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Low-noise sigma-delta analog-to-digital converter

Image Number 2 for United States Patent #6160506.

A sigma-delta analog-to-digital feedback converter arrangement (10) achieves low noise and a wide dynamic range by use of a directional coupler (18) as a summing device for generating the difference signal on which the forward sigma-delta analog-to-digital converter (32) operates. The feedback is provided by a digital-to-analog reconstructor (36), which applies the reconstructed analog signal to the tap (18.sub.3) of the directional coupler (18), in which the feedback signal is summed with the analog input signal. A low-noise amplifier (28) is coupled to the output of the directional coupler (18) at which the difference signal appears, so the dynamic range of the signal traversing the low-noise amplifier is small. The high isolation between the tap (18.sub.3) of the directional coupler (18) and the source of analog signals allows the use of an input bandpass filter (16).

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