Hi There,
Just wondering why in alot of pcb design guides they suggest separating
the digital and analog circuity
Chip makers who make ADCs etc usually specify that the digital ground and
analog ground connect only under the chip. This is often impractical to
do.
(i.e. grounding and supplies) and at
the same time have a commmon reference point somewhere in the circuit
(i.e. join analog ground to digital ground), I realise that the fast
digital switching could induce noise on some sensitive analog
components and separation is nesscesary but why then join both grounds
at one point?
Most things with digital and analog circuits also contain ADCs and DACs.
Neither of these work well if there is a difference between their digital
and analog grounds.
also as a separate but poss related question what is
meant by the term grond loops in pcbs and why avoid them?
These days, it is common to have digital stuff, some analog stuff and some
sort of power supply all on one PCB.
The digital area almost always determines the number of layers in the PCB
and that number is greater than the analog of power supply sections need.
You can take advantage of this.
In the digital section the Ground and Vcc layers work as AC ground planes.
They naturally form a capacitor. The bypass capacitors add to the
coupling between them. If the analog section is going to be run from the
same Vcc, it is a good idea to put some impedance in the way to keep the
digital sections currents out of the analog sections supply. If you can
prevent the current in the Vcc, you've mostly prevented it in the ground
plane.
In the analog section, you should use the extra layers to make things that
are AC ground for the analog. If the analog doesn't run on the digital's
Vcc, take that layer over as an analog supply.
If the analog section is processing lowish frequencies you want to add an
extra ground layer in that area to be the analog ground plane. You need
to make sure that no large currents can pass through this added layer.
This means that you often have to be careful about where and how many
places you hook the thing to the overall ground. Beware that the analog
circuits also make currents. It is also common practice to add even more
grounded copper on the top and bottom surfaces of the PCB. It doesn't
cost anything really to add it so go ahead.
In the power supply section, you again want to add more ground plane.
The best I've been able to come up with as a general description of what
to do is:
Imagine the power supply as being enclosed in a copper box with one hole
in it. In your thinking, all of the lines that come and go from the box
should pass in and out of this hole.
Right at the hole, they should have a capacitor to ground. Ideally each
line should also have some series impedance in it. The inductors normally
used in the supply design can be this impedance.
Now if you take this box and merge it with the top surface of the PCB, the
bottom of the box will become one of the added ground planes.
About chassis grounding:
You often will find you have competing requirements. For high
frequencies, you want the electronics solidly connected to the chassis.
For low frequencies, you may want only one point (near the signal entry
BNC for example). For safety, you may want no connection. For ESD you may
want some connection. I've used parallel RCs to make this all happen at
once. It adds a lot of extra capacitors though.
If this equipment is going to be rack mounted with other equipment, never
connect the PCB's plane to the chassis at both the front and the back
panel. If you have to, float a box within the chassis do so. Racks
almost always have 1 billion amps of current circulating in them. I have
measured voltages from front to back over 1.5Vp-p. Debugging a problem
that only happens when the screws are tight can take days (trust me).
