ADXL345 Die Analysis

Summary

The ADXL345 is a MEMS accelerometer made by Analog Devices. It’s a popular device among hobbyists because of its low cost, easy availability and rich feature set. But apparently you should be careful about where you buy them: one of my readers ended up with a bunch of ADXL345s that had significant offsets, measurement axes that didn’t work at all, and an inoperative freefall detection mode. After spending lots of time trying to get them to work, he decided to send them to me instead and hopefully find out what was wrong with them. The front and back look pretty normal at first glance. But if we slightly change the illumination angle, we can already spot something unusual. Behind the printed markings we find another set of laser markings. The top and bottom lines say the same thing (“945B” and “PHIL”), but the two lines in the middle are slightly different: “#934” instead of “#935”, and “2029” in place of “9697”. These numbers indicate things like the manufacturing date, a batch number or a factory indentifier and don’t affect the functionality or performance of the chip. However, the fact that there was a first set of numbers which was apparently deleted and replaced by another set is not a good sign. It could mean that this chip was part of a faulty batch meant to be destroyed, but ended up in the hands of an unscrupulous dealer who re-marked the top side and sold them as good parts. If we etch away the top plastic, we find this two-chip module on the bottom layer. There’s a MEMS chip in the middle and a readout chip on the right, connected to each other by nine bond wires. There’s another bond wire from the top of the MEMS chip to a pin that’s also used by the readout chip, which is most likely the ground pin. The readout chip is a typical mixed-signal IC. At the bottom there’s a row of bond pads that connect to the MEMS chip, with a three-channel analog-to-digital convert above it. The rest of the chip is a digital engine that drives the ADC, stores the results a...