AND8054 Datasheet, PDF (20/28 Pages) ON Semiconductor – Designing RC Oscillator Circuits with Low Voltage Operational Amplifiers and Comparators for Precision Sensor Applications

AND8054/D
V_{Q1_Base}
Q1
R
V_IN
C
V_CC
V_CC/2
–
+
V_EE
V_OUT
VMin_Limit + VQ1_base * VQ1_base–to–emitter
^ VQ1_base * 0.7 V
Figure 24. Single Supply Minimum Limit Circuit
Resistors and Capacitors
It is critical that the oscillator circuits use precision
resistors and capacitors with a small temperature coefficient
(TC) and low drift rate to minimize temperature and aging
errors. Long term stability is typically specified for resistors
and capacitors by a life test of 2000 hours at the maximum
rated power and ambient temperature. In general these
components have an exponential change in value for the first
500 hours of the test and are essentially stable for the
remainder of the test. Thus, a burn–in, or temperature
cycling procedure will significantly lower the drift error of
the resistors and capacitors.
Three types of precision resistors are available: metal
film, wirewound, and foil. Metal film and wirewound
resistors are available with a TC of ±10 to ±25 ppm/_C and
a drift specification of approximately 0.1 to 0.5%. Foil
resistors are available with a TC of ±0.3 ppm/_C and a drift
specification of less than 20 ppm. Errors with resistors are
caused by both environmental and manufacturing factors.
The major environmental factors causing changes in
resistance are the operating power and the ambient
temperature. Other environmental factors such as humidity,
the voltage coefficient (∆R vs. voltage), the thermal EMF
(due to the temperature difference between the leads and self
heating), and storage will cause relatively small errors.
Manufacturing induced errors from factors such as
soldering can cause a small change in resistance; however,
this error will not effect the component’s long term stability.
Two of the leading technologies of stable capacitors are
RF/Microwave multilayer porcelain and NPO (COG)
ceramic capacitors. The TC of porcelain capacitors is
specified at +90 ± 20 ppm/_C, while NPO ceramic
capacitors are available with a TC of 0 ± 30 ppm/_C. The TC
is specified over a temperature range of –55 to 125_C;
however, the specification is skewed by the relatively large
changes in capacitance at the extreme hot and cold
temperatures. Both types of capacitors have a drift
specification of 200 ppm or ± 0.02 pF, whichever is greater,
for a 2000 hour life test at 200% WVDC and a temperature
of 125_C. The major error term of capacitors is due to
temperature hysteresis and is specified as the retrace error.
Precision sensors use temperature compensation, thus a
change of capacitance with temperature can be corrected;
however, it is difficult to correct for hysteresis errors. Other
error sources are a result of the piezoelectric effect (∆C vs.
voltage and pressure), the quality factor (Q), and the
terminal resistance. These errors are relatively small
because the capacitors are designed for microwave
frequencies and are specified at a WVDC well beyond the
normal operating voltage of an op–amp circuit.
APPLICATION ISSUES
Remote Sensing
Often, it is necessary to remotely locate the detection
circuit from the sensor, and connect the sensor to the circuit
with a shielded cable. For example, an oil level sensor for a
gas turbine engine must operate at a temperature of 400°F,
which is well beyond the operating capability of standard
electronic components. In addition, a shielded cable is often
required to limit the noise sensitivity of the measurement.
The capacitance of shielded wire is typically 30 to 50 pF per
foot, while the sensor capacitance is usually less than 100 pF.
Thus, the electronic circuit must be insensitive to the cable
capacitance which will be much larger than the sensor
capacitance.
One approach to minimize the cable capacitance error is
to use a shielded cable and the virtual ground feature of an
operational amplifier when the non–inverting input is
grounded. This feature is inherent in the integrator and
inverter/differentiator circuits used in the state variable
oscillator. Because an operational amplifier has a high open
loop gain and input impedance, the differential voltage
between the inverting and non–inverting inputs is
essentially zero. Thus, the voltage potential at the inverting
input is equal to the ground potential at the non–inverting
terminal. The virtual ground approach forces a constant
voltage to appear across the cable capacitance; therefore, the
cable capacitance does not have to be charged or discharged
by the circuit and the oscillation frequency is not effected.
A constant DC level at the non–inverting input in the single
power supply configuration is equivalent to a virtual ground
because the AC level of the input terminals is equal to zero
volts. The remote sensing ability of the state variable
oscillator will be analyzed in detail in a future application
note.
Reference Design
The reference design for the absolute oscillator is shown
in Figure 25. The circuit uses the BiCMOS MC33501
operational amplifiers operated at a power supply of ± 2.5V.
In addition the circuit uses the dual supply limit circuit. The
operating voltage of the circuit could be lowered by
removing diodes D₁ and D₂, and adjusting the base voltages
of transistors Q₁ (V_{Pos_Limit}) and Q₂ (V_{Neg_Limit}). In the
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