A Bio-amplifier with Pulse Output Du Chen, John G. Harris and Jose C. Principe Department of Electrical and Computer Engineering, University of Florida, Gainesville, USA Abstract—A low-power fully integrated bioamplifier is presented that can amplify signals in the range from mHz to kHz while rejecting large DC offsets generated at the electrode-tissue interface. The novel aspect of this amplifier is that its analog output is represented by a series of pulses which provide a low-power, noise-resistant means for coding and transmission. The original analog signal can be reconstructed from the resulting pulse train with 13 bit precision at a remote site where power consumption is not so crucial. The fabricated analog amplifier exhibits a gain of 39.5dB from 0.3 Hz to 5.4k Hz. The power consumption of the whole system is less than 300 μW/channel from a 5-V supply. The fully integrated system was designed in the AMI 0.6μm CMOS process and it consumes 0.088 mm 2 /channel of chip area. Keywords: Bio-amplifier, CMOS, low power, pulse train I. INTRODUCTION The steady advance in MEMS technology has stimulated the rapid growth of electrode array micro-sensors used in biomed- ical applications, especially in the field of neural recording [1]. Implanted electrodes in sub-cortical regions are used to investigate the correlation between neuron population activity and associated subject behavior. Hardware for recording the signal directly from the electrodes must be small and low power to satisfy the implantation requirements for a large number of channels. One of the solutions is to use CMOS circuits to acquire and process the electrical signals transduced from implanted extracellular cortical electrodes. The extracellular neural signals have amplitudes of 50- 500μV [2], but large DC offsets arise across different record- ing electrodes due to electrochemical effects at the electrode- tissue interface. The magnitude of these DC offsets is about 1-2V [3], much larger than the neural signals to be measured. The frequencies of the brain waves range from 100Hz to 7kHz [4], while the Local Field Potentials (LFP) extend to below 1Hz. Thus, the ideal band-pass filter for neural recording must reject the DC offset while passing the LFP signal. Although the recording signal is analog, post-processing algorithms are more and more digitally based, which raises the need of translating the analog signals to digital representations through analog to digital converters (ADC) [4]. An on-chip ADC is required to enhance signal-to-noise ratio, increase robustness and provide a wireless transmission interface to reduce the risk of infection for chronic recording [5], [7], [6]. In the proposed circuitry, the analog output of the amplifier is translated to a series of asynchronous pulses which has better noise immunity than conventional analog signals in transmission and also eliminates the need for a traditional ADC. We have shown mathematically that the original bandlimited signal can be perfectly reconstructed solely from noise-free pulse timings. The pulse representation strategy tradeoffs a simpler, lower-power circuit with a more complicated signal reconstruction algorithm which is presumably run outside the body where power consumption is not such a critical resource. II. CIRCUIT DESIGN Fig. 1 shows the block diagram of the on-chip circuitry. Due to the very small input signal amplitude, it is necessary to preamplify the signal prior to other processing. The first stage is a pre-amplifier providing about 40dB gain at the passband and an AC coupling technique is used to reject the inherent DC offset. The second stage is an integrate-and-fire neuron which encodes the analog information in a pulse train. The voltage output of the amplifier is first converted into current and, by integrating this current, the amplitude information is encoded into an asynchronous digital pulse train. Bio-amp V-I Converter Integrator Comparator reset Delay Input Pulse Train Output + - + - Threshold Fig. 1. Block diagram of the on-chip circuitry A. Preamplifier The structure of the preamplifier was originally proposed by Harrison in [8]. Fig. 2 shows the schematic of the bio- amplifier. The midband gain Am is C 1 /C 2 , the bandwidth is approximately g m /(A M C L ), where g m is the transconduc- tance of the operational transconductance amplifier (OTA). C1 C1 C2 C2 CL Vout Vin Vref + - OTA VDD VSS Mc Md Ma Mb Fig. 2. Schematic of bio-amplifier 0-7803-8439-3/04/$20.00©2004 IEEE 4071 Proceedings of the 26th Annual International Conference of the IEEE EMBS San Francisco, CA, USA • September 1-5, 2004