More than a decade of research in the field of thermal, motion, vibration and electromagnetic radiation energy harvesting has yielded increasing power output and smaller embodiments. Power management circuits for rectification and DC-DC conversion are becoming able to efficiently convert the power from these energy harvesters. This paper summarizes recent energy harvesting results and their power management circuits.

Micropower energy harvesting

This paper presents a low power boost converter for thermoelectric energy harvesting that demonstrates an efficiency that is 15% higher than the state-of-the-art for voltage conversion ratios above 20. This is achieved by utilizing a technique allowing synchronous rectification in the discontinuous conduction mode. A low-power method for input voltage monitoring is presented. The low input voltage requirements allow operation from a thermoelectric generator powered by body heat. The converter, fabricated in a 0.13 μm CMOS process, operates from input voltages ranging from 20 mV to 250 mV while supplying a regulated 1 V output. The converter consumes 1.6 (1.1) μW of quiescent power, delivers up to 25 (175) μW of output power, and is 46 (75)% efficient for a 20 mV and 100 mV input, respectively.

A 20 mV Input Boost Converter With Efficient Digital Control for Thermoelectric Energy Harvesting

Closed-loop neuromodulation systems aim to treat a variety of neurological conditions by delivering and adjusting therapeutic electrical stimulation in response to a patient's neural state, recorded in real time. Existing systems are limited by low channel counts, lack of algorithmic flexibility, and the distortion of recorded signals by large and persistent stimulation artefacts. Here, we describe an artefact-free wireless neuromodulation device that enables research applications requiring high-throughput data streaming, low-latency biosignal processing, and simultaneous sensing and stimulation. The device is a miniaturized neural interface capable of closed-loop recording and stimulation on 128 channels, with on-board processing to fully cancel stimulation artefacts. In addition, it can detect neural biomarkers and automatically adjust stimulation parameters in closed-loop mode. In a behaving non-human primate, the device enabled long-term recordings of local field potentials and the real-time cancellation of stimulation artefacts, as well as closed-loop stimulation to disrupt movement preparatory activity during a delayed-reach task. The neuromodulation device may help advance neuroscientific discovery and preclinical investigations of stimulation-based therapeutic interventions.

/pdf/a-wireless-and-artefact-free-128-channel-neuromodulation-16ezgc2djj.pdf

A wireless and artefact-free 128-channel neuromodulation device for closed-loop stimulation and recording in non-human primates

A wireless electrical stimulation implant for peripheral nerves, achieving >10× improvement over state of the art in the depth/volume figure of merit, is presented. The fully integrated implant measures just 2 mm × 3 mm × 6.5 mm (39 mm
 3
, 78 mg), and operates at a large depth of 10.5 cm in a tissue phantom. The implant is powered using ultrasound and includes a miniaturized piezoelectric receiver (piezo), an IC designed in 180 nm HV BCD process, an off-chip energy storage capacitor, and platinum stimulation electrodes. The package also includes an optional blue light-emitting diode for potential applications in optogenetic stimulation in the future. A system-level design strategy for complete operation of the implant during the charging transient of the storage capacitor, as well as a unique downlink command/data transfer protocol, is presented. The implant enables externally programmable current-controlled stimulation of peripheral nerves, with a wide range of stimulation parameters, both for electrical (22 to 5000 μA amplitude, ~14 to 470 μs pulse-width, 0 to 60 Hz repetition rate) and optical (up to 23 mW/mm
 2
 optical intensity) stimulation. Additionally, the implant achieves 15 V compliance voltage for chronic applications. Full integration of the implant components, end-to-end in vitro system characterizations, and results for the electrical stimulation of a sciatic nerve, demonstrate the feasibility and efficacy of the proposed stimulator for peripheral nerves.

A mm-Sized Wireless Implantable Device for Electrical Stimulation of Peripheral Nerves

Closed-loop neuromodulation is essential for the advance of neuroscience and for administering therapy in patients suffering from drug-resistant neurological conditions. Neural stimulation generates large artifacts at the recording sites, which easily saturate traditional recording front ends. This paper presents a neural recording chopper amplifier capable of handling in-band artifacts up to 40 mVpp while preserving the accompanying small neural signals. New techniques have been proposed that solve the issues of low input impedance and electrode-offset rejection, which enable a DC input impedance of 300 MΩ and a dynamic range of 69 dB (200 Hz-5 kHz) and 78 dB (1-200 Hz). Implemented in a 40-nm CMOS process, the prototype occupies an area of 0.071 mm2/channel, and consumes 2 μW from a 1.2 V supply. The input-referred noise is 7 μV
 rms
 (200 Hz-20 kHz) and 2 μV
 rms
 (1-200 Hz). The total harmonic distortion for a 20-mV
 p
 input at 1 kHz is -74 dB. This paper improves the linearity by 14-26 dB, dynamic range by 11-28 dB, and input-impedance for chopped front ends by a factor of 11 as compared with the current state of the art, while achieving similar power and noise performance.

