The measurement of the tissue or bio-impedance (BIOZ) is a safe and power-efficient sensing modality that can be adopted for the acquisition of vital signals, such as respiration and heartbeat. A BIOZ readout IC with a wide-input impedance range is proposed. The IC supports vital signal acquisition through a two-electrode setup which requires a larger dynamic range than the conventional four-electrode setup. A digital-assisted baseline impedance cancellation method is implemented to measure small tissue impedance variations originating from respiration and heartbeat in the presence of larger baseline impedances. The proposed technique also mitigates the input-dependent noise behavior of the readout front-end by minimizing the effective input signal to the instrumentation amplifier (IA). The baseline cancellation loop further improves the noise performance by cancelling the reference current noise from the current generator (CG). Hence, one single solution—baseline cancellation—resolves two issues facilitating two-electrode BIOZ setups achieving similar impedance resolution performance compared with more four-electrode setups. The IC, fabricated in a 55-nm CMOS, can measure tissue impedances over a frequency range from 1 kHz to 1 MHz and can achieve a maximum input range of 24 $\text{k}\Omega $ (at the impedance measurement frequency) and a best-case resolution of 2 $\text{m}\Omega $ RMS (for a 100- $\Omega $ input). This increases to 14.0 $\text{m}\Omega $ RMS for a 2- $\text{k}\Omega $ input impedance. The ASIC consumes 18.9–34.9 and 31.4–154.7 $\mu \text{W}$ for the readout front-end and the CG, respectively. The power depends on the injected current amplitudes. A successful demonstration on the human body through a two-electrode (gel and dry) setup confirms the effectivity of the proposed work in a real use case.

A Bio-Impedance Readout IC With Digital-Assisted Baseline Cancellation for Two-Electrode Measurement

Continuous monitoring of fluid status through bio-impedance (Bio-Z) measurement of thoracic magnitude or phase is critical in reducing the mortality of chronic heart failure (CHF) patients. However, the stringent power constraints of implantable devices force the design of Bio-Z sensors to be highly challenging. We present a sub-10- $\mu \text{W}$ Bio-Z sensor IC operating from a 0.5 V-supply, sustaining a sub-1° of phase error even under 10% of supply and 0 °C–70 °C of temperature variations. Fabricated in a 65-nm CMOS process, this sensor IC exhibits a 15.28- $\text{m}\Omega /\surd $ Hz of input-referred impedance noise performance, occupying a 4.83 mm2. Its linearity performance shows that the magnitude and the phase of Bio-Z can be measured within the error of 2% and 0.4°, respectively, given that the magnitude of load impedance is less than 126 $\Omega $ , and thus, it is suitable for fluid status monitoring applications. Experimental measurement on the human chest demonstrates its capabilities of thoracic impedance variance (TIV) and impedance cardiography (ICG) monitoring.

A 0.5-V Sub-10-μW 15.28-mΩ/√Hz Bio-Impedance Sensor IC With Sub-1° Phase Error

An eight-channel 10-MHz wide-bandwidth electrical impedance tomography (EIT) IC is proposed for early breast cancer detection system. To increase the resolution of EIT images, the proposed IC has three key features: 1) wide-bandwidth instrumentation amplifier (WB-IA) to detect large impedance changes in cancer cells at a high frequency; 2) dual-mode driver (DM-driver) to obviate the complex switching network to reduce the noise and the system form factor; and 3) phase compensation loop (PCL) to efficiently correct the phase error for accurate images without artifacts. The proposed EIT IC occupies 16 mm2 in the 65-nm CMOS technology and consumes 9.6 mW for each channel. Thanks to the key features, the proposed breast cancer detection system with the dedicated EIT IC can operate up to 10 MHz with a small phase error of 4.32° and a state-of-the-art impedance resolution of 1.6 $\text{m}\Omega /\surd $ Hz eventually can detect a small-size target object of 0.5 cm and verify with the phantom experiments.

A 9.6-mW/Ch 10-MHz Wide-Bandwidth Electrical Impedance Tomography IC With Accurate Phase Compensation for Early Breast Cancer Detection

High common-mode rejection ratio (CMRR) with concurrent electrode offset rejection is essential for physiological signal acquisitions. This article presents a CMRR enhancement technique for ac-coupled instrumentation amplifiers (ACIAs), where the mismatch of passive components limits the CMRR performance primarily. A modified chopping structure is proposed to mitigate the mismatch effect of the pseudoresistors, and a successive-approximation based capacitor trimming loop is exploited. Fabricated in a 0.18- $\mu \text{m}$ CMOS technology, the ACIA draws $2.3~\mu \text{A}$ from a 1.2-V supply and exhibits 3.2- $\mu \text{V}\mathrm {_{rms}}$ input-referred noise over 0.5–400 Hz. The measured prototypes achieve > 110-dB CMRR at 50/60 Hz without any off-chip tuning.

An AC-Coupled Instrumentation Amplifier Achieving 110-dB CMRR at 50 Hz With Chopped Pseudoresistors and Successive-Approximation-Based Capacitor Trimming

Two-electrode ECG devices have gained popularity in the recent past to enable comfortable and long-term monitoring of cardiovascular health. As a ground or bias electrode is not used in a two-electrode ECG device, common-mode interference (CMI) caused by powerline coupling to the human body can be as large as a few tens of volts. Such a large CMI ruins the ECG recording, and thus the analog front-end of the ECG device must be immune to large CMI.

22.4 A 27.8μW Biopotential Amplifier Tolerant to 30V pp Common-Mode Interference for Two-Electrode ECG Recording in 0.18μm CMOS

Simultaneous measurement of Electrocardiogram (ECG) and bio-impedance (BioZ) via disposable health patches is desired for patients suffering from chronic cardiovascular and respiratory diseases. However, a sensing IC must consume ultra-low power under a sub-volt supply to comply with miniaturized and disposable batteries. This work presents a 0.6 V analog frontend (AFE) IC consisting of an instrumentation amplifier (IA), a current source (CS) and a SAR ADC. The AFE can measure ECG and BioZ simultaneously with a single IA by employing an orthogonal chopping scheme. To ensure the IA can tolerate up to 300mVpp DC electrode offset and 400mV pp common-mode (CM) interference, a DC-servo loop (DSL) combined with a common-mode feedforward (CMFF) loop is employed. A buffer-assisted scheme boosts the IA's input impedance by 7x to 140MΩ at 10Hz. To improve the BioZ sensitivity, the CG utilizes dynamic element matching to reduce the 1/f noise of the output current, leading to 35mΩ/√Hz BioZ sensitivity down to 1Hz. The ADC shows a 9.7b ENOB when sampled at 20ksps. The total power consumption of the AFE is 3.8μW.

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Yao Li

Papers

A 0.6V 3.8μW ECG/bio-impedance monitoring IC for disposable health patch in 40nm CMOS