Fast broad bandwidth bioimpedance measurement - the use of square wave excitation and non-uniform sampling

Square wave excitation is an excellent choice for excitation signal in broad bandwidth and fast impedance measurement. The advantages are simple scalability, easy generation and straight forward data reduction by non-uniform sampling. The often cited disadvantage of low signal energy at high frequency is only partially relevant because a high sampling rate at fast changing portions of the measured signal guarantees a sufficient signal-to-noise ratio throughout the frequency range determined by the sampling speed, the slew rate of the amplifiers used and the time duration of the excitation signal. Although briefly looking at spectra of other signals suggests their better performance, having a well-designed system from signal generation through sampling and processing up to proper data mining shows the advantages of square wave excitation, especially if speed and bandwidth is important. Here we describe a method for obtaining an estimate of a broadband spectrum over several orders of magnitude while only requiring a minimum of computing power.