Maker Pro
Maker Pro

Multilayer PCB - Shielding - Which supply on each layer?

B

Bill

Jan 1, 1970
0
Hi,

I need to design a 4-layer PCB for an analog circuitry that measures
EEG (10 uV to 1000 uV peak to peak, 0.5 Hz to 50 Hz).

My supplies are:
+5 V (for analog)
+3.3 V (for digital)
0 V (for analog and digital)
-5 V (for analog)

I know about (and have experience with) star topologies, to minimize
contamination from one section to others. That doesn't worry me.

I'm concerned about SHIELDING. I don't have much experience working
with uV signals, so I need to pay attention to what supplies/signals I
place on each layer.

I'm also not concerned about the +3.3 V supply, since it doesn't need
to power any IC close to the analog input.

I'm concerned mainly about +5 V, 0 V and -5 V. Where would you place
them, in the 4-layer PCB?

I was thinking of something like this:
Layer1 : 0 V plane + soldering pads for SMD parts.
Layer2 : +5 V and -5 V planes, sharing the total surface.
Layer3 : Signal tracks.
Layer4 : 0 V plane.

What do you think?

Thank you very much.
Bill
 
S

Son of a Sea Cook

Jan 1, 1970
0
Hi,

I need to design a 4-layer PCB for an analog circuitry that measures
EEG (10 uV to 1000 uV peak to peak, 0.5 Hz to 50 Hz).

My supplies are:
+5 V (for analog)
+3.3 V (for digital)
0 V (for analog and digital)
-5 V (for analog)

I know about (and have experience with) star topologies, to minimize
contamination from one section to others. That doesn't worry me.

I'm concerned about SHIELDING. I don't have much experience working
with uV signals, so I need to pay attention to what supplies/signals I
place on each layer.

I'm also not concerned about the +3.3 V supply, since it doesn't need
to power any IC close to the analog input.

I'm concerned mainly about +5 V, 0 V and -5 V. Where would you place
them, in the 4-layer PCB?

I was thinking of something like this:
Layer1 : 0 V plane + soldering pads for SMD parts.
Layer2 : +5 V and -5 V planes, sharing the total surface.
Layer3 : Signal tracks.
Layer4 : 0 V plane.

What do you think?

Thank you very much.
Bill


Make a bluetooth device for each sensor that sends the digitized value
to a receiver. Already having been quantified at the transducers removes
all error. Bonus: No need for shields, and small circuit size,at least
for the sensor senders. Also, you would incorporate a handshake in each
that would keep another nearby unit from passing false data to your "data
logger".

Have an inductive charging circuit for the battery device that powers
each "sensor xmitter". They would all charge overnight and after each
use while in the storage drawer of the receiver cart.

Sell tens of thousands of units.

Have a nice day...
 
B

Bill

Jan 1, 1970
0
Hi again,

Please read the last paragraph of the first article ("Input filter
prevents...") in this pdf:
http://www.bmed.mcgill.ca/reklab/ma...r_prevents_in-amp_RF-rectification_errors.pdf

It says: "You should build the RFI filter using a pc board with a
ground plane on both sides."

As I said, I don't have much experience with shielding uV, but that
made some sense to me. Intuitively, I think I would prefer having more
capacitive pickup, but it happenning to ground planes, where it
doesn't do any harm, than having less pickup, but it happenning to
signal tracks. Doesn't it make sense?

I now debugging will be more difficult (because the signal tracks will
be buried), but reducing interference is a higher priority for me.

I also understand Bill's comment (about a track right next to a
conductive plane radiating less at far distances --by image theory),
but I'm not sure that the benefit from that[*] will be higher than the
benefit from two ground planes at layers 1 and 4.

[*] Actually, from the reciprocal of that, since I'm not concerned
about radiating, but about picking up noise. By reciprocity, an
antenna that radiates less at infinity also picks up less from
infinity.

