question of connection of analog and digital ground?

[kimjin]

in my design ,there are two ad converter, they both used single 5v supply,how to connect the analog ground plane to digital ground plane?i'm puzzled for days.should i use a plane for analog and digital ground?
the design is a at96 6u eurocard,the bus can supply double 12v and single 5v supply, should i divide the power analog supply from digital supply at where? and how should i decouple the analog power and ground to digital power and plane?
i have been learnd that 3 cap should be placed in the analog power entrance,and 2 caps with a beam between them is need,how should i organize the power filter circuit ?
should i connect the annlog ground to digital at the power entrance or under the ad coverter?
should the analog power and ground be divided from the digital's with 2 inductance used?
i'm a newbie ,who can explaine these questions?thank you very much.

 

[House_Cat]

There are several good articles available on the web, take a look at these:

**broken link removed**
http://www.sigcon.com/Pubs/edn/adcgrounding.htm
http://www.sigcon.com/Pubs/edn/multipleadc.htm

 

[jdhar]

IF it is a simple PCB with very distinct analog and digital sections, you can split the ground plane under the important component, and connect them with a small 'bridge' under the component. However, if it is a mixed-signal application, separating analog and digital grounds is bad IMO. Since it's mixed signal, the whole board is a mix of analog and digital, and splitting the ground planes will only cause inductance loops and ground bounce.

 

[Iouri]

Digital and Analog ground should be conected under the ADC, just put SMD 0OHM resistor, also keep in mind you need to have gup around 75mil between grounds, also if it is multilayer board, make sure you have opening on other layers as well....


Regards,

 

[vdaniel]

I complete agree with "jdhar". Only once I have designed a board, where the digital and analog parts were neatly separated and it was possible to follow to the separate ground recommendation. In all other cases this recommendation seems to be a mockery, because analog and digital parts are usually mixed. So the best solution to reduce noises and interferences, even for a two layer board, is to make one (usually bottom) side a ground plane with as few other connection on it as possible. This can take a lot of time for a good design, but the result can be very good. You can have 27V PWM pulses and 12-bit ADC working on the same board.

 

[jdhar]

I complete agree with "jdhar". Only once I have designed a board, where the digital and analog parts were neatly separated and it was possible to follow to the separate ground recommendation. In all other cases this recommendation seems to be a mockery, because analog and digital parts are usually mixed. So the best solution to reduce noises and interferences, even for a two layer board, is to make one (usually bottom) side a ground plane with as few other connection on it as possible. This can take a lot of time for a good design, but the result can be very good. You can have 27V PWM pulses and 12-bit ADC working on the same board.

I agree too - I have FPGAs and 100 Mbit ethernet running on a 2-layer board just by making the back side a ground plane and decoupling. All high-speed interfaces now have an analog front-end, with the clocks included. If you split that plane anywhere, the return path for those clocks will be horrible and will cause a lot of problems. Now, ifyou have slow-moving analog signals, you can keep them away from digital signals and that will give you some benefit; but keep the plane solid.