An Electrochemical Magnitude of Impedance Readout Circuit with a Successive Approximation Analog-to-Digital Converter for Biomedical Signal Acquisition

• Main Authors: Bo-HanKe, 柯博涵
• Other Authors: Ching-Hsing Luo
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/j7k275

碩士 === 國立成功大學 === 電機工程學系 === 104 === This thesis proposes an integrated circuit design, which combines an impedance sensor and front-end readout circuit with Successive Approximation Analog-to-Digital Converter (SAR ADC), to efficiently measure urine Creatinine concentration (CC). The Creatinine concentration is a key index for diagnosis and can be used to prevent kidney disease and urethral infection. Due to different CC causing the variation of sensor’s impedance, we can convert the result into digitally quantized code by using the proposed front-end circuit with SAR ADC. Therefore, the impedance value can be calculated and analyzed via a simple formula transformation. Electrochemical Impedance Spectroscopy (EIS) is one of the methods for measuring dielectric and transport properties of materials. It has become the standard in recent years. This method is realized by sending some stimulating sine wave signals into interface circuit. These signals have different frequencies (eg. 1mHz~1MHz), and we can collect their frequency responses from the sensor output and analyze the results. The main purpose in this theise is to simplify the EIS circuit which contains three blocks：(1) Sensor interface circuit; (2) SAR ADC chip design and (3) Rectifier circuit design in the measurement board. In operation, the sensor and the interface are combined together. And a 100 Hz stimulating sinusoid signal is required to provide a current loop in sensor leading voltage variation, and then we can obtain the sensor impedance value. The sensor impedance (Z∠θ) can be estimated from the variation of output signal, where Z and θ, respectively, represent the impedance amplitude and phase. In this thesis, we only focus on the amplitude of impedance. The analog signal after the interface and full wave rectifier is converted to a DC signal. Through the SAR ADC, the impedance amplitude will be translated in a digital form. Then, we can calculate the impedance to make sure the reflection of Creatinine concentration. After describing calibration curve in impedance variation, the Creatinine concentration can be acquired. The proposed design is implemented in TSMC 0.18μm CMOS process, the chip includes impedance sensor interface circuit and SAR ADC. However, full wave rectifier is off chip. Under the condition of 31.25-KHz sampling rate, the SAR ADC measurement results show that the SNDR is 65.5dB and ENOB is 10.59bits. The chip area is 1.743×1.153mm^2(including IO PAD). The total power consumption is 3.19762mW where the chip power consumption is 199.62μW and the rectifier power consumption is 2.998mW. Impedance measurement linearity is 0.9999. Since four low-pass filters are added in the system for removing the PCB noise, the new linearity becomes 0.9997 which is still good for apllication. In conclusion, the the proposed design is vary suitable for the diagnosis of kidney diseases and urinary tract infection.