A High Dynamic-Range Neural Recording Chopper Amplifier for Simultaneous Neural Recording and Stimulation

Closed-loop neuromodulation is an essential function in future neural implants for delivering efficient and effective therapy. However, a closed-loop system requires the neural-recording front-end to handle large stimulation artifacts—a feature not supported by most state-of-the-art designs. In this paper, we present a neural-recording front-end that has an input range of ±50 mV and can be used in closed-loop systems. The proposed front-end avoids the saturation due to stimulation artifacts by employing a voltage-controlled oscillator (VCO) to directly convert the input signal into the frequency domain. The VCO nonlinearity is corrected using area-efficient foreground polynomial correction. Implemented in a 40-nm CMOS process, the design occupies 0.135 mm $^{\mathrm { {2}}}$ with an analog power of 3 $\mu \text{W}$ and a digital switching power of 4 $\mu \text{W}$ . It achieves ten times higher linear input range than prior art, and 79-dB spurious-free dynamic range at peak input, with an input-referred noise of 5.2 $\mu \text{V}_{\mathrm { {rms}}}$ across the local-field-potential band of 1–200 Hz. With on-chip subhertz high-pass filters realized by duty-cycled resistors, the front-end also eliminates the need of off-chip dc-blocking capacitors.

A ±50-mV Linear-Input-Range VCO-Based Neural-Recording Front-End With Digital Nonlinearity Correction

Closed-loop neuromodulation is essential for the advance of neuroscience and for administering therapy in patients suffering from drug-resistant neurological conditions. Neural stimulation generates large differential and common-mode (CM) artifacts at the recording sites, which easily saturate traditional recording front ends. This paper presents a neural recording chopper amplifier capable of handling in-band 80-mVpp differential artifacts and 650-mVpp CM artifacts while preserving the accompanying small neural signals. New techniques have been proposed that introduce immunity to CM interference, increase the input impedance of the chopper amplifier to 1.6 $\text{G}\Omega $ , and increase the maximum realizable resistance of duty-cycled resistors (DCR) to 90 $\text{G}\Omega $ . These techniques enable our recording front-end to achieve a dynamic range of 74 dB (200 Hz–5 kHz) and 81 dB (1–200 Hz). Implemented in a 40-nm CMOS process, the prototype occupies an area of 0.069 mm2/channel, and consumes 2.8 $\mu \text{W}$ from a 1.2-V supply. The input-referred noise is 5.3 $\mu \text{V}_{\mathrm {\mathbf {rms}}}$ (200 Hz–5 kHz) and 1.8 $\mu \text{V}_{\mathrm {\mathbf {rms}}}$ (1–200Hz). The total harmonic distortion for a 40-mV $_{\mathrm {\mathbf {p}}}$ input at 1 kHz is −76 dB. This work improves the input impedance by 5.3 $\times $ for chopped front-ends, linear-input range by 2 $\times $ , maximum resistance of DCR by 32 $\times $ , and tolerance to CM interferers by 6.5 $\times $ , while maintaining comparable power and noise performance.

An 80-mVpp Linear-Input Range, 1.6- $\text{G}\Omega $ Input Impedance, Low-Power Chopper Amplifier for Closed-Loop Neural Recording That Is Tolerant to 650-mVpp Common-Mode Interference

Modern neuromodulation requires closed-loop functionality, where neural recordings are used to adapt stimulation patterns in real time. A closed-loop system requires the neural sensing front-end to record small neural signals in the presence of large stimulation artifacts. The amplitude of artifacts can be a few 10s of mV, and their power is usually in the same frequency band as the signals of interest, requiring non-traditional adaptive filtering to attenuate the artifacts. This requires a sensing front-end that can handle large signals while maintaining the signal integrity of the accompanying small neural signals. State-of-the-art front-ends saturate beyond an input of ∼5mV and have limited linearity, making them incapable of handling large artifacts. This work presents a front-end that can tolerate up to ±20mV artifacts in the signal band of 1Hz to 5kHz. To digitize a 1mV neural signal to 8 bits in the presence of a 20mV artifact, the front-end requires a 12b linearity.

5.5 A 2µW 40mVpp linear-input-range chopper- stabilized bio-signal amplifier with boosted input impedance of 300MΩ and electrode-offset filtering

Chopping is an effective way to mitigate transistor flicker noise in biosignal amplifiers. However, chopping creates a ripple at the output of the amplifier, which is due to the upmodulated amplifier offset and flicker noise. Many techniques have been suggested in the literature to reduce these ripples. This brief presents a ripple-reduction technique that is far simpler than previous techniques, yet effective and area efficient while consuming no additional power. Simulations using a 65-nm complementary metal–oxide–semiconductor process show that the output ripples are attenuated by 78 dB, whereas all other performance parameters of the amplifier remain unchanged.

Hariprasad Chandrakumar

Papers

A High Dynamic-Range Neural Recording Chopper Amplifier for Simultaneous Neural Recording and Stimulation

A ±50-mV Linear-Input-Range VCO-Based Neural-Recording Front-End With Digital Nonlinearity Correction

An 80-mVpp Linear-Input Range, 1.6- $\text{G}\Omega $ Input Impedance, Low-Power Chopper Amplifier for Closed-Loop Neural Recording That Is Tolerant to 650-mVpp Common-Mode Interference

5.5 A 2µW 40mVpp linear-input-range chopper- stabilized bio-signal amplifier with boosted input impedance of 300MΩ and electrode-offset filtering

A Simple Area-Efficient Ripple-Rejection Technique for Chopped Biosignal Amplifiers