The development and implementation of a digital-signal-processor (DSP)-based portable impedance-measurement instrument is described. The circuit sinewave stimulus is generated by the device using a direct digital synthesizer at the desired measurement frequency. The sine is applied to a reference impedance in series with the unknown impedance. Two analog-to-digital converters (ADCs) acquire multiple samples of the two sine voltages, which are transmitted to the DSP. Sine-fitting algorithms determine the sine amplitudes, phases, dc components, and frequency. With these values and the reference-impedance values, the unknown impedance amplitude and phase are determined. Amplifiers, which are included in the device, extend the ADCs' input range, while a digital potentiometer enables the adjustment of the reference impedance to improve accuracy.
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Radil et al. (2007) studied this question.
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