Theory of operation of MS3110 high resolution capacitive sensor

[Chengjun Li]

@Gryd3: This is a continuation of numerous posts here and elsewhere by @Chengjun Li in an attempt to understand the internals of the Irvine Sensors MS3110 differential capacitance readout. Why he wants to know this makes about as much sense to me as wanting to know how the internal microcode of a microprocessor executes op-codes. It's certainly of academic interest, but in terms of application usefulness the MS3110 provides all the information you need to use it. Besides that observation, the internal nodes and circuitry of the MS3110 are mostly not accessible, unless you can probe the bare die. You can purchase bare dies for hybrid integration with MEMS devices, but the only easily accessible points are the bonding pads, which are of course available in the packaged product.

The only information I could find on the theory of operation of the MS3110 references a block diagram. As we all know (or should know) block diagrams are simplified explanations of what really goes on in the actual implementation. In this case no timing information is provided, so it's anyone's guess as to how it really accomplishes its "magic" of providing a DC signal proportional to the difference in capacitance presented at the input terminals, CS1IN and CS2IN. Perhaps someone could breadboard the block diagram to see how it works.

Sorry, I know repeatly open the same topic is boring.
The reason I want to know how does MS3110 work is because my advisor asks me to do this. He used to use the MS3110 when he was a student, but with a different transfer function. He said the circuit now shows no difference with the one he used but the transfer function is different, that's why he asks me, one who has no EE background to find out the reason. That's the reason I try to understand the circuit, which is also the reason I am still working on it at nearly 2 am right now. Now I think I can just show him your replies, although I am pretty sure he is not satisfied with this answer.

 

[KrisBlueNZ]

Repeatedly opening the same topic in NEW THREADS is not boring; it's INAPPROPRIATE.

Harald warned you after the third time you did it; this is the fourth time, so I'm giving you a week to think about WHY discussions on one topic should not be split into four separate threads.

Edit: I've merged the fourth thread into the original one and edited the title slightly.

 

[hevans1944]

Sorry, I know repeatly open the same topic is boring.
The reason I want to know how does MS3110 work is because my advisor asks me to do this. He used to use the MS3110 when he was a student, but with a different transfer function. He said the circuit now shows no difference with the one he used but the transfer function is different, that's why he asks me, one who has no EE background to find out the reason. That's the reason I try to understand the circuit, which is also the reason I am still working on it at nearly 2 am right now. Now I think I can just show him your replies, although I am pretty sure he is not satisfied with this answer.

Very sneaky on the part of your "advisor" to make you do the research he should have done as a student. Here is a "real world" fact: Intellectual Property (IP) is valuable but is often stolen. Patents are of some help to protect IP, but only if you have "deep pockets" to pay to defend your IP against theft by competitors. An alternative approach is the "trade secret" where you keep hidden the details of your IP while manufacturing and selling your product. The Coca-Cola beverage recipe is a famous example. There are many Coca-Cola imitations but none taste like "the real thing".

Not to help you in particular, but to satisfy my own curiosity, I searched the Web for information on how others have tried to sense small changes in capacitance. Measuring and integrating charge using pulse methods to measure small values of capacitance is described in the open literature, but it is fraught with details and problems that must be overcome before accurate results, stable with variations in temperature and accounting for parasitic effects, can be obtained. The engineers who designed the MS3110 did their "homework" and solved the problems, producing a viable product. I would rather purchase their product than try to build my own. This is also currently a niche market with perhaps a few thousand MS3110 devices sold annually. That could change, making the task of reverse engineering the MS3110 an economically attractive option... especially in certain Asian countries that do this routinely for high-volume products. So who can blame the manufacturer for not revealing all the details of how to copy their IP?

If MEMS devices requiring capacitance readouts become as popular as cell phones, perhaps the IP for the MS3110 will become available for license, or perhaps it already is available for license. In that case the masks and manufacturing details would be made available for incorporation in whatever product you wanted to design and everything would be made clear for your understanding. I haven't tried to contact the MS3110 manufacturer, but the principals of the company seem to have moved on to more advanced projects. Perhaps you should call them for answers to your questions. Start here.