Engineer experience: How to accurately collect complex impedance test data?

　　Impedance measurement is a difficult signal processing task that is becoming increasingly important in many sensor and diagnostic applications, such as medical sample analysis, biomedical tissue resistivity/conductivity, liquid environmental analysis, and sensor interfaces. Impedance conversion is traditionally accomplished with discrete solutions, but often requires advanced analog design techniques to extract the frequency response of the unknown impedance, as shown in Figure 1.

　　Figure 1. The ADI AD5933 is used in conjunction with an AFE to measure ground reference impedance in a coagulation measurement system.

　　When analyzing complex impedance measurements, you may encounter the following challenges:

　　How  to control ADC sampling frequency based on DDS output frequency (window sampling and coherent sampling)

　　How  to manage component selection

　　Develop  software to control DDS

　　Determine  the software required for FFT

　　How  to calculate the error budget

　　Determine  temperature effects

　　To measure the conductivity or impedance of a liquid the probe must be inserted into the liquid. You can measure not only the resistance of the liquid but also any capacitive effects that may be caused by metallic substances in the liquid. To switch between calibration and unknown impedance an external CMOS switch can be used.