1
\$\begingroup\$

I am in the process of designing a new ADC\DAC PCB with a digital interface and analog front end. I have tried to separate the different grounds in my circuit to reduce the noise as much as possible but I am not sure if I what I am doing is correct. Please see the diagram below

enter image description here

I have tried to ensure that all the GNDs are returned via a single return - anGND -> GND -> GND RETURN 1\2.

  • Should I separate my digital GND (route it to GND Return 1) from my analog ground (route anGND to GND return 2)? NOTE GND Return 1 & 2 are connected directly at source.
  • Should I give my +/-12V LDO a different ground?
  • Should I give my +5Vadda LDO a separate ground?
  • Should I incorporate an additonal "digital" GND for the +5.3V DC\DC ?

In previous designs we have used the same DC\DC +3.3V->+1.1V and have not seperated the digital and anlog grounds and it has worked fine. Here do you think I should seperate them? If yes which grounds would you recommend? What other recommendations do you have?

I am not sure why my diagram above can't be viewed. I have re-inserted it below as a jpg:

enter image description here

I changed the design slightly as below:

enter image description here

\$\endgroup\$
0

1 Answer 1

0
\$\begingroup\$

Well I do not see any diagram but from what I have learned:

  • only provide a single path for a current, so do not connect different grounds at different places
  • with analog and digital grounds connect em at a single point beneath the AD or DA Converter
  • depending on the design just short circuit the grounds at this point or use one or more capacitors to block dc but allow ac
  • for different voltages use a single ground plane and distribute buffer capacitors (normaly groups of 3 parts in parallel like 100n,10n,1n to improve frequency response) over your design (there are some comercial calculation programms where to place them, but when you place 5-6 groups to cover the overall area and then place some more near major sources and sinks... that should be sufficient in many cases) --> if your design needs more precise calculations you need to dig in literature and courses, then you really need to invest time and money

PS: seperating the digital and analog ground is a measure to reduce noise and such influence to the adc precision. You need this if you messure very low signal differences and it commes into play when doing fast conversion rates with fast signals. if you just want to distinguish 1V dc or 2V dc,... its maybe overengineered.

\$\endgroup\$
18
  • 1
    \$\begingroup\$ Segregating sensitive and noisy components to different areas of the same ground plane so noisy currents from one don't flow under the other is modern practice. It's really easy to make things worse by separating grounds than if you just did it thoughtlessly. Never ever pass a trace over a gap in planes. \$\endgroup\$
    – DKNguyen
    Aug 23, 2020 at 20:51
  • \$\begingroup\$ schnedan, Thank you so much for your comment. I have incorporated a 20 bit, 1-MSPS ADC so I think I need what ever methodologies I can to reduce the noise as much as possible. \$\endgroup\$
    – shmueld
    Aug 24, 2020 at 9:00
  • \$\begingroup\$ Yes with 20 bit, 1-MSPS ADC you are certainly in a range where layout matters. Then I would also opt for at least a 4 Layer Layout so you can route the data signals in the inner layers. Also I would advise to apply rules of good HF designs like minimize vias, no 90° corners, equal length of all signals , equal distances between all data signals,... And if you plan to operate the circuit in an enviroment with other electric fields, think about a shielding solution e.g. a metal cage. \$\endgroup\$
    – schnedan
    Aug 24, 2020 at 18:09
  • 1
    \$\begingroup\$ @schnedan: About those 90 degree angles... They are perfectly fine, except when they aren't - in which case 45 degree bends aren't any better. \$\endgroup\$
    – JRE
    Aug 27, 2020 at 11:33
  • 1
    \$\begingroup\$ Read the link,and the links in the answer. People did just that, and found that two 45 degree bends were as bad as one 90 degree bend for those cases where it mattered. That's frequencies above 2.5GHz, and rise times related. And it doesn't matter on FR4 boards. \$\endgroup\$
    – JRE
    Aug 27, 2020 at 11:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.