Radio Frequency (RF) & Wireless Design

Created by Steven Minichiello on 27 April, 2018

Recently I have been researching Analog to Digital Converters for bit resolution, types, conversion speed and for interfaces for direct conversion receivers.


Analog electronics seems to be a necessary nuisance for the digital designer these days. Its because of IoT and other sensors that the digital engineer is forced to understand and deal with the analog-to-digital conversion process.


But I wanted to survey the ADC landscape as we are now at 32b ADC resolution ! While this may not seem like anything, bear in mind that we now talking about 4 billionths of a voltage range !! Now considering that most devices are 3.3 volts, 4 billions of 3.3 volts is less than a nanovolt resolution; which is extremely tiny !


The issue is although you can do this, does it make sense, especially when you understand there in analog electronics there is something known as the noise floor : https://en.wikipedia.org/wiki/Noise_floor


which is made up of many various components :


In addition, this is noise on the bottom end (ground) but there is noise on your top end as well (power supply rail noise) from your voltage regulator. This is defined as part of the Power Supply Rejection Ratio :

https://en.wikipedia.org/wiki/Power_supply_rejection_ratio


So why am I even talking about this, well once you consider that you do NOT have a FULL voltage range, you have to understand what dynamic range you ultimately have. This means that you don't use the PSSR zone and you don't use the noise floor zone. What you have left is the zone for your dynamic range :

https://en.wikipedia.org/wiki/Dynamic_range


I have put together a graph here of the latest ADC parts from Digikey since everyone in the world seems to use them, although Mouser may also be worth comparing too ! The chart show that as you increase resolution for low voltages that you end up pushing into both the noise floor and the PSSR zone, so that your resolution gets smaller and smaller.



The only way to increase bandwidth of dynamic range is to increase your voltage swing area. This is important to understand especially for single chip MCUs since the internal ADC may be limited by external noise, regardless of ADC precision.


Its also worth noting that since the technology is advancing to higher bit resolutions, that it may be necessary to use higher voltages instead of pushing down to lower voltages. This may not be necessary for entire system, but just for the ADC front end. The trend with using batteries will mean that a boost converter will be necessary to get the higher range, but that the SMPS booster will also introduce additional noise, so its chicken-and-egg problem.


Your comments are welcomed.