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Semiconductor Components Industries, LLC, 2002
February, 2002 – Rev. 1
1 Publication Order Number:
AND8054/D
AND8054/D
Designing RC Oscillator
Circuits with Low Voltage
Operational Amplifiers and
Comparators for Precision
Sensor Applications
Jim Lepkowski
Senior Applications Engineer
Christopher Young
Engineering Intern, Arizona State University
INTRODUCTION
The design of RC operational amplifier oscillators
requires the use of a formal design procedure. In general, the
design equations for these oscillators are not available;
therefore, it is necessary to derive the design equations
symbolically to select the RC components and to determine
the influence of each component on the frequency of
oscillation. A design procedure will be shown for two state
variable oscillator circuits that can be used in precision
capacitive sensor applications. These two oscillators have
an output frequency proportional to the product of two
capacitors (C
1
*C
2
) and the ratio of two capacitors (C
1
/C
2
).
The state variable oscillators have been built using ON
Semiconductor’s new family of sub–1 volt operational
amplifiers and comparators. The MC33501, MC33503, and
NCS2001 operational amplifiers, and the NCS2200
comparator are the industry’s first and only commercially
available analog components that are specified at a voltage
of 0.9 volts. These components can be powered from a single
NiCd, NiMH or alkaline battery cell. The wide operating
temperature range of –40C to +105C makes these devices
suitable for a wide range of applications.
ON Semiconductor’s family of low voltage operational
amplifiers and comparators help solve the analog limitations
that have resulted from the industry’s movement to low
power supply voltages. The ON Semiconductor family of
analog components provide a solution for the analog I/O
interface circuits that are required for emerging low voltage
DSP and microcontroller ICs.
There are a number of advantages that result from
lowering the power supply voltage such as lower power
consumption and the reduction of multiple power supplies.
Low voltage analog design also results in new challenges for
the designer and care must be taken to transfer existing
higher voltage circuits to the lower voltage levels. For
example, device parameters such as the bandwidth and slew
rate decrease as the voltage is reduced and are modest in
comparison to traditional devices operating at voltages such
as ±10 V. Also, there is a limited voltage swing range
available at low voltages; however, this problem is
minimized by the rail–to–rail single voltage range of both
the input and output signals of the ON Semiconductor
devices.
The MC33501 and MC33503 are designed with a
BiCMOS process, while the NCS2001 and NCS2200 are
implemented with a full CMOS process. The main attributes
of these devices are their low voltage operation and a full
rail–to–rail input and output range. The rail–to–rail
operation is provided by using a unique input stage that is
formed by a folded cascade N–channel depletion mode
differential amplifier. A simplified schematic of the
MC33501 and MC33503 is shown in Figure 1.
http://onsemi.com
APPLICATION NOTE
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