Low Frequency CMOS Sinusoidal Oscillator for Impedance Spectroscopy

Impedance measurement as a function of frequency is being increasingly used for the detection of organic molecules. The main building block required for this is a sinusoidal oscillator whose frequency can be varied in the range of a few kHz to tens of MHz. Considering the low frequency end of the range, the design challenge is to get an on-chip oscillator without the need for large external components. The design of an Integrated CMOS Oscillator Circuit based on the Wien Bridge is presented. It provides both in-phase and quadrature outputs needed for the determination of the real and imaginary parts of complex impedances. A novel circuit for detecting and controlling the amplitude of oscillation without the need for on-chip low frequency filtering is described. Since this oscillator is used in an inexpensive portable hand-held instrument, both power consumption and chip area must be minimized.