Why is there a 0R resistor linking GND and AGND in analog voltage reference circuit?

Question

This is related to another question I've just posted (What's the purpose of a ferrite bead inductor on this circuit?), regarding the battery charger described in the AVR450 Application Note - Battery Charger for SLA, NiCd, NiMH and Li-Ion Batteries, which one day I hope to build.

On page 40, there's a schematic showing the MCU connections (picture below). Marked in red is a 0Ω resistor that is puzzling me. I suspect that it is just a wire jumper linking AGND and GND. But I don't understand why there's a jumper there.

My questions:

1. What does the jumper represent?
2. Why are AGND and GND separated like that?

Answer 1

Digital circuits are noisy, but they can (mostly) handle their own noise without noticing. Analog circuits notice noise a lot; in fact, they have to pass noise just like a signal because they really can't tell the difference.

The best way to keep digital noise out of analog circuits is to keep them separate, both physically and electrically. But they have to be connected somehow in order to convert from one to the other, hence the jumper in exactly one spot, which is probably next to the converter on the physical board.

Answer 2

I'll repeat what I wrote in your previous question and expand on it a bit:

R33 may have been a thought to allow for a ferrite bead in the analog ground (usually not a good idea) or it may be used as a net tie to enforce a single point connection between the analog ground nets and the ground.

In communication between the engineers and layout persons it's sometimes easier to give two nets that are connected together separate names and join them at a single point, either with a physical 0R (0\$\Omega\$) resistor or with something called a net tie, which appears as copper on the board but looks like a component joining two nets in the schematic.

Obviously the net tie costs less than a 0R resistor (shorting jumper).

The other idea is to try to put some impedance between analog and digital ground- and that's almost always a bad idea. If the difference between the two grounds exceeds more than a few hundred mV at any time, bad things will happen. It's usually best to tie them together closely and to a solid ground plane if possible.

Answer 3

1. The jumper represents a zero ohm resistor. It is used on a PCB like a jumper. If it is installed then AGND and GND are shorted together. If it is not installed then they are isolated.

2. The two grounds are isolated from each other like that to deal with potential noise issues. If you have problems with the MCU injecting noise into the ground (which can upset sensitive analog circuits) then you can remove the 0 Ohm resistor and isolate them. This can have other problems (such as ground loops) but sometimes it is necessary.

It is simpler from a PCB design standpoint to just short the grounds together (by installing the 0 Ohm resistor). It is more complicated (and error prone) to design your board with two ground planes.

The jumper is there so that both configurations can be done (separate or unified ground planes).