continuous time v.s discrete time Sigma-delta converter

[kuohsi]

Hi,all
Does anyone know the difference betwen continuous time & discrete time SDC??
Thank yuo!!

 

[tsb_nph]

Hi,
The main difference is that the loop filter in the forward path is a switched capacitor (discrete time) filter in a discrete time sigma-delta, while the loop filter is a continuous time (gm-C, LC or active RC) filter in a continuous time adc.

The signal is sampled at the ADC input for the discrete time ADC, while the signal is sampled at the comparator input for the continuous time ADC.

Frequency tuning is a problem for continuous time but not for the discrete time.

Advantages of continuous time : lower power, higher frequency of operation, inherent anti-alias filtering
Disadvantages of continuous time: more sensitive to clock jitter, excess loop delay and requires some mechanism to tune the cutoff/center frequency.

This is a very brief overview. If you have any specific questions, we can discuss further.

Bharath

 

[kuohsi]

Disadvantages of continuous time: more sensitive to clock jitter, excess loop delay and requires some mechanism to tune the cutoff/center frequency.

Bharath

Thank you very much, Bharath!!
Another question:What is the "loop delay"mention to???
Thanks!!!

 

[tsb_nph]

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