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AND8054 Arkusz danych(PDF) 6 Page - ON Semiconductor |
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AND8054 Arkusz danych(HTML) 6 Page - ON Semiconductor |
6 / 28 page AND8054/D http://onsemi.com 6 Ratio State Variable Oscillator The ratio state variable oscillator [6] is used for dual capacitive sensors when the oscillation frequency is proportional to the ratio of sensor capacitances C3 and C4 (i.e. freq. α C3 / C4). The block diagram and schematic of the ratio circuit are shown in Figures 7 and 8. This circuit consists of two integrators and a differentiator circuit. The integrators formed by amplifier A1 and A2 are identical to the integrators used in the absolute circuit. Amplifier A3, resistors R3, R4 and R5, and the sensor capacitors C3 and C4 form the differentiator stage which provides a 180 _ phase shift. The values of resistors R3, R4 and R5 are selected to set the break frequencies of the differentiator stage, so that the gain of the stage is equal to –C3/C4. Resistor R5 provides a DC current path through capacitor C3 in order to initiate oscillation at power–up. Because R5 is relatively large (MΩ), it can be replaced with a three resistor “Tee” network in order to use readily available resistors, as shown in Figure 9. Limit Circuit Integrator Differentiator Integrator Figure 7. Ratio Oscillator Block Diagram V1 V2 V3 6 θ = 90° 6 θ = 90° Limit Circuit Figure 8. Ratio Oscillator Schematic The differentiator amplifier is formed by the dual sensor capacitances C3 and C4. C1 C2 C4 R2 R3 R4 A3 A2 A1 R1 V1 V2 V3 R5 C3 ⇔ C1 C2 C4 R2 R3 R4 A3 A2 A1 R1 V1 V2 V3 R5a R5b C3 R5c R5a R5b R5c Figure 9. Ratio Oscillator Schematic with R5 Tee Network A Tee network provides a method to replace a large resistor (i.e. M Ω) with three small resistors (i.e. kΩ). R5_equivalent + R5a ) R5b ) R5aR5b R5c + – + – + – + – + – + – 6 θ = 180° R5_equivalent |
Podobny numer części - AND8054 |
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Podobny opis - AND8054 |
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