Common Mode choke 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

 

[John Popelish]

AJ wrote:

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?

It gives you a ground that has instantaneous voltage variations from
the ground on the other side of the choke.

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 agree. A simple inductor in the positive side of the supply would
make more sense.

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

I think it is a good idea to draw a schematic of your circuit and all
its external connections, with each wire drawn as an inductor. Then
try to visualize (or actually simulate) what external sources of noise
and internal sources of current variation (like those produced by that
switcher) will do to the voltage across those inductors. If the DC
currents are significant, or the circuit has large DC sensitivities,
include resistance in each connection, so you can think about where
the DC drops will be, also.

Just throwing filter components or ground planes at a circuit without
having a specific purpose for each (based on circuit analysis) is
unlikely to provide anything like an optimal use of the available
choices. Every resistive drop and inductive voltage needs to be
accounted and compensated for or the compromise consciously chosen.

 

[John Woodgate]

I read in sci.electronics.design that AJ <itwasme3133@hotmail.com> wrote
(in <yRTMe.3379$FA3.2476@news-server.bigpond.net.au&gt about 'Common Mode
choke and grounding', on Thu, 18 Aug 2005:

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?

Yes: to be kept separate from unfiltered ground, otherwise the CM choke
is effectively short-circuited (in BOTH legs).

I was thinking about connecting the unfiltered ground to
the Chassis

Correct, and connect it to nothing else.

and running a ring around the outside of my PCB on both layers.

No, keep it OFF the board entirely. It would act as an antenna, spraying
the smog that you CM choke filtered out back into your signal circuits.

I was then going to connect the digital ground the filtered output after the
choke and connect the analogue ground like described above

Not described clearly enough for me. Connect it to the filtered ground,
the same as for the digital ground.

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..

See above. It can't be right to do that, AFAICS.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Ken Smith]

In article <yRTMe.3379$FA3.2476@news-server.bigpond.net.au>,
AJ <itwasme3133@hotmail.com> wrote:

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.

So far so good but what are the I/O connections on this device. The wires
entering and leaving the device are where the trouble comes in. You
really need to consider this before you go any further.

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.

Imagine your switcher being put inside a box. All the connections of the
supply go through one single hole in the side of the box. Each wire has a
capacitor just inside the box that RF bypasses it to the box. Each wire
then passes through a core and has a second bypass capacitor outside the
box. Even the ground wires of the supply are handled this way. The box
has a ground wire attached to it near the opening and no other connections
to it.

If you do this, there is no way for RF created inside the box to get
outside the box. This concept is what you are trying to duplicate with
the common mode chokes etc.

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.

If anything, you want to run the filtered ground to the rings around the
PCB. Breaking the ring and putting a resistor in the break can be used to
make the ring lossy if you want to be extreme about things.


--
--
kensmith@rahul.net forging knowledge