Frequency Measurement For Small Capacitance Detection Asic Design

Accelerometer is used widely for measuring acceleration in civil and military field.At present,there are several type of accelerometers such as Piezoelectricity Electric Charge Accelerometer, IC Piezoelectricity Electric Accelerometer, Compressive Resistance Accelerometer and Alteration Capacitance Accelerometer.The main part in Alteration Capacitance Accelerometer is capacitance measurement circuit.Foreign countries have produced accelerometer that have tiny volume and high performance.However,our country is not able to produce such accelerometer due to the lack of technique.MEMS (Micro Electromechanical System ) has become one of the most rapidly development technologies.Along with the rapid development of MEMS techniques,capacitive sensor is used widely whose capacitance changes small,which has put forward the new request to the measurement technique.The common measure for capacitance measurement is that convert capacitance to voltage,electric current or frequency.The area of polar plate of the capacitor becomes smaller and smaller,and the total capacitance of micro capacitive sensor is usually several pico farad only.As a result,its change amount is smaller. For such capacitance measurement,there are several method such as resonance,charge/discharge,AC bridges,and so on.These method are all implemented by analog device with analog output.With the maturity of the digital ICs,some digital method comes up.In this paper,a mixed method with the combination of advantages of analog and digital is discussed.It consists of two parts,analog and digital.First capacitance is converted to frequency by analog part, which makes the relations between capacitance and frequency, then high accuracy and instantaneous frequency measurement is accomplished by digital part based on the multiple period synchronization method. It has some advantages such as high accuracy, high speed, easy to integrate, simple structure, anti-interference and so on. All these advantages demonstrate that this method has practical value.