AND8054 Datasheet, PDF (5/28 Pages) ON Semiconductor – Designing RC Oscillator Circuits with Low Voltage Operational Amplifiers and Comparators for Precision Sensor Applications

AND8054/D
CIRCUIT DESCRIPTIONS
Absolute State Variable Oscillator
The absolute state variable oscillator is used when the
measurement is proportional to either one or two capacitors
(i.e. freq. α C₁*C₂). The block diagram and schematic of the
absolute circuit are shown in Figures 5 and 6. This circuit
consists of two integrators and an inverter circuit. Each
integrator has a phase shift of 90_, while the inverter adds an
additional 180_ phase shift; thus, a total phase shift of 360_
is fed into the input of the first integrator to produce the
oscillation. The first integrator stage consists of amplifier
A₁, resistor R₁ and sensor capacitance C₁. The second
integrator consists of amplifier A₂, resistor R₂ and sensor
capacitance C₂. Resistor–capacitor combinations R₁ and C₁,
and R₂ and C₂, set the gain of each integrator stage, in
addition to setting the oscillation frequency. The inverter
stage consists of amplifier A₃, resistors R₃ and R₄ and
capacitor C₄. Capacitor C₄ is not essential for normal
operation; however, it ensures oscillator startup under
extreme ambient temperature conditions.
Limit Circuit
Integrator
V₁
6 q = 90°
Integrator
V₂
6 q = 90°
Inverter
V₃
6 q = 180°
Figure 5. Absolute Oscillator Block Diagram
C4
Limit Circuit
C1
C2
R4
R1
A1
–
+
R2
A2
–
R3
V₁
+
V₂
A3
–
+
V₃
The absolute sensor capacitances C1 and C2 are used by the integration amplifiers.
Figure 6. Absolute Oscillator Schematic
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