How does the Zoom ADC exactly zoom in (Practically)?

I am trying to understand the actual working of this specific Zoom ADC (diagram below), which is a 2-Step ADC with coarse (SAR) ADC and a fine (Sigma-Delta) ADC. I understand it theoritically but could not think practically on how it exactly zooms. I could write down step by step in brief on what i understood and where i got stuck (highlighted in bold).

I am referring to the below architecture: As per my understanding, it operates as follows:

1. The ADC in the forward path called a coarse ADC samples the input to a few MSB.
2. Then the output of the coarse ADC (Y coarse) is used to adjust the references of the DAC.
3. Then the sigma delta loop turns on and then a fine conversion (the remaining bits) are found using the usual Delta Sigma modulation and a decimation.

In a high level, the coarse ADC sets some reference and the fine ADC kind of wiggles small around the reference until it reaches the correct average value.

Conceptually, it looks like this: I understand the working of a normal single bit sigma delta loop where the feedback DAC outputs a 0 or 1 (VCM or Vref) depending on the output of comparator. But I could not think on how changing this Vref to a different value would zoom.

If anybody could guide on how to think this zooming process practically, it would be helpful.

• I think you are just too hung up on the term "zoom". If you take the coarse value and add the sd value, you get the best approximation you can, as simple as that – PlasmaHH Oct 19 '18 at 20:50
• @PlasmaHH I understand that, but how adjusting the feedback voltage in DAC does this operation? – sundar Oct 19 '18 at 20:54
• imagine the first stage sets the reference voltage to exactly the input voltage, then the difference the second stage adds is 0. Now make that reference voltage deviate a tiny bit from the input, thus the second will only result in that tiny bit etc. – PlasmaHH Oct 19 '18 at 21:22