The design of a novel differentially fed dual-band implantable antenna operating at 402-405 MHz Medical Implant Communication Services (MICS) band and 2.4-2.5 GHz Industrial, Scientific, and Medical (ISM) band is introduced. The proposed implanted antennas are for both planar and flexible implantation scenarios. Biocompatible material parylene-C is adopted to cover the implanted antenna. The size of the proposed antennas including the encapsulation for planar and flexible case is 179.0 mm3 and 186.3 mm3 respectively. The Specific Absorption Rate (SAR) distribution and the radiation pattern at both frequencies induced by the implanted antenna are evaluated. The performance of the communication link between the implanted antenna and an external half-wavelength dipole at two resonant frequencies is also presented. In vitro test in minced pork is performed to test the reliability of the antenna in the real implantation cases.

Design and in Vitro Test of a Differentially Fed Dual-Band Implantable Antenna Operating at MICS and ISM Bands

This paper presents a fully implantable 100-channel neural interface IC for neural activity monitoring. It contains 100-channel analog recording front-ends, 10 multiplexing successive approximation register ADCs, digital control modules and power management circuits. A dual sample-and-hold architecture is proposed, which extends the sampling time of the ADC and reduces the average power per channel by more than 50% compared to the conventional multiplexing neural recording system. A neural amplifier (NA) with current-reuse technique and weak inversion operation is demonstrated, consuming 800 nA under 1-V supply while achieving an input-referred noise of 4.0 μVrms in a 8-kHz bandwidth and a NEF of 1.9 for the whole analog recording chain. The measured frequency response of the analog front-end has a high-pass cutoff frequency from sub-1 Hz to 248 Hz and a low-pass cutoff frequency from 432 Hz to 5.1 kHz, which can be configured to record neural spikes and local field potentials simultaneously or separately. The whole system was fabricated in a 0.18-μm standard CMOS process and operates under 1 V for analog blocks and ADC, and 1.8 V for digital modules. The number of active recording channels is programmable and the digital output data rate changes accordingly, leading to high system power efficiency. The overall 100-channel interface IC consumes 1.16-mW total power, making it the optimum solution for multi-channel neural recording systems.

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A 100-Channel 1-mW Implantable Neural Recording IC

A simple and robust “cell-based” method is presented for the design of variable-gain amplifiers (VGAs). The proposed unit cell utilizes a unique gain compensation method and achieves accurate dB-linear characteristic across a wide tuning range with low power consumption and wide bandwidth. Several such highly dB-linear unit cells can be cascaded to provide the required gain range for a VGA. To prove the concept, single-cell, 5-cell, 10-cell and 15-cell reconfigurable VGAs were fabricated in a standard 0.18 µm CMOS technology. The measurement results show that the 10-cell VGA achieves a gain range of 38.6 dB with less than 0.19 dB gain error. The 15-cell VGA can either be used as reconfigurable VGA for analog control voltage or tunable PGA for digital control stream, with the flexibility of scaling gain range, gain error/step and power consumption. For the VGA at highest gain setting, it consumes 1.12 mW and achieves a gain range of 56 dB, gain error less than 0.3 dB.

Cell-Based Variable-Gain Amplifiers With Accurate dB-Linear Characteristic in 0.18 µm CMOS Technology

A novel circularly polarized antenna is proposed at 902–928 MHz Industrial, Scientific, and Medical band for implantable applications. By properly positioning the feed and shorts, either right-hand circular polarization property or left-hand circular polarization property can be realized. Slow wave concept is utilized by loading patches to the radiated loop antenna to achieve miniaturization. Thus, a compact size of 13 mm $\,\times\,$ 13 mm $\,\times\,$ 1.27 mm is obtained. Compared to the unloaded loop antenna of the same size, the centre frequency shifts from 1.93 GHz to 882.5 MHz, which suggests a miniaturization of 54.4%. The simulated results show that a wide bandwidth of 18.2% can be realized with $\vert S_{{11}}\vert$ below ${-}10$ dB and axial ratio below 3 dB. The simulated realized gain is ${-}32$ dBi at 915 MHz. The measurement is carried out in both skin-mimicking gel and pork, and a bandwidth of 27.8% and 29.4% can be achieved with $\vert S_{{11}}\vert$ below ${-}10$ dB, respectively. The measurement of $\vert S_{{21}}\vert$ reveals that circular polarization can be obtained for the proposed configuration.

Miniaturized Circularly Polarized Loop Antenna for Biomedical Applications

A novel differentially fed dual-band implantable antenna is proposed for the first time for a fully implantable neuro-microsystem. The antenna operates at two center frequencies of 433.9 MHz and 542.4 MHz, which are close to the 402-405 MHz medical implant communication services (MICS) band, to support sub-GHz wideband communication for high-data rate implantable neural recording application. The size of the antenna is 480.06 mm3 (27 mm × 14 mm × 1.27 mm). The simulated and measured bandwidths are 7.3% and 7.9% at the first resonant frequency, 5.4% and 6.4% at the second resonant frequency. The specific absorption rate (SAR) distribution induced by the implantable antenna inside a tissue-mimicking solution is evaluated. The performance of the communication link between the implanted antenna and external half-wavelength dual-band dipole is also examined.

