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Solid-state accelerometers are surface micromachined polysilicon structures built on top of a silicon wafer. Using nanotechnology, silicon springs suspend a small mass over the surface of the wafer creating a resistance against acceleration forces. Under acceleration, the small mass deflects. The deflection is measured using a three-layer silicon differential capacitive MEMS sensing element. The differential capacitor consists of two plates fixed to the wafer surface and a plate attached to the moving small mass. The fixed plates are driven by 180° out of phase square waves. An acceleration will deflect the beam and unbalance the differential capacitor, resulting in an output square wave whose amplitude is proportional to acceleration. Phase sensitive demodulation techniques are then used to rectify the signal and determine the direction of the acceleration. This rectified sgnal is either directly taken off of the chip (ratiometric output) or further pulse-width modulated (PWM output). The advantage of having a PWM output is that the accelerometer can be directly interfaced to a microprocessor without using an A/D converter. The microprocessor has to merely determine the duty cycle of the waveform to calculate the acceleration. Not very accurately, but good enough for very low cost applications. For our purposes we’ll want to use an A/D converter and get higher accuracy.
Here’s a quick summary. The details are in the second table below.