🚀 go-pugleaf

On 24 Dec 2006 22:55:41 -0800, "Vitaliy" <m.vitaliy@gmail.com> 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

More layers won't necessarily improve speed or reduce noise. If you
increase trace capacitance (from thinner dielectrics) it might make
things worse.

10 or even 40 MHz isn't very fast for a photodetector; I've got 180
MHz from cheap opamps. What detector device are you using? Increasing
the photodiode back-bias should help speed a bunch, maybe 2:1 if you
go from 5 volts to something like 20.

What's the application? How much noise do you see, in equivalent
optical power?

John