Hi everyone,
I have an analog grounding question. It is asked a lot but it often seems more opinion than facts. In my application I have a 24 bit ADC and I need to sample a low voltage signal (-10 to 10mV). I made a shielding cabinet around the ADC and then thought about how to ground the thing. I have a 3.3V low noise LDO for the analog stuff and a DC-DC Converter for the 1.8V digital section which operates from 1 to 2MHz switching frequency. So I want to make sure that the analog section is not affected by switching noise (there is bluetooth on board too, also GPS which has a high dynamic range of current consumption all operating from the 1.8V rail).
So here are a couple of things I thought about and do not know if they make sense or not:
1) To route out the analog inputs I used the bottom layer of the 4 layer board and made a track above it (called it guard, it should be a shield effectively) and connected this track only at one point. This shielding track is currently twice as wide as the analog input track. Is this nonsense or useful? (it is semi-transparent on the image)
2) Worst part: Grounding. I first made a common ground not separating analog ground and digital ground. Then I thought I should separate them since digital ground might have switching noise and a lot of people say it is absolutely essential to do so. On the other hand James Bryant of Analog devices has this article that states that AGND and DGND should be joined right at the chip level. (
http://www.analog.com/static/imported-files/rarely_asked_questions/RAQ_groundingADCs.pdf)
So I wanted to ask for some inputs on this issue...whats your take on this? I've attached a screenshot of the analog section
Daniel