Re: Photodiode TIA

On Thu, 16 Nov 2006 05:18:02 GMT, Robert Baer
<robertbaer@xxxxxxxxxxxxx> wrote:

John Larkin wrote:
On Thu, 16 Nov 2006 00:03:43 GMT, Robert Baer
<robertbaer@xxxxxxxxxxxxx> wrote:


John Larkin wrote:


On 15 Nov 2006 08:57:13 -0800, wellies@xxxxxxxxx wrote:



I realise there is quite a bit of advice in this group on TIA's but i
thought i'd save myself some time and get an off the shelf TIA,
designed to work with PIN photodiodes in high speed applications.
However i'm now confused. I'm only working with 160Mbps, but everything
either stops at 155 or goes up to 622 due to various fibre optic
standards. I'm looking at using the max3665 which runs up to 622Mbps,
reading the data***, the device has a bandwidth of 470MHz. Reading on
it then states:

"In general, the bandwidth of a fiber optic preamplifier
should be 0.6 to 1 times the data rate. Therefore, in a
622Mbps system, the bandwidth should be between
375MHz and 622MHz. Lower bandwidth causes pattern-
dependent jitter and a lower signal-to-noise ratio,
while higher bandwidth increases thermal noise."

Does this mean that the thermal noise would be large in my system if i
used it for 160Mbps? Or is this not really an issue? I was going to use
the Optek OPV480 and reverse bias to around 20V and feed it straight
into this chip, or it is easier just to design my own transimpedance
amp around a single opamp and feedback resistor and capacitor?

Thanks

Andrew


A lot depends on how much light you have. If it's microwatts, the
noise budget will be tight. If's it's in the milliwatt range, you'll
probably overload the Maxim and might be better off making your own
TIA. I've had good results using a current-mode opamp as a TIA when
there's lots of light.

What's the capacitance of the OPV480? That's a serious issue at high
speeds.

John


You forgot that Maxim only advertises stuff that is not available.


But they always have samples. A few months ago, in deep trouble, I had
all my engineers and some outside friends and family request Maxim dac
samples. Turns out you can get 18 samples of this dac per request if
you play with the part number options. We got enough to get out of
trouble.

John

Sure no way to run an airline.....

There have been threads on this. The engineering concensus seems to be
"don't design with Maxim parts."

If I browse my parts/purchasing database, most of the Maxim parts have
a note like "LONG LEAD!" or "10 WEEK LEADTIME" and occasionally
"OBSOLETE NO SUB"

Maybe they cater to a small number of huge-quantity OEM customers or
something.

John



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