Method and apparatus for extending the spurious free dynamic range of an analog-to-digital converter

摘要

An A/D system (10) provides an expanded SFDR when compared to the SFDR of individual A/D(s) that make up the A/D system. In response to an analog input signal(s) whose amplitude would result in a narrower SFDR for a first A/D (12), the A/D system uses an amplitude adjuster (14) which receives the analog input signal and produces an amplitude-limited signal with resulting signal distortion (S + D) to the first A/D (12). By amplitude-limiting the analog input signal, the A/D system produces the resulting signal distortion but reduces or "clips" the amplitude of the analog input signal, thereby improving the SFDR performance of the first A/D by reducing the spurious distortion produced by the first A/D. The signal distortion (D) resulting from the amplitude adjustment is routed to a second A/D (15). The first A/D converts the amplitude-limited analog signal to a digital signal (S<1> + D<1>) with an expanded SFDR due to the lower amplitude of the adjusted analog signal. The second A/D (15) receives the portion of the analog input signal removed by clipping which can be referred to as the signal distortion (D). The second A/D converts the signal distortion, which typically has a lower amplitude than the adjusted analog signal, to a digital signal (D1) with at least an acceptable SFDR. The A/D system combines (16) the resulting digital signals from the first and second A/Ds to produce the desired converted digital signal with an extended SFDR. The SFDR is extended because the relative difference is increased between the amplitudes of the digital output signal and the spurious distortion when compared to the amplitudes of the digital output signal and the spurious distortion if the analog input signal has been converted by the first A/D without amplitude adjustment. <IMAGE>