New Method And Realization Of High Accurate Impedance Measurement Based On DSP

Impedance measurement plays an important role in the distributed parameter analysis of sensor, instrument and PCB(Printed Circuit Board).Various types of techniques and instruments (ac bridges, oscillators, time-domain techniques and amplitude-phase measurements)have been developed over many years. However, all these methods require the power source to be low distorted sinusoidal periodic wave, which restrict the expansion of measurement range and the improvement of accuracy because it's difficult to gain low distorted sinusoidal signal. A novel impedance measurement technique based on the theory of digital signal processing is presented in this paper. Where, the power source can be any periodic signal and its signal-to-noise ratio can be very low (SNR≤ 5 dB); A special digital quadrature sampling arithmetic with high mirror frequency rejection is adopted to get the in-phase and quadrature (I&Q) sequences which are used to confirm the parameters of the impedance under measurement; The dynamic range is improved by adopting digital filtering method with good frequency-selection performance and linear phase character, which ensures a high accuracy even when the voltage or current of the unknown impedance is very feeble. Thus the measurement range is also greatly extended. The arithmetic analysis, computer simulation and experimental results are presented, all of which demonstrate this method take on the characters of wide measurement range and high accuracy. The above-mentioned measurement technique has been implemented in a prototype apparatus which is mainly composed of a Texas Instrument DSP, a double-channel Σ-?A/D converter and a host personal computer. The DSP system where the procedures of digital filter, quadrature sample and parameter calibration are carried out connect to PC through EZ-USB bus controller. All the applied programs are written in assemble language in order to increase the performance of the system. Experimental results show this method take on the characters of wide measurement range and high accuracy. Eventually factors affecting measuring accuracy are analyzed.