From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2002-01-29 20:15:35
Dear Andreas:
Follow this word picture. This does not specifically refer to your circuit, but you will have your answer.
Pick a spot on the chassis ( frame, box ) and install a long screw, the long part inside the box. Put a lock washer and a nut to hold the screw, nextput a lug with the green wire from the three prong power plug, add a nut, then put a lug coming from the digital power ground there, put on a nut, next put a lug from the analog ground, put on a nut. Keep this going for however many different systems there are. This works well for any size system. It is sometimes called a star ground. The point is that there should not be any ground loops with this method. A shielded box should have a ground wire "and" a shield wire and the box must be "insulated" from chassis. I have worked with systems that had 20 or more lugs on this screw. I have done quite a bit of EMI, RFI, EMP work and people always try to come of with ways of not doing it right.
I did a design for a fancy piece of test equipment. The prototype worked well and all the production units failed the power supply ripple spec. It took week of fussing before they called to blame me, but the prototype worked. They even removed the capacitors from the prototype and that didn't help any. Eventually I found out that some smart production worker gotten an award for saving 3 feet of heavy wire that just ran from the front to the back and right back again. That extra wire was part of a star ground systemand very important!
So you have to decide where your "star" ground will be located. It could be on the PC board. I have seen some PC boards with a very large round padwith small traces terminating to the circumference.
Making the ground plane heavier only helps a little sometimes. You would think that a heavy gauge chassis will be the answer, but even then you can have serious ground loop problems.
> Now, for the question: How should I route the ground
> trace in the center?
> Should it stay the way it is, going directly to the
> ground pins (13-14) on U200
I don't know what all the parts are, but on a quick inspection the way you have it looks correct. You should look at it from the star ground concept and see.
+++++++++++++++++++++++++++++++++++
> I've managed to cut the costs a bit.
This is not the time for that!
Let me state my opinion about price again. FIRST make something that works, THEN re-evaluate the design and possibly give people some cost options.
I think that this $100 price is a totally unrealistic goal. I would be happy to pay $1000 for a modular expandable concept where the software source is available to be modified as needed.
Please get some hardware working. It looks real close. Let me know if I can help.
Then we will see if we have any programmers in this group.
Juergen P. (Jim) Meissner
Check out my Website at www.MeissnerResearch.com
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----- Original Message -----
From: Andreas Robinson
To: buildcheapeeg_at_yahoogroups.com
Sent: Tuesday, January 29, 2002 1:54 PM
Subject: [buildcheapeeg] TinyEEG - ADC grounding question and new cost estimate
Hello,
A question for Jim and Joerg, or anyone else who has
the knowledge:
I have completed all the layouts, but there are some
rough spots left. I've attached an image that shows
part of the layout on the ADC-board.
To the left, labeled U200, is the ADC, and to the
right, labeled U201 is the microcontroller.
At the bottom of the picture you see three grey pads
labeled VIN-, GND and VIN+ where you connect the
power, +/-5 volts.
Now, for the question: How should I route the ground
trace in the center?
Should it stay the way it is, going directly to the
ground pins (13-14) on U200
or
should I connect it directly to the analog ground
plane to the left
or
the digital ground plane to the right?
or
fill the whole surface below pins 13-14 with ground
plane, letting all currents flow whichever way they
want?
Any suggestions are greatly appreciated!
If you feel you need more details, please let me know.
Status report on my project:
I've managed to cut the costs a bit. Changing the
filter to 4th order (needs higher sampling frequency,
but that is a good tradeoff) and reducing the number
of different part types has allowed me to reduce the
cost to around $145 for a 2-channel device, and $195
for a 4-channel device.
The input stages would cost around $28 each, if you
could buy resistors and capacitors from Digikey in
singles. That's not possible now, so the actual cost
will be higher.
The ADC-part, including the fiber-optic receiver,
costs about $60-$80 (depending on how you count those
resistors and capacitors) so a cheaper 10-bit variety,
using a TinyAVR and regular opto-couplers could
possibly save $40 on the 2-channel version.
Regards,
Andreas
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