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Analog Fundamentals: Instrumentation for impedance measurement

-November 03, 2013

When creating designs, engineers must meet or exceed their objectives for instrumentation and measurement equipment. Having accurate and correct instruments for test and measurement methods and solutions is crucial for the design process. To better equip design engineers, we will discuss the challenges of precision data acquisition specifically in a complex impedance measurement system.

 

Impedance measurement is a difficult signal processing task and is becoming more important in many sensor and diagnostic-related applications, such as medical sample analysis, resistivity/conductivity of biomedical tissues, liquid condition analysis and sensor interface. Impedance conversion is traditionally accomplished using discrete solutions, but usually requires a high level of analog design skill to extract frequency responses of the unknown impedance as seen in Figure 1.

 

 


Figure 1. ADI’s AD5933 Used with AFE for Measuring Ground-Referenced Impedance in Blood-Coagulation Measurement System

 
 

Below are some of the challenges to consider when analyzing a complex impedance measurement:

  • How to control ADC sampling frequency with respect to DDS output frequency (windowing
    vs. coherent sampling)
  • How to manage component selection
  • Developing software to control DDS
  • Determining software required for FFT
  • How to calculate error budget
  • Determining temperature effects
 

To measure the conductance or impedance of a liquid, you would put the probes down into the liquid. You can measure not only the resistance of the liquid but also any capacitive effects which may be due to metallic content in the liquid. In order to switch between the calibration and the unknown impedance, you could use the external switches using CMOS switches.


To dig deeper on the challenges of precision data acquisition related to complex impedance measurement systems, tilt measurements and more, tune in to my ADI colleague Walt Kester  during his “Instrumentation: Test and Measurement Methods and Solutions” presentation at ADI’s Virtual Design Conference, which can be viewed on-demand until January 25, 2014.

 

Let me know if you have any questions!

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