What is the material the membrane is made from on the LPS28DFW?

[ch701x]

I've been working with a LPS28DFWTR pressure sensor and noticed it's pretty sensitive to sunlight. As in, the pressure will climb significantly when the sun shines on it. Because the membrane appears to heat much faster than the temperature sensor it, the reported pressure isn't temperature compensated. This isn't really a problem but I'm curious what material is used to make the membrane?

[Eleon BORLINI]

Hi @ch701x​ ,

the membrane of the pressure sensor at the device core is in silicon, so it is not influenced by the sunlight (in terms of wavelength or heat).

It could probably be a side effect due to residual charges. Is the application board ground well connected?

-Eleon

[ch701x]

Well that breaks my theory. I'm certain it's not a grounding issue. We've seen this on a number of sensors soldered to different boards in a couple different ways, so it seems pretty consistent regardless of installation.

Could you elaborate more on the residual charges?

[SSher.3]

I confirm that LPS28DFWTR is not just sensitive to sunlight, it is sensitive to ANY light! It is actually pretty crazy. Feels like it's some p-n junction exposed to light through transparent silicone confusing readings?

Attaching video here:

https://www.veed.io/view/af431c82-304c-4966-b9d4-a96c8cc29d36?sharingWidget=true&panel=share

It's a simple setup where I measure both pressure/temp using LPS28DFWTR  and convert it to altitude.

As you can see on the display, when I cover membrane and flash on the sensor - pressure readings are fine.

When membrane is facing bright light, readings of pressure are jumping quiet crazy, white temperature still OK.

I used bright Cree cold led on video, then I also tested with IR led (850nm) and I think it got even worse.

Looks like design issue, and it has to be black silicone instead of transparent? Unless it has an Easter egg - built-in ALS sensor? :D

[ch701x · Accepted solution]

Thank you for the video. I guess I was right but for the wrong reasons. We've "solved" the issue by simply flipping over the housing they are in to keep them out of the sun.

In recreating your test with the flashlight on my phone, I've actually induced I2C failures and inaccurate readings multiple times. Something very strange is happening when these sensors are exposed to bright and intense light.

Thanks again.

[SSher.3]

Hey, by the chance, what formula are you using for altitude conversion? I think it would be nice to have formula in some app.note or so, but I found none. In datasheet it says "temperature compensated" but I believe it's about die itself but not a altitude temp.compensation? I'll get back to laptop in several hrs and will post my formula that I use, but wasn't able to verify altitude due to pressure fluctuations caused by light.

[ch701x]

Unfortunately, our use case does not require us to calculate altitude. If I did though, I would use this one: https://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html