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We propose a time-domain technique that significantly improves resolution of an RF-DAC. As an alternative to resorting to various resolution improvement attempts in the amplitude domain or through quantization noise shaping, pulsewidth modulation (PWM) of a single unit switching device is employed with fine timing accuracy easily afforded by advanced CMOS technology. The PWM is categorized into centered PWM and noncentered PWM depending on the relative pulse position, and their performance and implementation methods are compared. The technique is examined in the context of a commercial EDGE polar transmitter realized in 65 nm CMOS, which employs an amplitude modulator with basic 10-bit amplitude resolution limited by the RF-DAC switching device mismatches. The proposed architectures with centered PWM and noncentered PWM achieve the worst case resolution improvement of 2.2 bits and 2.5 bits, respectively, assuming 20-ps worst case time granularity of the PWM signal controls.