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What word and fraction length would you suggest ? Does it make sense to use a higher word length than the signal input is?
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I would suggest you use as many fractional bits as you need, no more. Its best to work out what range you require and what precision you need to do this (often, this is done in something like matlab first)
Also, keep in mind tricks you can do with hardware multipliers.
If you have a number in the range -256 to 255 (9 bit signed) and you need to multiply it by a number in the range -1 to 1-1/256 (this is also 9 bit signed)
you can use a 9 bit multiplier for this, instead of zero padding and using an 18 bit multiplier, because all 0s are always implied, giving you a 2^n bias on each number. Therefore your 18 bit output will have 10 bits magnutude (with 2 sign bits) and 8 bits fractional.
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