TAILIEUCHUNG - Biosensors Edited byPier Andrea Serra Part 10

Tham khảo tài liệu 'biosensors edited bypier andrea serra part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 218 Biosensors Fig. 8. Die micrograph of the ULCA circuit. Fig. 9. DC gain and bandwidth as functions of input currents ranging from -1 to 1 pA. The black squares represent the dc gain whereas the circles illustrate the bandwidth. The DC gain as a function of input currents ranging from -1 to 1 pA is plotted in Fig. 9 which is lower than the designed value of 20 dB and implies about 2 of the overall mismatch on transistors M0 M1 Ms and M4 due to fabrication variations. The gain error over the input range is less than . One can certainly increase the transistor dimensions to further reduce the mismatch and achieve better linearity however trading with the bandwidth performance. The bandwidth has been characterized by analyzing the step response of the ULCA which is cascaded by a commercial transimpedance amplifier and the result as a function of input Integrated Biosensor and Interfacing Circuits 219 currents is also illustrated in Fig. 9. It can be seen that the bandwidth basically linearly increases with the input current levels on both positive and negative directions from around 20 kHz to 1 MHz. The minimal bandwidth of 15 kHz occurs in the low-input cases satisfying the 10 kHz requirement in the application. The overshoot of the bandwidth is because the offset of opamps AN1 and AP1 and the threshold mismatch shift the actual quiescent current of Mo and Ms up to around 2 nA estimated value from the designed value of nA. A larger bandwidth may slightly degrade the noise performance and sensitivity but it could be adjusted by somewhat tuning the biasing current Iref down for compensation. The IRO is analyzed by keeping the input open while measuring the mean current at the output and dividing by the gain which is found to be pA at VOUT V. The IRO corresponds to the offsets of opamps and mismatches at the output node however it is not critical for the application. One can either slightly modify VOUT for compensation or do the calibration after

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