High Sensitivity Electret Accelerometer With Integrated FET

Electret accelerometers with high sensitivity and low equivalent noise level are presented. An integrated field effect transistor is used as a high impedance voltage amplifier. The voltage is generated between a metallic seismic mass and a backplate, covered by a charged electret film. These two electrodes are separated by a soft cellular polymer ring. Upon application of an external acceleration, the inertial forces of the seismic mass change the thickness of the ring and thus the distance of the electrodes. Sensitivities of up to 600 mV/g and equivalent noise voltages as low as 80 μg were measured with the electret accelerometers. High quality accelerometers with flat frequency responses can either be obtained with completely closed or opened air gap volumes. For each of these two implementations, an analytical model is presented, assuming the absence of damped air streaming effects. Both model variants are discussed and calculated data from the models are compared with measured results obtained with an accelerometer operated with various internal static pressures on the cellular spacer ring.