A broad bandwidth mixed analog/digital integrated circuit for measuring complex impedances; Principles and design considerations

A broad bandwidth measuring system for complex impedances is presented. The measuring principle, based on synchronous detection, is worked out mathematical. The final circuit is realized as a mixed analog/digital BiCMOS integrated circuit. The output of the IC is a modulated 100 kHz frequency and RS232. The phase accuracy requirement of the system is mainly determined by parameter deviation of transistors which share the same IC. The phase accuracy of the circuit is according to PSPICE simulations 0.07´ at 20 MHz. This enables to measure a capacitance of 100 pF, in parallel with a conductance of 100 mS, with an accuracy of ± 1 pF. The requirements are based on the application of the IC in a sensor for water content and ionic concentration measurements in soil or other agricultural substrates.