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An Acid Trip

When you think of a high-acid environment, what do you think of? That can of soda? Lemon juice? Your stomach? Battery acid? Well, to review, neutral pH is 7. Your stomach will have a pH of 1.5 to 3.5, presumably depending on how much soda or lemon juice or Thai-spicy tom yum gai you just had. Coca-Cola Classic is around 2.5. Lemon juice: 2.0. Battery acid: 1. It’s hard to imagine electronics functioning in a bath of any of those tongue ticklers.

But there’s an environment that’s even worse. One within which an electronic component must operate. And you might never expect it. In fact, many folks didn’t expect it. It can have a pH of 0.5 or less. It’s car exhaust. And it’s not all gaseous. It can make mincemeat of your chip in a couple months. Who knew…

Certainly not the many folks trying to build pressure sensors for that environment. Sensata has just announced a relative pressure sensor for this application – and even they didn’t realize this when they started.

There are actually a couple things that they’ve done that others have had trouble with. One is simply getting this to work in such an environment; the other is doing it with a single element – other folks apparently use two pressure sensing elements (and measure the difference between them).

The Sensata approach is a two-port sensor, with one sealed port on each side of a piezo-resistive element. Each port exposes the pressure of one of the environments to be compared, so the element is, by definition, providing the relative pressure difference. The element, which, of course, has a backside etch so the membrane is accessible to front and back, is mounted in a ceramic carrier.

Then there’s the bit about the acid. There are lots of things that can be attacked, most of which are on the logic that makes sense out of the raw pressure measurement. In other words, CMOS. Much of it can be protected by suitable passivation, but you still have to get the signals in and out, and that takes metal. And, no matter how much you protect everything else, that metal isn’t going to like lots of acid.

So rather than having that logic on the same chip as the element or an ASIC collocated with the element, they put the ASIC away from the element, outside of the corrosive environment. That minimizes the part that has to be robust and protects the delicate bits.

You can find more in their release

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