文档介绍：M AN866
Designing Operational Amplifier Oscillator Circuits
For Sensor Applications
The ratio oscillator provides an output frequency that is
Author: Jim Lepkowski
proportional to the square root of the ratio of two capac-
Microchip Technology Inc.
∝ 1/2
itors (., freq. (C4 / C3) ). The ratio oscillator can
be used to cancel the effect of a fluid-level sensor with
INTRODUCTION a varying dielectric constant, such as an oil level sen-
sor. An oil level sensor consists of two capacitances
Operational amplifier (op amp) oscillators can be used that are formed by tubes where the fluid serves as the
to accurately measure resistive and capacitive sen-
dielectric media. The measurement capacitor (CMEAS)
sors. Oscillator design can be simplified by using the is partially covered by fluid and detects the level of the
procedure discussed in this application note. The deri- oil in the tank. In contrast, pensation sensor
vation of the design equations provides a method to
(CCOMP) pletely buried in the fluid. When the
select the ponents and determine the influ-
ratio CMEAS / CCOMP is calculated, the dielectric con-
ence of ponent on the frequency of oscilla- stant of the oil is canceled. Air pressure and accelera-
tion. The procedure will be demonstrated by analyzing tion sensors can also use the ratio sensor to minimize
two state-variable RC op-amp oscillator circuits. the error that occurs from the variance of the dielectric
constant over temperature.
SENSOR APPLICATIONS
State-variable oscillators are often used in sensor con- TRANSDUCER SYSTEM
ditioning applications because they have a reliable A block diagram of a typical sensor system is shown in
start-up and a low sensitivity to stray capacitance. The Figure 1. The oscillation frequency can be found by
absolute and ratio state-variable oscillators can be counting the number of clock pulses (., MHz) in a
used to accurately detect both resistive and capacitive time window tha