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).
As per my understanding, it operates as follows:
- The ADC in the forward path called a coarse ADC samples the input to a few MSB.
- Then the output of the coarse ADC (Y coarse) is used to adjust the references of the DAC.
- 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.
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.