About noise in analog GND and digital GND .

[huzuhong525]

hi all ,
As i know ,AGND and DGND is separate in board ,and they get together finally .
Could somebody tell me some info about it ?which noise is more in AGND & DGND ?etc

thank you very much .

Added after 16 minutes:

DGND has lots of high frequency noise .

Added after 1 hours 19 minutes:

Anybody can help me ?

 

[RF-OM]

It is not easy question. Do not expect that somebody will tell you a dozen of words and solve all your problem. You need to study Signal Integrity, Layout and High Frequency Design. Check the books. The best authors are Bogatin, Johson, Ott, Brooks and Montrose. In high speed and RF designs there is no such things as ground. There are return paths. AGND and DGND are just a means to provide these return paths. It is very interesting area but it is not easy and require studying.

 

[huzuhong525]

Thanks for your suggestion .
I got some info about it on my own,and i just want to know what others understand about it .

 

[RF-OM]

This is what I see every day. One engineer ask another and got answer that may be not right for this particular case or even totally wrong. Instead of checking well known books people often trust some other sources. EMC problems are not very obvious as, for example the Ohm's law. It hard to find mistake right now and often it is too late when design is done. I think that there are only two proven ways to learn EMC. The first is good books and the second is good courses from well known teachers. Any other ways will put engineer in trouble. Unfortunately it is very often when engineer even with 25 years of experience is making wrong boards and doing too many prototyping rounds.

 

[ParagKaria]

Your query can be justified in simple words as below.

FIRST we will see why the AGND and DGND are kept seperate(For SI issue) and then we will see why they are met at one end on the board(FOR EMI/EMC issue).

FIRST THING COMES FIRST::

The Analog signal is considered low frequency and low amplitude compered to that of digital signals.
Analog signals can be in the range of few kHz to MHz. and respectively low amplitude and the rate of change is gradual.
While digital signals are switching signals and changes level from 0 to 1.8V/3.3V/5.5V depending upon the logic levels. The digital signal useally are in few hundred MHz and the rate of change is sudden. It jumps to high or low levels in fraction of seconds.
This switching is coupled to power and ground and thus the return path is formed.

Now, this switching can easily gets coupled to analog circuit and the actual analog signal gets distorted and signal identity gets lost.
So, both the PWR and GND for analog and digital are kept seperate.

THE SECOND THING::


The basic rule for the radiation can be worded as "Larger the loop area, more radiation can be expected"
Now, as you have different PWR and GND for Analog and Digital, you have larger return current loop on the board. This inturns increases the radiation.
And boards fails for the EMI/EMC safeguard norms.
This can be avoided by shortning the loop area. But where to short is the main issue here.
The best place can be at the bellow the ADC/DAC chip on the board as it has both the Analog and Digital circuit and PWR/GND.
In case the ADC/DAC chip is not available on the baord they can be shorted at the supply entry to the board via a FB.

Hope this explanation will solve your doubt regarding AGND and DGND on the board.

Thanks for reading.

Regards,
Parag.

 

[huzuhong525]

thank you all .