tsb_nph said:
Thank you very much, Bharath!!
Another question:What is the "loop delay"mention to???
Thanks!!!
In a discrete time ADC, the loop filter is discrete - which implies the signals are sampled and then processed by the filter at discrete time steps - usually at either rising or fall edge of the clock driving the switched capacitor filter.
The DAC signal feeding back to the discrete filter is also processed only at discrete time instants by the filter. When the comparator clock samles its input signal, it gives a quantized output (1 or 0), then the digital signal is fed back at discrete time steps by the DAC to the filter. Because of the discrete nature of feedback, any propagation delay in the feedback path (prop delay of comparator + DAC switching time) does not affect the system.
In a continuous time filter, the signals are processed continuously - which implies that it requires that the feedback DAC signal to be continuous in time and it expects the feedback DAC signal with no delay (or sometimes a predetermined fixed delay).
But the propagation delay of the feedback elements (comparator + DAC switching time) causes extra time delay and is usually referred to as "excess loop delay". This is harmful because it changes the loop transfer function (and noise transfer function) and affects SNR, stability etc.
In summary, the loop delay occurs fundamentally because the continuous time nature of loop filter expects the feedback signal in continuous time mode (and not at discrete time steps).
I hope this explanation is clear.
Bharath