本設計提出一個結合感測器的電路架構，來量測尿液中肌酸酐(Creatinine)的濃度，用於腎臟與尿道的診斷與預防感染。其中不同濃度的肌酸酐，會造成不同阻抗值的感測器變化，利用電路讀取出阻抗變化值，並結合三角積分調變器，輸出數位化的量測結果。
交流電化學阻抗量測法(Electrochemical Impedance Spectroscopy, EIS) 是一種量測感測器傳導與介電特性的方法之一，並已成為近幾年來阻抗量測法的主流。原理是把不同頻率(eg. 1mHz~1MHz)的弦波刺激訊號，透過介面電路給於感測器上，分析其中的響應結果，得以建構從低頻到高頻的感測器阻抗模型，並分析其中的各項參數。
EIS阻抗量測法的目標在於建立感測器阻抗模型，由於本設計的目標是量測尿液中肌酸酐的濃度，而肌酸酐的濃度會改變感測器的阻抗值，因此本設計基於EIS中的類比方法量測方法，利用鎖定放大器來實現，並修正其近似誤差，讓量測結果線性度與誤差都能符合感測器的規格。此設計包含下面四個部分，來完成整個阻抗量測的功能：感測器介面電路、相位切換器、弦波轉方波比較器、三角積分類比數位轉換器。實際操作只需要輸入一個100 Hz的弦波刺激訊號於感測器介面電路，輸出訊號會因為感測器的阻抗變化，而產生1)振幅變化和2)相位變化。經由介面與截切電路，轉換出截波的訊號出來，再經過二階Cascade of Resonators with Distributed Feedback (CRFB)架構的三角積分調變(Sigma-delta Modulation ,SDM)轉換，由後端讀取出這兩個參數的數位值，經換算之後可得到感測器的阻抗變化值，由此測量出肌酸酐的濃度數值。
此晶片採用TSMC 0.18um COMS的製程實現這個阻抗式讀取電路，取樣頻率為192 kHz，SDM量測到的SNDR為70.25dB，有效位元(ENOB)為11.38 bits。整體阻抗量測的線性度為0.9998，而相位量測的線性度為0.9717。本設計的線性度上有不錯的表現，對於阻抗量測有-9.6%到-6.8%的相對誤差，但相位量測的相對誤差則最多有60%。而功率消耗和晶片面積分別為1.22mW 與 1.224*1.222mm^2，核心晶片面積為 0.765*0.778mm^2。因此，本設計的阻抗量測技術可做為肌酸酐的讀出電路，以利尿道和腎臟的病變診斷。

A combining impedance sensor and front-end read-out circuit with sigma-delta modulator (SDM) is proposed in this design. This read-out circuit is designed to measure urine Creatinine concentration, which is for diagnosis and preventing from kidney disease and urethral infection. Different Creatinine concentration will cause variation of sensor impedance. By using this front-end circuit with SDM, the Creatinine concentration will be read out and quantized in to digital signal for DSP processing.
Electrochemical Impedance Spectroscopy (EIS) is one of the measurements for measuring dielectric and transport properties of materials. EIS has become de facto standard in these years. EIS is based on sending different frequency (eg. 1 mHz~1 MHz) stimulating signal (sine and cosine wave) into impedance sensor, collecting frequency response and analyzing the result. Through this analysis, sensor impedance model can be built and approached.
The main function of this front-end impedance read-out circuit is to measure urine Creatinine concentration. Therefore, different from completed EIS measurement, which built out the whole impedance model, this design base on analogue approach and realized by Lock-in Amplifier. This design fixes the approach error to fit for sensor accuracy specification. This design only requires a 100 Hz sine wave stimulating signal on sensor interface. Output signal varies in response to sensor impedance variation with its 1) amplitude variation and 2) phase variation. Passing through interface and chopper circuit, the output signal is processed and converted by second order Cascade of Resonators with Distributed Feedback (CRFB) SDM into digital signal for back-end to calculate amplitude and phase variation in response to urine Creatinine concentration is acquired.
This design is implemented in TSMC 0.18 m CMOS process with 192 kHz sampling rate. The SDM measurement result shows that the SNDR is 70.25 dB, and ENOB is 11.38bits. System impedance measurement linearity is 0.9998, and phase measurement linearity is 0.9717. This design has good performance on linearity in measurement, and impedance relative error is -9.6 % to -6.8 %. But phase relative error is up to 60%. The power consumption of this design is 1.22 mW and the area of the chip is 1.224*1.222mm^2. Chip core area is 0.765*0.778mm^2. In conclusion, the impedance measurement of the proposed design can be used as a read-out circuit of Creatinine for the diagnosis of kidney diseases and urinary tract infection.

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