Parallel-path digital-to-analog converters for Nyquist signal generation

Nyquist-rate digital-to-analog converters (DACs) can generate frequencies up to half the sampling frequency. It is, however, impractical to generate such high frequencies. Due to its nature, the converter will not only generate the desired signal itself but also, often undesired, image frequencies. For frequencies near the Nyquist frequency, an image with almost exactly the same amplitude appears very close to the signal. An extremely steep filter is required. Therefore, real-life systems do provide oversampling to locate the Nyquist image further away from the wanted signal. In practice, the signal frequency has to be reduced if the maximum sampling rate is reached. We propose a technique that conversely removes this Nyquist image so that only further away Nyquist images with lower amplitudes have to be filtered off. The proposed technique is applied to the design of a dual 6-bit binary current-steering DAC running at 250 MS/s.