AND8054 Datasheet, PDF (3/28 Pages) ON Semiconductor – Designing RC Oscillator Circuits with Low Voltage Operational Amplifiers and Comparators for Precision Sensor Applications

AND8054/D
TRANSDUCER SYSTEM
A wide variety of different circuits can be used to
accurately measure capacitive sensors. The design choices
include switched capacitor circuits, analog multivibrators,
AC bridges, digital logic ICs and RC operational amplifier
oscillators. The requirements for a precision sensor circuit
include high accuracy, reliable start–up, good long–term
stability, low sensitivity to stray capacitance and a minimal
component count. State variable RC operational amplifier
oscillators meet all of the requirements listed above; thus,
they form the basis for this study.
A block diagram of a capacitive sensor system is shown
in Figure 2. The oscillation frequency is found by counting
the number of clock pulses (i.e. MHz) in a time window that
is formed by the square wave oscillator output (i.e. kHz) of
a comparator circuit. The counter circuit can be
implemented with a digital logic counter circuit or by using
the Time Processing Unit (TPU) channel of a
microprocessor. If necessary, temperature correction can be
accomplished by implementing a curve fitting routine with
data obtained by calibrating the sensor over the operating
range. An analog IC sensor can be used to monitor the sensor
temperature or for very precise applications a second
oscillator could be built with a platinum resistive
temperature device (RTD) sensor.
In addition, it is often important for the sensor system to
compute the ratio of two capacitors. Calculating the ratio of
the capacitors reduces the transducer’s sensitivity to
dielectric errors from factors such as temperature. In other
cases, such as in an air data quartz DP pressure sensors, the
desired measurement is equal to the ratio of two
capacitances (C_MEAS / C_REF). Furthermore, dual sensors are
typically designed to double the C_MEAS in capacitance,
while C_REF may vary less than one percent. Thus, the
transducer’s accuracy is increased if a circuit such as the
ratio state variable oscillator can directly detect the C_MEAS
to C_REF ratio.
C_MEAS
C_REF
RC Op–Amp
Oscillator
Comparator
Algorithm:
Count the number of clock pulses in a time window set by oscillator pulses.
Clock
Signal
Oscillator
Signal
Counter
Circuit
Temperature
Sensor
EEPROM:
Temperature
Compensation
Coefficients
Clock
Micro–Processor
Figure 2. Block Diagram of Capacitive Sensor Application
SENSOR APPLICATIONS
RC operational amplifier oscillators can be used to
accurately detect both resistive and capacitive sensors;
however, this paper will only analyze capacitive applications.
The three basic configurations of capacitive sensors and their
attributes are shown in Table 1. The absolute and dual
capacitive sensors will be used with the absolute and ratio
oscillator circuits, respectively. Differential capacitive
sensors typically are not used in precision applications;
therefore, they will not be analyzed in this paper.
http://onsemi.com
3