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A Piezo-sensor Readout Integrated Circuit for Multifunctional Electronic Skins

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Title
A Piezo-sensor Readout Integrated Circuit for Multifunctional Electronic Skins
Author
Kim, Seungmok
Advisor
Kim, Jae Joon
Issue Date
2018-02
Publisher
Graduate School of UNIST
Abstract
Nowadays, the term smart device has become a common term around us. A large number of smart devices appear on the market in the form of wearable electronics, and the size of market has steadily increased in recent years. Among them, healthcare and medical related wearable electronics shows the steepest growth. In this sense, this paper proposes a multifunctional electronic skin which can detect bio-signals of human body. The compound of polyvinylidene fluoride and reduced graphene oxide (PVDF-rGO) was used as a sensing material. It has a piezoresistive characteristic by adding reduced graphene oxide. This characteristic can be utilized to detect static pressure such as respiration and pulse wave. In addition, the piezoelectric property of PVDF was maximized by adopting interlocking microdome structure. Therefore, sufficient performance is guaranteed even with the adition of rGO. Generally, there are two types of readout integrated circuits for reading piezoresistive and piezoelectric properties of PVDF-rGO: resistance-to-digital converter (RDC) and charge-to-digital converter (QDC). The resistance-to-digital converter is also divided three types: resistor array based RDC, current DAC based RDC, and correlated double sampling based RDC. The most common form of charge-to-digital converter can be categorized based on the number of amplifiers used, referred to as single-AMP based QDC and multi-amps based QDC, and will be introduced in the paper. The readout integrated circuits so far introduced in past papers are insufficient to apply to PVDFrGO. Therefore, a novel structure of piezo-sensor readout integrated circuit should be needed for wearable healthcare platform. This paper focuses on minimizing power consumption by sharing blocks with as many common factors as possible in the process of implementing the necessary circuits to detect multi-bio-signals. Especially, considering ADC structure, the single ended signal can be converted to pseudo differential signals that means no additional circuit is needed for converting analog voltage to digitized bits. Due to the nature of PVDF-rGO, it is important to bias a constant voltage. The proposed ROIC structure can guarantee the reliability of the signal read through biasing the constant voltage at any circumstances. Furthermore, in order to maximize the linearity of the input pressure and the output voltage by experimentally measuring the inverse relationship of the pressure and the resistance within a certain range. All of the implementations are designed in forms of on-chip including the passive components of low pass filter. It means that a smaller, better-performing platform can be implemented. In addition to implementing the circuit, the prototype was built so that the user can finally recognize his or her bio-signal information through microcontroller and Bluetooth module. Simultaneous measurement of ECG and pulse wave was made within a single chip. The peak time delay of ECG and pulse wave can be calculated and this gives the possibility of measuring blood pressure in real time with minimum restraint.
Description
Department of Electrical Engineering
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