THE CORE IDEA
A mass on a spring
Every accelerometer starts with the same basic structure: a proof mass suspended from a spring.
When the sensor accelerates, the proof mass lags behind and is displaced in the direction of sensing. That displacement can be measured piezoelectrically or capacitively.
THE PHYSICS
A mass-spring-damper system
Newton's second law, F = ma, is the starting point. The proof mass (m) is held by a spring with constant k, and its motion is damped by a factor b.
NATURAL FREQUENCY & Q
How the mass wants to move
A stiffer spring or a lighter mass raises the natural frequency. A higher Q means the system rings longer once disturbed.
SENSITIVITY
Sensitivity vs. bandwidth
A softer spring or heavier mass gives more displacement per g of acceleration — higher sensitivity. But that also lowers the natural frequency and shrinks the usable bandwidth.
Example: a 50g ADI accelerometer at 24.7 kHz has a maximum static displacement of just 20 nm. At a lower 1 kHz design, that displacement grows to about 1.2 µm.
NOISE
The noise floor: TNEA
TNEA stands for Total Noise Equivalent Acceleration — the fundamental noise floor of the measurement.
It's caused by air molecules randomly colliding with the proof mass. A real ADXL accelerometer has a noise floor around 0.005 g/√Hz.
DESIGN RULE OF THUMB
Stay above resonance
Design your accelerometer's resonant frequency to be higher than the highest frequency component of the acceleration signal you want to measure.
UNIT 18 STUDY COMPLETE
Ready for the Fab Challenges?
You've covered Sensing Physics, Sensitivity & Noise.