The data sheets for analog 4-quadrant multiplier-on-a-chip chips are remarkably vague as to what goes on inside the chip. I'm thinking specifically of the Anaalog Devices AD633 and the Burr-Brown MPY634, which are (at my level of viewing) similar in overall function and specs (but different in some details.)
- One-quadrant multiplying doesn't seem too hard. Take the logs, add the logs, exponentiate the log. Basic building blocks are things I think I understand pretty well.
- Four-quadrant multiplying is still somewhat a mystery. The AD633 just says it has a "translinear core". I have about as much understanding of that as if they had said "dilithium crystals". The MPY634 data sheet doesn't even say that much, it just jumps right into an equation without relating it to any internal functions.
- Maybe a four-quadrant multiplier can be done with a one-quadrant multiplier, some absolute-value-taking circuits, and some comparators, and a final multiply-by x1 or x-1 depending on the comparators.
- One clue that the MPY634 and AD633 don't use the method I suggest in (3) is a lack of symmetry in the X and Y inputs. One input always has a linearity of maybe 4 times better linearity than the other. This indicates to me that internally there is some assymetry that isn't necessarily implied by my simple suggestion.
- Maybe the "translinear core" is something like a Gilbert cell. Or is it just a clever application of the Barrie patent that describes how things like the AD603 work? (See my thread here from September about how the AD603 works.) Even then that only gets you two quadrants... but there is an assymetry in the input such that maybe some other trick comes in.
So how many of my 5 thoughts above are completely and hopelessly wrong? All 5? :-)
If there's some Barrie patent that explains all this, I'd love to read it. When I've been pointed directly towards them in the past, they were always a joy to read. But I've got an exceptionally thick head and unless I'm pointed towards a specific one my eyes still glaze over in the claims section :-(