Tim Tears It Apart: Measurement Specialties Inc. 832M1 Accelerometer

So, yesterday the outdoors turned into this.

This much snow in a few hours gets you a travel lockdown...

This much snow in a few hours gets you a travel lockdown…

Not quite the snowpocalypse, but it was enough that a travel ban was in effect, and work was closed. What happens when we’re stuck in the house with gadgets?

All right, I’d like to tell you that’s the reason, but this actually got broken open accidentally at my work a little while back. Sorry for the crummy cellphone-cam pic and not-too-exhaustive picking at the wreckage.

Today’s patient anatomical specimen is a fancy 3-axis piezo accelerometer from Measurement Specialties Inc. This puppy retails for about $150, so this is a ‘sometimes‘ teardown.

The insides of the 832M1 showing two sensor orientations and the in-package charge amplifiers

The insides of the 832M1 showing two sensor orientations and the in-package charge amplifiers

One thing that comes to your attention right away is holy shit, there’s an entire circuit board in there. In retrospect, I probably shouldn’t be too surprised by this. It appears that these are full-on piezoelectric sensors (not e.g. piezoresistive), which are a bit dastardly to read from without a charge amplifier inline. On the circuit you can see three identical-ish copies of a small circuit that is almost certainly that, with a small SOT23-5 opamp in each. The part’s total quiescent current consumption is billed at 12uA, so that’s a paltry 4uA per circuit.

Here you also get a gander at the acceleration sensors themselves. Each is ‘glued’ by what appears to be low-temperature solder paste to its own metal pad on the ceramic substrate, with more of the same used to bond together the parts of the sensor itself. These consist mainly of a layer of gray piezoceramic material sandwiched between two chunks of metal. The larger of these acts as a proof mass, compressing or tensioning the piezoceramic layer when the part is moved on the axis normal to it. The metal ‘buns’ double as electrodes. There are (were) three such sensors in different orientations, one per axis, but the middle one broke off and flew across the room when the package was cracked open.

Like most piezoceramics, the sensors inside are affected by thermal changes, and become more sensitive with increasing temperature. The designers appear to account for this and provide some measurement headroom over the nominal value (this bad boy is a 500G accelerometer) so that the full quoted range can be measured even at the maximum specified operating temperature. This means at room temperature, where it’s less sensitive, you can actually measure accelerations of maybe 30-40% higher than the nominal value before the output limits (with appropriate calibration and reduced resolution of course). At very cold temperatures, even quite a bit higher measurements are possible, with the same caveats.

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