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question on AFE5809's coefficient output test mode

Hello,

In the datasheet it describes coefficient test mode as below,

>>>The Input to the decimating filter can be replaced with a sequence of "one impulse" and "zero" samples,
where "one impulse(that is 0x4000)" is followed by (16 x M) "zeros (that is 0 × 0000)". 

However I got some confusing results by using test vectors provided in EVM package.

1.decimation factor=4, below is the original filter plot,

below is output of coefficient test mode which has been decimated by 4,

Question: how does the negative impulse response come from?

2.M=16 original filter

test result of coefficient test mode output which has been decimated by 16,

Additional question: how to explain the distance between response? In M=4, the distance is 64, however in M=16 the distance is 32.

And maybe the root question is: what's the real input sequence in coefficient test mode?

Thanks!

B.R.

River