I have a problem of the parallel data D<17:0> of AD9957. In the data sheet of AD9957, the data-words applied to Pin D<17:0> are expected to be coded as twos complement (default). For example, if the data is -7, then Pin D<17:0> is 111111111111111001，but when the data is a decimal, such as -0.5, then wha's the PIn D<17:0>? I don' know where to put the decimal point. In another words, I don't understand how the AD9957 recognise a decimal.