Why does the Derivative x(t) VI different than Derivative x(t) PtbyPT VI?

[GS]

Why does the Derivative x(t) VI use Y = (Xi+1 - Xi-1) / 2dt whereas the Derivative x(t) PtbyPT VI uses Y = (Xi+1 - Xi-1) / dt? It seems to me that dividing by 2dt isn't right.

George

[johnsold]

George,

x[i+1] - x[i-1] is the difference across two sample intervals, hence the 2*dt, because that is the time difference between the x samples. In the point by point VI an uninitialized shift register is used to retain the previous sample so it is calculating x[i] - x[i-1], a single dt interval.

Lynn

[bkk]

Could i ark you about using Derivative x(t) VI function ?

coz i'm using this function but it's not work ! i don't know why?

the output of the function always zero even the input is the signals.

i compose the example.vi that i try to find the derivative.