Non-linear DAC implementations in DDFS

A technique to reduce ROM size and therefore power dissipation in direct digital frequency synthesizers (DDFS) is to use a non-linear DAC to approximate the sine function. Piecewise-linear and piecewise-quadratic approximations were investigated for a 12, 14 and 16-bit non-linear DAC in terms of the required ROM size, achievable spurious-free dynamic range (SFDR) and implementation complexity. Results show that 94 dB SFDR can be achieved using a 16-segment quadratic approximation, DAC resolution of 14-bits and a 5-bit squaring circuit. The required ROM size is only 256 bits.