NI data acquisition device, for PC display. A software user interface is programmed with LabView to display and record the captured data. Details of each part are described below. A. Wireless Head Stage The wireless head stage, as shown on the left of Fig. 2, can record from 4 channels. Signals are amplified, filtered, and selected through the custom-designed front-end IC. The architecture and specifications of this IC are explained in Section III. Since the neural signal is usually in the order of microvolts, an operational amplifier (STMicroelectronics TSV991) is used to achieve additional gain of 10. The small package (SOT23-5, 2.8mm × 2.9mm) makes TSV991 a good choice for this application. To block DC current at the amplifier output, an AC-coupling circuit is used between the amplifier and the FM transmitter. In this system a 433MHz Tx2 FM transmitter (Radiometrix) with bandwidth of 90 KHz is used. It requires only one external passive device, and it has a long transmission distance (tens of meters in buildings) with reasonable power consumption (9mA from 3.1V power supply). The physical dimension, 32mm × 12mm × 3.8mm, makes the transmitter the largest device on the head stage. The size of the planar antenna (Linx Technology) is 28mm × 13.7mm, which is similar to the FM transmitter. By stacking the antenna on top of the transmitter, additional area for the antenna is avoided. The total power consumption of this wireless head stage is 28mW. It is powered by two button batteries on the back side of the head stage. One 357 silver oxide button battery (Energizer) provides the supply voltage level of 1.55V, and has 175mAh capacity. The battery life time of this system is longer than 15 hours and hence it is suitable for the experiments monitoring neural activity over a long period of time. B. Receiver Board The receiver (Fig. 2) consists of a 433MHz Rx2 FM demodulator (Radiometrix) and an amplifier. Since the output signal of the FM demodulator tends to have high frequency noise, a low pass filter with cut-off frequency between 10 KHz to 50 KHz is applied to filter out the noise. According to the specification of the FM demodulator, the full range AC output level is only 400mV. To increase the amplitude a 1~10x variable gain amplifier is built using TSV991. We employ NI-6009 data acquisition device (National Instruments) as the interface between the receiver board and PC due to its small size (63mm × 85mm ×23mm) and compatibility with LabView software. Also, it can use the power from USB cable to supply the 5V and 2.5V power to the receiver board; so no external power supply is required. The receiver board is designed to match the position of the connectors on NI-6009 and it can be placeed on NI-6009 directly. NI-6009 has 14-bit high precision analog-digital converter with up to 48 KHz sampling rate. However, the voltage level of the signal fed into the ADC is smaller then the ADC input full-range, so the equivalent precision is 10-bit. The data captured by NI-6009 is fed to PC via USB2.0 interface. C. User Interface Software A user interface is designed to handle the data captured by NI device using a LabView 8.4 (National Instruments), as shown in Fig.3. This application software has several functions. Lower waveform window displays all the recorded data for a longer period of time. The upper waveform window is for spike detection. The spike detection is achieved by a user defined threshold voltage. Whenever the input signal is higher than the threshold voltage, the graphic window updates its value with the new spike waveform. This application software can also record the input data. Figure 2. Wireless head stage and receiver board (batteries at the back side). Figure 3. User interface software. 1761 III. FRONT-END CHIP Fig. 4 shows the block diagram and microphotograph of the custom designed 4-channel front-end chip. It consists of preamplifiers, an analog multiplexer, a 2nd amplifier, and an output buffer. The entire signal path is fully differential to eliminate the common mode noise. The preamplifier based on [7] is composed of a gain-stage and an output stage to drive the analog multiplexer. Fig. 5 shows the schematic of the preamplifier and OTA. Because there is often DC open-circuit potential at the electrode-electrolyte interface, the amplifier should have large DC blocking capacitors at the input [7] and this makes the preamplifier have large biasing resistors. The biasing resistors are implemented using NMOS in subthreshold region. The gate voltage for those NMOS, VB, is provided