ABSTRACT
The Nyquist-rate analog-to-digital converter has no choice but to trade high power for low quantization noise. When it is operated at oversampling rates, the in-band quantization noise is lowered by the oversampling ratio as the quantization noise is spread evenly over the Nyquist band. If it is placed inside a feedback loop, its quantization noise can be even further suppressed by the loop gain. A modulator is basically an active feedback filter made of a high-gain loop filter that performs either as a low-pass filter or band-pass filter. The loop stability is affected by all delay elements in the loop, including the quantizer’s half clock delay. For stability, zeros are commonly inserted to warrant phase margin at the unity loop-gain frequency. The digital output spectrum repeats per sampling frequency and is made of the low-frequency input signal and the shaped high-frequency quantization noise.
