A broad-bandwidth mixed analog/digital integrated circuit for the measurement of complex impedance

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 o 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