| 摘要 |
Prior detection circuits for measuring capacitance differences have commonly included error source, such as P-N junctions, in the DC signal path. The present invention provides an improved detection circuit for measuring the capacitance difference between first (C1) and second (C2) capacitive elements. The detection circuit comprises a first capacitor (C3) serially connected to the first capacitive element at a first common node (36) to form a first series circuit, and a second capacitor (C4) serially connected to the second capacitive element at a second common node (38) to form a second series circuit. The detection circuit also includes switch a circuit (32) connected between the common nodes and a reference potential, a driver (30) for applying a drive signal across each series circuit, a line for activating the switch circuit (48) and a difference detector (34) for measuring the voltage difference between the common nodes. The switch circuit has a first state in which the common nodes are connected to the reference potential and a second state in which the common nodes are isolated from the reference potential and from one another. The drive signal comprises a series of first voltage transitions (104, 106) operative to vary the total voltage drop across each series circuit. The line for activating the switch circuit changes the switch circuit from the first state to the second state at or prior to each first voltage transition. Therefore after each first voltage transition, the difference between the voltages of the first and second common nodes is a measure of the difference between the capacitances of the first and second capacitive elements. A similar technique may be applied to the measurement of the capacitance of a single capacitor.
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