AND8054 Datasheet, PDF (18/28 Pages) ON Semiconductor – Designing RC Oscillator Circuits with Low Voltage Operational Amplifiers and Comparators for Precision Sensor Applications

AND8054/D
Dual Power Supply Limit Circuits
Figure 18 shows the clamping function of the limit circuit
for a dual power supply application. A simple dual supply
voltage limit circuit can be created by using two
back–to–back zeners as shown in Figure 19. There are
several performance limitations with this circuit that result
from the relative large junction capacitance, leakage current
and temperature coefficient of a zener diode. These
limitations result in a distortion of the output signal and an
error in the oscillation frequency. In addition, this circuit’s
low voltage operation is limited to the value of the zener
diode’s clamping voltage (V_Zener) plus the forward voltage
drop (V_f) of the second zener diode. Zener diodes are
available in voltages of about 1.8 volts, while their forward
voltage drop is typically 0.7; therefore, this circuit is not
useful for voltage limiting applications below 2.5 volts.
The minimum voltage range of the back–to–back zener
diode limit circuit can be reduced by adding two resistors to
the limit circuit as shown in Figure 20 [4]. The clamping
value of this circuit is a function of the zener diode
breakdown voltage multiplied by the ratio of the resistors.
This circuit is solves the low voltage limitation of the
back–to–back zener limit circuit; however, this circuit is not
suitable for the integrator amplifiers of the oscillator when
resistor R2 is replaced by a capacitor.
D1
D2
V_IN and V_OUT
V_IN
V_{Pos_Limit}
V_OUT
V_{Neg_Limit}
Figure 18. Dual Power Supply Clamping
R2
R1
V_IN
V_CC
–
+
V_OUT
V_EE
VLimit +" (VZener ) Vf)
Figure 19. Back–to–Back Zener Diode Limit Circuit
D1
D2
R1a
V_IN
R1b
R2
V_CC
–
+
V_OUT
V_EE
VLimit
^"
(VZener
)
Vf)
*
R2
R1b )
R2
Figure 20. Back–to–Back Zener Diode Limit Circuit
with Voltage Ratio Resistors
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