by an on-board simple voltage divider with a variable resistor and this enables adjustable low frequency roll-off of the preamplifiers. Experimental results show that this low-frequency roll-off can be adjusted from 0.01 Hz to 100 Hz. High frequency roll-off can be adjusted from 2 KHz to 20 KHz in 16 steps by changing the load capacitance CL. The gain of the preamplifier is determined by the ratio of the two capacitors, C1/C2, and is set to 100. A fully differential and self-biased operational transconductance amplifier (OTA) is used in preamplifiers and it enables the preamplifier to have common mode rejection ratio (CMRR) of 90dB and power supply rejection ratio (PSRR) of 80dB with input equivalent noise of 4.9µVrms. The input transistors are sized large enough to minimize the 1/f noise. Each preamplifier draws only 2µA and each buffer draws 20.3µA to drive the analog multiplexer. An analog multiplexer connects one selected channel to the 2nd amplifier by external control signals. The 2nd amplifier provides additional gain of 7 to 10 also according to the external control signals. The 2nd amplifier together with the output buffer consumes 40.6µA. The chip was fabricated in 0.35µm CMOS process. The die size of the front-end chip is 1.9mm × 2.7mm (core size is 1.5mm × 1.0mm). Figure 5. Schematic of the preamplifier and OTA used. IV. TESTING RESULTS The bench testing of the system was performed first and Table I illustrates the performance of this system in comparison with other previous works. An ex-vivo recording using Helix Aspersa snail was done to verify the proper operation of the system. In this experiment, a snail was dissected, and extracelluar action potentials were recorded successfully. Plastic suction electrode with an uninsulated 0.01Ǝ diameter stainless-steel wire was used as a recording electrode and an Ag-AgCl pellet ground electrode was placed into saline as a reference electrode. In order to record extracellular compound action potentials (CAPs), nerve stumps were inserted into electrode tip and suction was applied to obtain a tight seal. Fig. 6 shows the recorded signal. The system was also used for in-vivo recording experiments to record the extracellular action potentials from a live rat. Fig. 7 shows the rat wearing the system with electrodes connected. The system successfully recorded neural signal for two days and part of the recorded signal is shown in Fig. 8. Figure 4. Block diagram of the custom designed front-end chip. Figure 6. Recorded extracelluar action potentials from a disected snail brain. 1762 TABLE I. BENCH TEST RESULTS IN COMPARISON WITH OTHER WORKS Publication System Weight Dimension (cm) Power Consumption Power Supply Level Amplifier Channel # Input-refered Noise Overall Gain Technology Data Telemetry Telemetry Frequency Telemetry Bandwidth Distance [4] [5] [1] [6] [3] 2.2g 2.1x 2.1x 0.16 2.05mW ±1.5V <1g 0.5 x 0.5 x 1 5.8mW - 66g 6.5 x 3.1 x 6 50mW 3V 3.1g(w/o batt.) 2.5 x 1 x 0.5 ±1.4V - a few ounces 20g 3.4 x 7.1 3.9 x 4.4 x 1.2 130mW 28mW 3.7V ±1.55V 7 1 1 7.1µVrms 3.12µVrms 43.7dB 34dB 80dB 1.5µm CMOS 0.35µm CMOS 1.5µm CMOS 2 TI TLV2262 4 - 8 4 5.94µVrms 5µVrms 80dB 70dB 0.6µm CMOS 0.35µm CMOS FM with TDM 94~98MHz 150KHz 0.5m FM 88~108MHz 5KHz Few m FSK with TDM 433MHz 9.6Kbps >50m FM 3.2GHz 10KHz <1m IEEE 802.15 2.4GHz 250Kbps >20m [2] QFSK 2.4GHz 250kbps 1.8m This work FM 433MHz 90KHz >10m V. CONCLUSION A wearable neural recording system with wireless telemetry for monitoring live animal biopotentials was presented in detail. This system amplifies the signal, filters, transmits wirelessly, receives and stores in PC. The wireless head stage was implemented using custom designed front-end chip which has adjustable channel gain and bandwidth and COTS FM transmitter chip. The wireless capability with small power consumption and the minimal size and weight of the head stage make this system suitable for monitoring the behavior of live animals. The user-interface software enables users to be able to monitor and store the waveforms for later stage signal processing such as spike sorting and classification. Figure 8. Recorded neural signal from the live rat in Fig .7. REFERENCES [1] [2] [3] [4] Figure 7. A live rat wearing the wireless recording system for in-vivo extracellular recording from the brain [5] ACKNOWLEDGMENT The authors would like to thank Jerry Tian and Jiping He of Arizona State University for the rat experiments. [6] [7] 1763 S. Farshchi, A. Pesterev, E. Guenterberg, I. Mody, J.W. 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