Hello JasonSung,
I'm not sure that my reply is what you want.
And, although you may already know,
>1.How does the low-pass filter work in digital signal process?
I think the low-pass filter works as anti-aliasing filter for down-sampling.
>2.Does the decimation rate ND only effect the capture time? (page 12)
I think ND affects filter's cutoff frequency and filter's group delay.
They are very important for selecting ND.
Best regards,
Piyo