# 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 Commented Oct 19, 2018 at 20:50
• @PlasmaHH I understand that, but how adjusting the feedback voltage in DAC does this operation? Commented Oct 19, 2018 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. Commented Oct 19, 2018 at 21:22

## 1 Answer

The way that diagram indicates it works it that the SAR gets the rough value +/- 1 bit of the coarse value, then subtracts that from the signal to create a remainder. That is then fed to the Delta Sigma whose range spans just 2 bits worth of the coarse range, and adds the remaining less significant bits to the total result. The zoom I think is the implication that the second ADC has a much smaller span than the first. Practically, it might be done by adjusting the upper and lower limits of the second ADC rather than the subtraction, but the end result would be the same. The issue with this arrangement is that the coarse ADC needs linearity far better than its resolution would demand alone for the additional bits of the second ADC to have any precision.

• Thank you, so, by the comments from Plasma11 and your answer, it seems changing the reference voltage close to input would somehow get the remaining lsb. Ok. I will try to simulate it in SPICE later then. Commented Oct 19, 2018 at 21:46
• I tried to simulate this in simulink today and somehow got what you meant. But I implemented the coarse ADC logic in the feedback path with incremental approach - and then used it to adjust the reference - which adjusted the resolution as well. Thanks! Commented Oct 24, 2018 at 17:33