Analog GND <-> Digital GND = how to place both GNDs o

[nicleo]

gnd digital

When I come to ADC (high resolution) design, I've always been told to isolate the Analog GND and Digital GND. Few questions as follows:
1) Are both GNDs PHYSICALLY isolated on PCB?
2) How to place both GNDs on PCB?

Many Thanks!

 

[Circuit_seller]

ground noise zero ohm resistor

Hi

I recommend you to use one GND not seperate GNDs.
Please refer to henery ott's paper about this topic.
"Partitioning and Layout of a Mixed-Signal PCB", Henry W.Ott, Printed circuit Design, June 2001

Regards

 

[nicleo]

main gnd digital gnd

Circuit_seller, the article is pretty good, thank you. By the way, are you a PCB layour designer?

 

[Circuit_seller]

analog gnd digital gnd pdf

Hi

I am not a specialist in PCB Design.

Regards

 

[CADPCB]

Re: Analog GND <-> Digital GND = how to place both GN

Use them as splite planes , separate but connected with 0 ohm res or
just shorted in so called "THE STAR" place.


.............................. ............................
. . .
. . .
............ .
.
.
.....................................................................

Regards

 

[bimbla]

Re: Analog GND <-> Digital GND = how to place both GN

I will follow CADPCB's method.
From the powersupply point, two zero ohm resistors (jumpers) will provide ground to analog and digital sections. This way they remain terminated at a single point and yet do not share common path.

bimbla.

 

[ted]

Re: Analog GND <-> Digital GND = how to place both GN

The grounding requirements for analog and digital are quite different, and therefore one often has to use for instance that "single point" or "star" or "zero-ohm-resistor" concepts to keep track what is grounded and where.

However, the essence in my experience cooks down to some very basic rules:
1) Low level analog signals are easily disturbed and collect unwanted noise
2) Logic signals are noisy in their nature, and generate also noise over the impedance of ground coductor.

Therefore, you have to keep the digital side ground return currents from generating (ground) noise to those low level analog signals. But also:

3) Digital ground returns are part of the (usually quite high speed) digital signal channel. Too much impedance there may cause digital side ground bounce, even in degree that disturbs the digital side.
Therefore, the digital ground should always follow a low-impedance path towards the other end of the digital signal path.

By separating the analog ground one can solve the issues 1 and 2, but care has to be taken to not to weaken the digital side return path. Therefore, if possible, the joining point of these two grounds should generally be under or close to the mixed mode chip (for instace an A/D converter or such), while the digital side is directly grounded to the "main" ground plane or similar.

Personally I prefer the ground connection in the PCB itself, and not by a zero-ohm resistor component. In most PCB layout systems you may solve the issue in several ways. For instance by having only one "GND" net for both, and manually tracking the different "sides" of it, or having a dummy component, which has a built-in short circuit (such as "2D line" in PADS), and placing it between "Analog GND" and "GND".

Good Luck,
Ted

 

[elcielo]

Re: Analog GND <-> Digital GND = how to place both GN

"Partitioning and Layout of a Mixed-Signal PCB", Henry W.Ott, Printed circuit Design, June 2001

http://www.hottconsultants.com/pdf_files/june2001pcd_mixedsignal.pdf

 

[Rayengine]

Re: Analog GND <-> Digital GND = how to place both GN

Both GNDs should be in shape of star and only connect in the source (power supply ground) to avoid interaction between GNDs.