analog and digital ground

Thread Starter

abc77

Joined May 10, 2017
59
hi,

i have a schematic in which there is digital ground and analog ground symbol as shown below. the analog ground is for mic and speaker. the value of R5 resistor is not given or is zero resistor!!

can i simply connect the analog and digital ground together, that is same potential?

1.png
 

Papabravo

Joined Feb 24, 2006
12,301
hi,

i have a schematic in which there is digital ground and analog ground symbol as shown below. the analog ground is for mic and speaker. the value of R5 resistor is not given or is zero resistor!!

can i simply connect the analog and digital ground together, that is same potential?
That depends. It would help to see the rest of the schematic. The zero ohm resistor is placed there so that there is an option to have them connected or separate. Normally they would be connected together at the power supply off board, or right at the power connector. The reason for doing this is to make the digital noise travel all the way back to the power supply before having a chance to effect the analog ground.
 

MrChips

Joined Oct 2, 2009
19,154
Generally, in any sensitive mixed signal application, you want to keep the analog and digital grounds separate. They are eventually joined back at the power supply. If your application is not fussy, then it doesn't matter.
 

WBahn

Joined Mar 31, 2012
24,572
Aside from the noise considerations of the physical layout, the reason why there is a zero ohm resistor between your GND node and your AGND node is that these are both globally named nodes and since a node can only have one name (in any simulator I've ever worked with) you can't directly connect them together. So to have them simulate as the same node, you need to connect them with a component that will never have any voltage across it regardless of the current.

The two common ways of doing that are with a zero ohm resistor or with a DC voltage source set to 0 V. In either case, you can measure the current through the component to determine if your schematic mixes your analog and digital circuits improperly. If the current is nonzero, then you've got something that is drawing from a source referenced to one ground but returning the current to another. Eliminating any of those is the first step in managing your supplies and grounds. Beyond that it is more physical than schematic, though you can model your various sections of ground (and power) rails and planes as resistors (and inductors and capacitors, if you really need to) to get a feel for where your currents are actually going to go -- but this is a lot easier said than done.

Another way to connect nodes is to have them not be global and instead pin them out and connect them together with a wire at a higher level. This approach has a number of pros and cons and usually the cons win out.
 
Top