A Research On Signal Detecting In Bio-impedance Measurement System

With the development of science and medical technology, Electrical Impedance Tomography (EIT) has become a major bio-medical engineering research topics. Biological organs always have the electrical properties (receptivity, permittivity, etc.), which we often get by measuring the voltage or current that generated by electrode array on the surface. As the voltage or current is tiny weak when measured, many researchers are face to weak signal condition and detection problem. It is necessary to condition and detection the weak signal. But it's a hard work. Not only the amplitude of the weak signal is very small, but also the message of the weak signal is easy submerged in the strong noise. It is also a key reason that the accuracy of an ADC in whole system cannot reach the accuracy of ADC in manual, even the accuracy of the ADC is very high. Weak signal condition has great important significance which uses the way of modern electronic and signal process to get useful signal.Based on bio-impedance measurement system, several common signal detection algorithm has been analyzed, the key factors which effect on ability of signal detection has been studied. Common Digital Phase-Sensitive Demodulator usually uses periodic sum to filter, in order to avoid that, the D-DPSD(Double Digital Phase-Sensitive Demodulator) algorithm is proposed. This algorithm uses two orthogonal signal to cross the unknown network, and calculates them with another two orthogonal signal. Whenever high frequency is generated, it would be removed soon by two source signal with liner calculation. When simulated, the D-DPSD algorithm is more advanced than common DPSD on precision and bandwidth, which has great supportiveness to bio-impedance measurement system.Signal condition is the most forehead of bio-impedance measurement, which should be considered while design. To achieve high SNR, this article implies a weak signal condition PCB board by matching impedance, controlling CMRR and choosing reasonable component. By measuring actually, this board can detect weak signal range fromμV to mV level, and outcome spurious free dynamic range can reach higher than 50 dB. On the aspect of CMRR, the outcome spurious free dynamic range is higher 8dB than before. And the signal gain can be controlled between -11.5dB to +20dB. The performance of whole system can be improved by signal condition, and can miss the requirement of signal detecting and pre-processing.