Op-Amp bridge oscillator
Operational amplifier (op amp) oscillators can be used to accurately measure resistive and capacitive sensors. Oscillator design can be simplified by using the procedure discussed in this application note. The derivation of the design equations provides a method to select the passive components and determine the influence of each component on the frequency of oscillation. The procedure will be demonstrated by analyzing two state-variable RC op-amp oscillator circuits.
The appropriate op amp to use in a sensor oscillator is determined by the required accuracy and acceptable distortion of the oscillation frequency. The design equations assume that the amplifiers are ideal. However, op amps have a finite gain bandwidth product (GBW), a limited slew rate (SR) and full power bandwidth (fP). the non-ideal characteristics of the amplifier will lower the oscillation frequency at high frequencies and may also result in a design with poor start-up characteristics.
Note that the total harmonic distortion specification of the amplifiers is critical for oscillators that are used as sine wave references. However, the shape of the waveform is not critical in most sensor applications because only the frequency of the output is measured. Several general design rules can be used to select an op amp for an oscillator circuit. One of them is GBW should be a factor of 10 to 100 higher than the maximum oscillation frequency.