ARMDuino simple impedance meter shield.

 Bioimpedance measurement is an emerging tool in the field of biomedical engineering. It consists in studying the passive electrical properties of biological materials to indirectly determine certain physiological aspects. These measurements usually are employed as a method for monitoring physiological variations. This monitoring method presents three main advantages. First, it is a simple technique that can be applied with just two electrode setup. Also it requires low-cost instrumentation and is able to monitoring in real time.

Impedance meter based on the AD5933 IC, The AD5933 IC is designed as a complete system for the impedance measurement. It consists of a 27-bit direct digital synthesis (DDS) sine excitation voltage (Vout) generator, a digital-to-analogue converter (DAC) and a programmable gain amplifier (PGA1) which

determines the Vout amplitude. The receive stage consists of a current-to-voltage converter (CVC), a programmable gain amplifier (PGA2) and a low pass filter. The data processing block consists of a 12-bit analogue-to-digital converter (ADC) and a 1024 point discrete Fourier transform (DFT) engine with a sample windowing unit and a multiply-accumulate unit (MAC). As the result of DFT, the AD5933 gives a complex number; its modulus and argument should be roportional to the magnitude and phase of the measured current through an unknown impedance Zx which - for a constant amplitude of the voltage excitation - will

also be proportional to the admittance of Zx. An I2C interface provides communication with a microcontroller.

The measurement system is equipped with a female goldpin socket for connecting measured objects directly or for an additional analogue frontend board  which contains a DC offset canceller , an additional socket for the measured device under test (DUT) and an external CVC with selectable feedback resistors RFB.

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