| 摘要 |
In order to improve the signal-to-noise ratio of a known sigma-delta analogue-to-digital converter A, the input to the loop filters 3, 4 (which can be regarded as an error signal between the analogue input and, ignoring imperfections in the digital-to-analogue converter 8, the analogue equivalent of the digital output), is filtered, 10, amplified, 11, and converted to a digital signal in a second sigma-delta converter B. This digital signal is combined with the digital output of the analogue- to-digital converter A to produce an output of the two stage converter with improved signal-to-noise ratio. Phase and amplitude errors introduced before the signals are recombined are corrected for by an equalisation network which is calibrated at regular intervals by means of a test generator 18. The same technique can be employed to produce a digital-to-analogue converter, and a digital modulator, with improved signal-to-noise ratio. <IMAGE>
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