Common mode chokes and grounding

[AJ]

Hi,

I am looking for opinions on grounding techniques, particularly in
automotive circuits. I have designed a circuit that uses both digital and
analogue circuits and I was planning to connect these grounds at a single
point to attempt to keep digital noise away form the analogue section. I am
also using a switch mode power supply operating at 300KHz so I was thinking
of using a common mode choke on the input to help EMC. I was wondering the
best way to connect this, I am a bit confused......

I connect positive and ground to one side of the choke so on the other side
I will have a filtered positive and a filtered ground. Does this then give
me a 3rd ground? I was thinking about connecting the unfiltered ground to
the Chassis and running a ring around the outside of my PCB on both layers.
I was then going to connect the digital ground the filtered output after the
choke and connect the analogue ground like described above Some circuits I
have seen appear to have the ground connected together before and after the
choke so I cant see what the pint is of filtering the ground in the first
place in that example..

I hope I have been clear on what it is that I am trying to do and really
appreciate any feedback

Best Regards

AJ

 

[Harold Ryan]

Hi AJ:

Try to use star grounding where all the grounds connect together at one
point. Usually the AD converter is the grounding point because they are not
isolated like in a PC. Make sure only one ground and power trace connect to
this point. This is also where the power supply connects. Also connect the
filter caps like 0.01uf to each of the electronic sections. The trick is to
separate the currents from one section from another. As for the switcher,
its impossible to remove the ringing from the analog circuits due to RF from
the wires near by. Always use a liner for analog if possible. The choke
should be used to separate the digital and switcher from the analog
circuits. The switcher and the digital circuitry are both noisy.

The ring around the board only causes trouble like an AM antenna. The
protection you want is to protect the board from RF as it passes thru the
board, not from the edges. In other words, add a ground plane and a Vcc
power plane with traces on the other 2 layers. If you are making a 2 layer
board, create a ground plane with signal traces on the other side. Its ok
to run a few traces on the ground side but you need a ground plane.

Harold