Wireless Neural Recording

Text Box: Wireless recording of the neural signals from a large number of recording sites is highly desired because a growing number of neuroscientists have become interested in visualizing the extracellular activities of hundreds to thousands of single neurons in awake, freely moving animals. High site-count neural recording systems are currently hardwired and the tethering effects of the wires attached to implanted electrodes interfere with natural animal behavior and bias the overall results. Considering that neural signals have a bandwidth of about 10 kHz, a wideband telemetry link in the order of tens of MHz is needed to wirelessly record from a large number of sites, simultaneously. So far most of the reported wireless neural recording systems have been battery powered, and therefore, not fully implantable except for a short period of time. The objective of the present research is to develop an inductively powered 15-channel wireless implantable neural recording system for long-term in vivo experiments.

Text Box: The electrical connection to the neural tissue is formed through either a group of metal microwire electrodes or a micromachined silicon microelectrode array. For every recording channel, a low-noise low-power amplifier (LNA), which is capable of amplifying signals from millihertz to kilohertz range, is used to amplify the acquired neural signals. A capacitive highpass filter at the input of every LNA rejects the large DC offset generated at the electrode-tissue interface but not low-frequency evoked potentials that may contain significant physiologic information. 15 identical neural recording channels plus a constant reference voltage (MARK) that marks the beginning of each frame are multiplexed by a 16 to 1 multiplexer that is controlled by a 4-bit counter. The counter is run at 320 kHz by a local ring oscillator, taking 20k samples/sec from every channel. This sampling rate should be enough for reconstruction of the neural signals which have a bandwidth of 8~10 kHz. A sample and hold (S&H) circuit follows the TDM to stabilize the acquired samples before pulse width modulation (PWM). The PWM is dedicated to convert the analog signal at the output of the S&H to a pseudo-digital signal that is more robust against noise. Using a pulse width modulator instead of an analog to digital converter (ADC) results in less power consumption and less complexity in the implantable device.

Text Box: 15-Ch Wireless Neural Recording ASIC: A 3 mm × 3 mm 15-channel wireless neural recording system on a chip (SoC) implemented in AMI-0.5 um standard CMOS process.

Text Box: A voltage controlled oscillator (VCO) converts the PWM signal to a frequency shift keyed (FSK) carrier in the industrial, scientific, and medical (ISM) band. Due to the short range application of the system (within the animal cage), the VCO output can be directly applied to a miniature patch antenna with a proper off-chip matching circuit. A commercial ISM-band receiver will be used as the external part of the system. The received PWM signal will be directly converted to digitized samples using a high frequency counter on a PC data acquisition card. Finally by demultiplexing the TDM samples, the original neural signals are reconstructed. The wireless neural recording system also contains a receiver coil followed by an on-chip rectifier, filter, and regulator that provide the rest of the implant with a clean DC supply. The power carrier frequency is selected to have minimum interference with the neural signals and ISM carrier.

Text Box: Related Publications: M. Yin and M. Ghovanloo, “A Clockless Ultra Low-Noise Low-Power Wireless Implantable Neural Recording System,” Submitted to the IEEE Symp. on Circuits and Systems (ISCAS), May 2008. M. Yin and M. Ghovanloo, “Wideband Flexible Transmitter and Receiver Pair for Multi-channel Wireless Neural Recording Applications,” IEEE-MWSCAS 2007 IEEE-NEWCAS 2007, pp. 85–88,Aug 2007. M. Yin and M. Ghovanloo, “A low-noise preamplifier with adjustable gain and bandwidth for biopotential recording applications,” IEEE International Symposium on Circuits and Systems (ISCAS), pp.321–324, May 2007. M. Yin and M. Ghovanloo, “Using Pulse Width Modulation for Wireless Transmission of Neural Signals in a Multi-channel Neural Recording System,” IEEE International Symposium on Circuits and Systems (ISCAS), pp.3127–3130, May 2007. M. Yin, R.M. Field, and M. Ghovanloo, “A 15-channel wireless neural recording system based on time division multiplexing of pulse width modulated signals,” IEEE-EMBS Special Topic Conf. on Microtechnologies in Med. and Biol., pp. 221-224, May 2006.

Text Box: 2-Ch Wireless Neural Recording ASIC: A 2.2 mm × 2.2 mm 2-channel wireless neural recording system on a chip (SoC) implemented in AMI-1.5 um standard CMOS process.

© 2007 Maysam Ghovanloo

Text Box: 4-Ch Clockless Wireless Neural Recording ASIC: A 1.5 mm × 1.5 mm 4-channel wireless neural recording system on a chip (SoC) with no running clock implemented in AMI-0.5 um standard CMOS process.