AND8054 Datasheet, PDF (21/28 Pages) ON Semiconductor – Designing RC Oscillator Circuits with Low Voltage Operational Amplifiers and Comparators for Precision Sensor Applications

AND8054/D
typical application, capacitors C₁ and C₂ would be the
sensor capacitances.
The reference design for the ratio circuit is shown in
Figure 26. This circuit uses the CMOS NCS2001
operational amplifiers operated at the single power supply
of 0.9V. In addition the circuit uses the single supply limit
circuit. In the typical application, Capacitor C₃ functions as
the C_MEAS sensor while C₄ serves as the C_REF sensor.
The single supply Vcc/2 reference voltage was obtained
by using a resistor divider network. The values of the
resistors R₉ and R₁₀ were obtained by finding the input
impedances of the integrator circuits formed at amplifiers
A₁ (R₁ || C₁) and A₂ (R₂ || C₂). The input bias current of the
CMOS amplifier is specified at only 10 pA; therefore, it is
not necessary to balance the impedances at the
non–inverting and inverting terminals of the amplifiers. In
most applications, the non–inverting terminal can be
connected directly to the reference voltage. Figure 27 shows
a voltage follower circuit that could be used to provide a
more stable reference voltage with the additional benefit of
a low output impedance.
The NCS2200 comparator is used by the ratio oscillator
design to convert the oscillator’s sine wave output to a
square ware digital signal. The NCS2200 is available in both
a complementary and an open drain output configuration.
The reference design used the open drain configuration to
form a zero crossing detector.
Table 2 lists the calculated and measured oscillation
frequency for the reference designs. The calculated
frequency was obtained by measuring the R’s and C’s and
using these values with the oscillation equations.
The measured frequency of the absolute and ratio
oscillators was approximately ±1% different than the
calculated frequency. This error between the measure and
predicated oscillation frequency is probably due to the
capacitance of the limit circuits, which is not included in the
frequency equations. The reference designs used standard
NPN, PNP transistors and diodes; selecting high frequency
or RF devices would minimize the oscillation error of the
limiting circuit.
D1
MPS2907A 1N4001
Q1
V_{Q1_Base} = 0.5 V
V_{Q2_Base} = –0.5 V
D2
1N4001
Q2
MPS2222A
C1
240 pF
C2
240 pF
R1
39 k
– ^A1
R2
39 k
+
MC33501
R3
– ^A2
10 k
+
MC33501
C4
22 pF
R4
10 k
– ^A3
+
MC33501
NOTES:
1. Power Supply Voltages for amplifiers A1, A2, and A3 are V_CC = 2.5 V, V_EE = 2.5 V
2. V_{Pos_Limit} = 1.9 V and V_{Neg_Limit} = –1.9 V
Figure 25. Reference Design – Absolute Circuit
Table 2. Reference Designs Oscillation Frequency
Circuit
Absolute Oscillator
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{Ratio Oscillator}
Calculated Oscillation Frequency
16.6 kHz
16.5 kHz
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21
Measured Oscillation Frequency
16.4 kHz
16.3 kHz