Interface for MEMS-based rotational accelerometer for HDD applications with 2.5 rad/s² resolution and digital output

A 0.6 μm BiCMOS interface for microelectromechanical systems (MEMS) based rotational accelerometers is presented. It is housed in an inexpensive standard SO-24 plastic package with a capacitive rotational accelerometer sensor produced using MEMS technology. This sensitive interface chip includes the analog-to-digital conversion, filtering, and interface functions. The analog-to-digital conversion is realized through a single-bit electromechanical Σ-Δ loop able to detect capacitive unbalancing as low as 50 aF (50×10^-18 F). The produced bitstream is then processed by a digital chain and made available through a standard 3.3 V (5 V tolerant) three-wire serial bus. The signal bandwidth is about 800 Hz, the sensitivity is 2.5 rad/s², with a full-scale sinewave of 200 rad/s² and a signal-to-noise ratio peak of 38 dB over 30-800 Hz. Through the serial bus, it is also possible to program device characteristics including gain, offset, filter performance, and phase delay. The complete sensor is used in a feed-forward compensation scheme to cancel external disturbances acting on computer hard-disk drives so as to steadily keep the read-write heads on track: this allows greater track densities and better speed performance.