timmc6
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I am currently design a digital Sigma Delta for a Frac N PLL and I saw that if the accumulator size is not large enough, there will be an error when it tries to wrap around. For example, if your required MOD value is 128, and you have a 3rd order MASH architecture, it is possible that the last accumulator will need to hold a value of 256. So the required accumulator size would be 8 bits instead of 7 to hold a maximum value of 256. If 7 bits are used and the accumulator needs to hold something larger than 128, it will wrap around and not generate a carry, throwing off the whole SD operation. Hope this helps.
Tim
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