A BROAD-BANDWIDTH MIXED ANALOG-DIGITAL INTEGRATED-CIRCUIT FOR THE MEASUREMENT OF COMPLEX IMPEDANCES

A broad-bandwidth measuring system for complex impedances is presented. The measuring principle, based on synchronous detection, is worked out mathematically. 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 of the system is 0.07-degrees at 20 MHz and is mainly determined by parameter deviation of transistors that share the same IC. The circuit enables measurement of a capacitance of up to 100 pF with an accuracy of +/-1 pF, in parallel with a conductance of up to 100 mS. Apart from the impedance measuring circuit, the IC can process signals from other sensors, such as for temperature or pH. The design criteria for the IC are derived from the primary application in a sensor for water content and ionic concentration measurements in soil or other agricultural substrates.