why the quantised of DAC will affect the amplitude of the output waveform of AD 9850?
The output of the D/A converter is a series of steps, in which the voltage moves directly from the previous value to the next. This is also known as "zero-order hold".

The output spectrum would be perfectly flat if the output were a series of infinitesimally-narrow impulses. The zero-order hold is equivalent to putting such a series of impulses through a filter whose impulse response is a rectangular pulse whose width is equal to the sample period. The frequency response of such a filter is the dashed sin(x)/x line shown in Figure 5, which is why the amplitudes of the harmonics in the output spectrum are affected in the manner shown.