A problem is analyzed related to quadrature sequences digital demodulation algorithm due to frequency difference exists between reference signal and measured signal which can cause inaccuracy of data and sacrifice the image quality. An approach solving the problem is proposed based on extracting impedance amplitude directly by multiple samplings and taking the arithmetic average value for imaging by electrical impedance tomography( EIT) . An EIT system based on FPGA is developed to verify the feasibility of extracting impedance amplitude directly. The experimental results indicate that the average accuracy of the EIT system can reach 0. 082% and SNR ( signal to noise ratio) is 60.3 dB and it can image out the correct rods with accurate size and location,the spatial resolution can reach 0.29%,which demonstrate the methods feasible and reliable. The accuracy and stability of the system are increased and the image quality is improved. Non-demodulation and self-diagnosis methods provide effective solu-tions for EIT system and can boost of the EIT technology application.%探讨了正交序列数字解调算法中参考信号与测量信号频率不一致引起的解调结果不准确影响成像质量的问题,提出无需数字解调直接提取幅值进行电阻抗成像的方法：通过多次采样比较和算术平均滤波法获取幅值进行EIT成像。通过构建以FPGA为核心控制器的EIT系统,验证了直接提取阻抗幅值方法的可行性。实验结果表明：系统测量精度达0.082%,信噪比为60.3 dB,能准显示物体的大小和位置,空间分辨率达到0.29%,验证了方法的可行性,提高了数据采集精度和系统的稳定性,改善了图像成像质量,该方法为EIT技术的完善及系统性能的提升提供了一种有效的解决方案。
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