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ADXL362 and Photosensitivity

Question asked by Superluminal on Jul 7, 2014
Latest reply on Jul 9, 2014 by Superluminal

Hi all,

 

I’ve been prototyping with the ADXL362 and it would appear that the accelerometer is photosensitive!

Let me explain, once calibrated etc. the XYZ values behave themselves in my office, with Z showing approximately 1g and XY at roughly 0g when horizontal.

The device is attached to a small UAV project that I’m working on which requires outdoor testing. During this testing I could not get the accelerometer associated parts of the project to work as expected. After some analysis I discovered that the XYZ bias values were significantly shifted during flight testing.

Initially I assumed s motor was causing the issue and that I either had to improve the mounting of the accelerometer or use a part with a better “vibration rectification” specification.

It turns out that after some analysis, the ADXL362 was well within specification with respect to the UAV vibration levels. So the problem was somewhere else. After lot’s of code checking and test flights designed to log the accelerometer outputs etc. I discovered that the bias levels changed abruptly as soon as I took the UAV outside into direct sunlight. I should also add at this point that the PCB on which the accelerometer is mounted is exposed (but is protected by transparent heat shrink).

So was it temperature related? No... (more testing) However by using a small “hand operated sun shade” to cover only the ADXL362 I could instantly (within one sample period) modulate the bias values on all axes (by approximately 1g for the Z axis and by similar amounts for X and Y). This was a big surprise! So much so that I that I had to get a colleague up to speed on the experiment and
convince him of the effect.

I checked on your web site and discovered that the black packaging used on the ADXL362 is made from Slica with 0.5% added black carbon (for colour I assume). i.e. a compound (Silica) that is not completely opaque to infra-red, of which the Sun produces plenty!!

The fact that the ADXL362 is also extremely low power is another possible reason why it may (or may not...) be susceptible to light/infra red photosensitivity etc.

Pardon the pun, but can anyone shed any light on what is happening here? Is the part photosensitive?

In production the accelerometer will be encased so this isn't an issue, but I would still be interested to know what is going on!

Many thanks!

Max

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