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A Fully-Differential Accelerometer

Accelerometers are used for a wide variety of applications (which we’ll look at more specifically soon). Particularly demanding are automotive applications, not least because of the harsh conditions and huge amount of noise that they must tolerate. That noise can be electrical or simply “ambient vibration” that is not of interest.

Electrically, differential signaling is often used to reject common-mode noise. But one paper at ISSCC took the concept all the way back to the proof mass: a team from Robert Bosch split the proof mass, working then with what are nominally two identical half-masses. This sets up a differential signal flow from the get-go.

Of course, the masses aren’t going to be exactly the same; various differences are averaged out by swapping back and forth (to oversimplify). Chopping is also used to boost near-DC noise up away from the frequencies that matter, although the process does result in some noise being moved into the way instead of out of the way. In order to minimize the effect of this, they used a pseudo-random signal for chopping so that the energy of this noise is, to use their word, “smeared” across the spectrum, rendering it largely impotent.

You can find circuit details and results in the ISSCC proceedings, paper 22.1

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