Baseline Cancellation Technique in Scalable Peak Detectors for Impedance Measurement ICs

Abstract

Electrical impedance tomography (EIT) is a non-invasive and radiation-free imaging technique to create impedance images of a certain target area. It has been used in numerous biomedical applications, such as lung-ventilation monitoring, cardiac activities monitoring, breast cancer monitoring, and cerebral blood flow monitoring. In addition, neural EIT has also been introduced recently with the advancement of underlying technologies. Neural EIT involves the medical imaging of the brain to capture and analyze electrical impedance changes in brain tissues, contributing to improved diagnostics and monitoring of brain functions When neural depolarization occurs, tissue impedance reduction happens due to ion opening. This impedance change can be imaged by EIT, and neural functions can be analyzed by the reconstructed image. The feasibility of neural EIT for monitoring peripheral nerves has been demonstrated successfully. However, from the circuit point of view, two requirements should be addressed to increase the efficacy of employing EIT. First, circuits that can support real-time EIT imaging are required. In particular, the throughput of the readout front-end is a key performance metric for implementing real-time monitoring devices. Second,in order to circumvent the drift and distortions in acquired signals, a continuous level calibration known as baseline cancellation is also required. This thesis presents an EIT application to address the above-mentioned challenges. The proposed technique uses a peak detector to obtain impedance magnitude every cycle of input frequency. After acquiring the peak, the peak detector is reset to a DC baseline voltage. The signal is further amplified which is swinging between the amplitude and the reset baseline, allowing to measure small impedance variations even with a large baseline. The proposed IC, fabricated in a 180-nm standard CMOS process, can measure impedance variations of >0.1% baseline, while achieving high throughput of 100 kS/s . The scalable design allows a wide frequency range operation from 100 Hz to 100 kHz with a power consumption from 31 μW to 39 μW from a 1.2-V supply.