Impedance Measurement With Sine-Fitting Algorithms Implemented in a DSP Portable Device

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.