Differentially Fed Dual-Band Implantable Antenna for Biomedical Applications

A low-power variable-gain amplifier (VGA) based on transconductance (gm)-ratioed amplification is analyzed and designed with improved linearity. The VGA has the merits of continuous gain tuning, low power consumption and small chip area. However, the linearity performance of the gm-ratioed amplifier is usually poor. We analyze distortion in gm-ratioed amplifiers and propose to improve the output linearity by applying load degeneration technique. It is found that theoretically the output linearity can be improved by 8.5 dB at the same power consumption. We also analyze gain, bandwidth and noise performance of the gm-ratioed amplifiers. Two VGAs based on gm-ratioed amplification are designed and fabricated in a 0.18-μm CMOS process-one with load degeneration only and the other with both input and load degeneration. The VGA with load degeneration only achieves gain of -20 to 41 dB, bandwidth of 121 to 211 MHz, and input and output P1dB up to - 17 dBm and 0.65 dBm, respectively. The VGA with both input and load degeneration achieves gain of -37 to 28 dB, bandwidth of 76 to 809 MHz, and input and output P1dB up to - 2.63 dBm and 2.29 dBm, respectively. The two VGAs consume a similar amount of power, which is about 3 to 5 mW from a 1.8-V supply. For the same bias condition, the proposed load degeneration improves the output linearity by more than 15 dB.

A Low-Power Variable-Gain Amplifier With Improved Linearity: Analysis and Design

For real-time monitoring of brain activities, a highdata-rate, low-power, and highly mobile neural recording system is desirable. This paper presents a complete chipset for a 100-channel wireless neural recording system, which consists of 3 ICs - a neural interface (NI) IC and a wireless power RX and data TX IC for an implant unit (IU), and a wireless data RX IC for an external head unit (EHU). With a dual S/H NI architecture and a burst-mode (BM) wideband (WB) FSK TX, the IU achieves a 100-channel recording and wireless transmission at 54.24Mb/s while consuming only 6.6mW. Using power coupling with optimal resonant load transformation and high-efficiency rectifier and LDO circuits, the whole wireless power link achieves 40% efficiency over 1cm distance with 0.5cm tissue in between. The EHU needs to transmit the RF power lower than 30mW to operate the IU. The EHU is implemented using a crystal-less BM WB FSK RX consuming only 14.4mW at 27.12Mb/s.

100-Channel wireless neural recording system with 54-Mb/s data link and 40%-efficiency power link

A burst-mode frequency-shift keying (FSK) transmitter is presented using injection locking technique for wireless neural signal recording. An initial single current pulse is injected to the LC tank to start up the oscillator quickly and, more importantly, to set the phase of the free-running oscillation at known value. We prove that the free-running oscillation phase is linearly related to the logarithm of the initial current pulse magnitude. With the free-running oscillation phase fixed, a gated reference signal can be injected at the optimum timing to achieve fast settling and consequently realize the high-data-rate burst-mode FSK transmission. The duty cycle of the reference signal is controlled to be 33% to reduce the required reference signal power by 73% approximately. With burst-mode operation and direct modulation by changing the self-resonance frequency of the oscillator, low power consumption and high energy efficiency is achieved. The transmitter is implemented in a 0.18-μm CMOS technology. It achieves a maximum data rate of 54.24 Mbps while consuming 3.4 mW from a 1.8-V supply, resulting in the high energy efficiency of 62 pJ/bit.

A 3.4-mW 54.24-Mbps burst-mode injection-locked CMOS FSK transmitter

A method and an apparatus for a duty-cycled injection locked oscillator is provided for frequency shift keyed (FSK) signal transmissions. The oscillator includes a resonance LC tank and a first switching device. The first switching device is coupled to the resonance LC tank and injects an initial current pulse with a predetermined pulse magnitude into the resonance LC tank. The initial current pulse also fixes an initial phase of the duty-cycled injection locked free-running oscillator in response to the predetermined magnitude of the initial current pulse to enable fast settling of injection locking and high data rate operation of the duty-cycled injection locked oscillator. The oscillator also includes a second switching device, such as a differential pair of switching devices. The second switching device is coupled to the LC resonance tank for injecting a gated periodic reference signal having a duty cycle modified to reduce power of the reference signal by approximately seventy-five per cent.

Method and apparatus for a duty-cycled harmonic injection locked oscillator

A low-power variable-gain amplifier (VGA) with integrated received signal strength indicator (RSSI) for a low data-rate impulse radio ultra-wideband (IR-UWB) receiver is designed and implemented in a CMOS 0.18 µm technology. The VGA achieves a gain range from −9 dB to 63 dB and bandwidth from 31 to 187 MHz. An input P1dB of −15.4 dBm is achieved at zero-dB gain. The RSSI has a dynamic range more than 70 dB. The RSSI linearity error is within ±1.2 dB for an input power from −77.5 dBm to −36.2 dBm. The VGA with integrated RSSI draws 5.4 mA from a 1.8V single supply and the test chip occupies an area of 970 µm × 750 µm, including two huge output buffers and digital gain control circuit.

Zhiming Chen

Papers

A Low-Power Variable-Gain Amplifier With Improved Linearity: Analysis and Design

100-Channel wireless neural recording system with 54-Mb/s data link and 40%-efficiency power link

A 3.4-mW 54.24-Mbps burst-mode injection-locked CMOS FSK transmitter

Method and apparatus for a duty-cycled harmonic injection locked oscillator

A CMOS low-power variable-gain amplifier with RSSI for a noncoherent low data rate IR-UWB receiver