SIN(x)/x correction (inverse SINC function)
Does that mean that I can also produce a sinc signal with this DDS?
No - there is a sibc filter correction for the DAC build in - please see page
22 of the datasheet
INVERSE SINC FILTER
The sampled carrier data stream is the input to the digital-to-analog converter
(DAC) integrated onto the AD9857. The DAC output spectrum is shaped by the
characteristic sin(x)/x (or SINC) envelope, due to the intrinsic zero-order
hold effect associated with DAC-generated signals. Because the shape of the
SINC envelope is well known, it can be compensated for. This envelope
restoration function is provided by the optional inverse SINC filter preceding
the DAC. This function is implemented as an FIR filter, which has a transfer
function that is the exact inverse of the SINC response. When the inverse SINC
filter is selected, it modifies the incoming data stream so that the desired
carrier envelope, which would otherwise be shaped by the SINC envelope, is
restored. However, this correction is only complete for carrier frequencies up
to approximately 45% of SYSCLK.
Note also that the inverse SINC filter introduces about a 3.5 dB loss at low
frequencies as compared to the gain with the inverse SINC filter turned off.
This is done to flatten the overall gain from dc to 45% of SYSCLK.
The inverse SINC filter can be bypassed if it is not needed. If the inverse
SINC filter is bypassed, its clock is stopped, thus reducing the power
dissipation of the part.