Electrical Characterization in Time Domain – Sample Rate and ADC Precision

Abstract

Fast impedance measurements for characterization of time variant objects use broad bandwidth excitation signals and time discrete sampling to obtain the response as a function of time. Despite the opportunity of direct processing in time domain, in most cases Fourier transform is used in order to present the result as impedance spectrum. Besides common noise sources, quantization of the response but also the creation of the excitation signal is a bottleneck with regard to measurement precision. Fast ADC with high sampling rate have lower precision while high resolution ADC (e.g. 16 bit or 24 bit) comprise speed and finally bandwidth of the measurement chain.

Theoretically, the bandwidth is limited by the sampling theorem. Practically, precise results are only obtained up to a tenth or even less of this frequency due to increasing uncertainties at higher frequency.

Approaching the theoretical limit of the method requires high ADC-precision and a bandwidth of the measurement chain reaching the Nyquist frequency. Proper design of time domain based impedance measurements and post processing of the data ensures significant readings over the desired frequency range.