# ENOB and oversampling ratios for delta-sigma ADCs (in relation to CS5343)

I came across the CS5343 ADC the other day and was impressed with its specs (relative to its cost) but then became confused when I dug deeper and read the datasheet.

It's nominally a 24-bit 96 kHz ADC, but the datasheet lists a dynamic range of only 98 dB and a "-92 dB THD+N". The 92 dB figure gives an "effective number of bits" value of 15 using the normal arithmetic. Even the dynamic range would only correspond to 16 bits worth.

So I guess the first part of my question is: why the heck would anyone choose to market this product as a 24-bit ADC?

I then read a bit more about how delta-sigma ADCs work (along with noise-shaping). I partially understand them now but I clearly have serious gaps relating to the practical details, including the decimation. For example, the CS5343 has a master clock rate of up to around 40 MHz (when digitising at around 100 kHz) - specifically, a 36.864 MHz clock can be used to give a 96 kHz sample rate, using a divider of 384x. Now, I'm stuck here. My feeble understanding of the 1-bit datastream that emerges from a delta-sigma would imply to me that if you want to achieve 24-bit resolution, you need to count the number of "1s" that occur in each set of 2^24 samples. That would mean a multiplier of 2^24 (i.e. more than 16 million) between the data rate and the master clock rate, not 384. (It also implies a master clock rate of around 1.6 THz!)

So here's the second part of my question: given that this 2^N multiplier (where N is the number of ADC bits) is clearly not present in real-world ADCs, can anyone point out the breakage in my italicised text above, or a link which explains it?

• Your understanding is correct for a first order DSM : by the time they got to 3rd order modulators and noise shaping they can do much better with much less. Worth reading the classic Bob Adams papers from about 1990 if you can find them. Oct 3, 2020 at 22:08
• Thanks Brian. I forgot to note that it's described in the datasheet as a 3rd order modulator, and I did wonder if that hid some of the magic I was missing. Will hunt for those papers :) Oct 3, 2020 at 22:11