So... I don't know what to do. Are you guys convinced of what you say?
Can you convince me, too? :-(


Thank you,
Bill
 
S

Son of a Sea Cook

Jan 1, 1970
0
Really makes *NO* sense to have all of that radiation (bluetooth
transmit & receive plus inductive radiation to the PCB) any where near
those microvolt inputs and circuitry.....


Sorry, but I'll bet that examination rooms are already RF rich, and I'll
also bet that there is no influence on low impedance transducers at all,
particularly when the value gets converted right at the source. From
then on, the data that gets sent is digital, and 100% error free. We can
sense picoamp fluctuations in a PMT to see a missile launch 80 miles
behind an aircraft without error introduction from local RF, I think we
can handle this.

So, the transducer and ADC would be right there local to the sensor (ie
no interference whatsoever), and the signal sent over the air is the
quantisized values from sampling to sampling and are digital, so they
cannot carry error either. Absoluetely a better solution. No more need
for wired patch sensors. Far better, and it IS or WILL BE the future.
Why not have it be him that goes wireless with body sensors?

Transparent Aluminum, anyone?
 
S

Son of a Sea Cook

Jan 1, 1970
0
Wouldn't having shielding planes surrounding sensitive signals be better?

Why would one ever want sensitive signals taking long paths? Nowadays
converting a signal to digital is cheap, even when 8 or 16 channels are
involved.

Hell, a simple (or complex) data logger would be an excellent PCB layout
to examine.
 
B

Bill

Jan 1, 1970
0
Why would one ever want sensitive signals taking long paths? Nowadays
converting a signal to digital is cheap, even when 8 or 16 channels are
involved.

No one wants sensitive signals taking long paths, but sometimes you
need to amplify before A-to-D conversion is possible. And sometimes,
you need to amplify so much, that you need to break that gain into
several stages, not to have output-to-input feedback. That's how you
end up needing shielding.
Hell, a simple (or complex) data logger would be an excellent PCB layout
to examine.

It depends on the voltage levels and frequency bands it deals with. My
application is not RF, but it deals with uV, so I'd rather learn from
RF PCBs than from PCBs of data loggers.
 
B

Bill

Jan 1, 1970
0
So, the transducer and ADC would be right there local to the sensor (ie
no interference whatsoever), and the signal sent over the air is the
quantisized values from sampling to sampling and are digital, so they
cannot carry error either. Absoluetely a better solution. No more need
for wired patch sensors. Far better, and it IS or WILL BE the future.

I also think that is the future, and there are several companies that
start offering wireless electrodes. I don't know how they do the
strong filtering that is needed at the input to avoid saturation of
the first stage, due to (self) HF interference.
 
B

Bill

Jan 1, 1970
0
If you really need 10 uV and 0.5 Hz, it has to be thermally shielded,
not just ground-plane-against-capacitive-coupling shielded.
Keep all wiring to the first amplifier/preamplifier short,
and maybe random breezes causing temperature fluctuations
won't dominate your signals.

Copper wiring, aluminum metallization on ICs, gold bond wires,
Kovar leads on IC packages... you have lots of dissimilar
metals, and there will be thermocouple voltages.

Hey, thanks for bringing this up. I never thought of it.
I'll hope thermal changes won't be as fast as 0.5 Hz.
 
P

Paul Keinanen

Jan 1, 1970
0
Hey, thanks for bringing this up. I never thought of it.
I'll hope thermal changes won't be as fast as 0.5 Hz.

When testing your circuit and closely inspecting something on the PCB,
don't be surprised, if the voltages fluctuate with your breathing
every few seconds :). Any transparent semiconductor package can cause
problems due to ambient light variations.

Paul
 
A

Artemus

Jan 1, 1970
0
Any transparent semiconductor package can cause
problems due to ambient light variations.

Even hermetically sealed packages can be a problem. Way back
when I was a test engr for a semi manufacturer I discovered that the
TO-99 packages we were using leaked light up the frit seal around
the legs. This wreaked havoc with Vos and Ib on OA's and IA's.
Art
 
Top