sigma-delta versus nyquist rate ADC

[nathanee]

nyquist adc

Hi,

Both of sigma-delta and nyquist ADC have SND, SNDR, THD, SFDR specification.
But, why sigma-delta has extra OL(over-load) and DR(dynamic range) spec?

Is there any phyical meaning of these?

Thanks in advance.

BR,
-nathan

 

[naalald]

nyquist rate adc

Hi,
Delta Sigma ADCs reach to so higher max SNDRs by two major techniques: One is oversampling and the second is Noise shaping. The max SNDR is given by this equation in Delta Sigma ADCs (according to Shreier book),

in which the Bq is the quantizer bits, the OSR is the oversampling ratio and the n is the order of the loop filter.
Since the power of the quantization noise is constant for a specific quantizer in a delta sigma ADC, by increasing the sampling frequency the in-band quantization noise power decreases (see the picture), and consequently the SNDR increases. Furthermore, by Shaping the noise with an n-th order loop filter you move the quantization noise power to higher frequencies than the nyquist frequency so that you can get better resolutions.
This picture depicts a 0, 1st and second order noise shaping. You see the difference between the in-band quantization noise power of the three loop filters.

 

[safwatonline]

nyquist-rate adcs

OL:Sigma delta are closed loop systems
DR : is for both types

 

[safwatonline]

sigma delta second nyquist band

to add: the DR in high order SD ADC is important cause of stability issues