🚀 go-pugleaf

PCB stack is an invention to minimise the EMI radiation and improve the
speed of signals talking in terms of microwave or high frequency
propagation.
Jumping from 2 to 4  layers will not gave you many improvements in EMI
as long the major signals are still routed on top and bottom. The first
stack with some performance is a 4 layer stack (counted from top to
bottom)as PWR-signal-signal-GND and 6 layer stack
(top-signal-PWR-GND-signal-GND-bottom or other similar versions) where
top and bottom are mixed signal/ground planes
Will be no noise improvement if +5 and -5V routes becomes planes, could
be worse.
In analogic and RF design the supply lines are usually routed radially
from the source (supply) to destination. This not means a contiguous
supply plane could be worst as long is in the neigborhood af quiet
ground planes.

best regards,
Vasile Surducan
senior application engineer
National Institute R&D for Isotopic and Molecular Technologies
Cluj-Napoca
Romania



Vitaliy wrote:
> Hello,
>
> My appplication is light detection -> using photodiodes and
> transimpedeance amplifier (OPA657 from Texas Instruments). At this
> point I have 2-layer working design. I need to increase the bandwidth
> of the detector (from 10MHz to 40MHz) and try to reduce noise (also
> reducing noise is always welcome, of course). I'm thinking about using
> two inner layers for +5V and -5V power layers (I'm using free space on
> top and bottom layers for ground layer). +-5V is currently connected to
> the board from external power supply by soldering two wires (one for
> +5V and one for -5V). I am using +-5V to bias photodiodes and to power
> the opamp. The software I'm using is OrCAD. Even if going to 4 layer
> from two wouldn't give me a significant improvement in terms of
> reducing the noise level, I would like to know the answers for the
> following questions.
>
> 1) Will traferring +-5V to separate layers (rather than just have the
> power traces with filters) work? Since the inner layer is most likely
> to be .5oz copper as opposed to 1oz copper of outer layers. There is no
> way to specify that in gerber file, right? Will creating a power layer
> give me more stable voltage (and high capacitance)? That's why people
> are using multilayers as opposed to two layers, right? I guess I will
> still need some decoupling capacitors at the vias.
>
>
> 2) The price differential between 2 and 4 layer boards is quite steep,
> and I can easily fit a few prototype boards on one board and then cut
> them (or have it cut by manufacturer). What is the easiest way to place
> array of differemt designs on one board? I only have access to OrCAD,
> but it can generate gerber files, so maybe some other software can be
> used. Are the manufacturers ok with this? I would prefer them to cut
> the boards, if possible, since I probably wouldn't be able to do it as
> well without special equipment. (Something tells me they woudn't be
> willing to make x by x board, and then cut it in y pieces, but just
> checking). I do realize I might be able to place three designs side by
> side and not require any cutting, but that might not always work for
> me.
> 
> Merry Christmas,
> Vitaliy