A System On Chip design for fast time domain impedance spectroscopy

Impedance spectroscopy is a powerful technique that can be employed to determine the physical properties of different materials. In principle the technique can be implemented in the time domain using pulsed signal excitation rather than sweeping across frequencies. The advantage is particularly apparent when the measuring instrument is either traveling through the material medium rapidly, as in the case of a satellite moving through space plasma, or if the medium is for example a fluid that flows past an instrument quickly. Here we describe a Time Domain Impedance Probe (TDIP) circuit design that is used to measure the absolute density of ionospheric plasmas. A preliminary version of this instrument was flown on a sounding rocket, but here we outline the system and circuit design that is being implemented for a Low Earth Orbit (LEO) micro-satellite. The design employs a bridge architecture together with a software adaptive filter and LMS algorithm for fast calibration and data compression. We propose that the design can be generalized, and we present a System On Chip (SOC) concept based on the time domain architecture. The proposed concept appears to be well suited to ultra-fast time domain spectroscopic measurements, but does have some inherent limitations such as increased noise. We suggest that this unavoidable shortcoming can be somewhat mitigated through repetitive pulsing and averaging during the measurement process.