Re: Choosing voltage opamp

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#2061

From: redbelly
Date: Sun, 24 Sep 2006 04:30

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Vitaliy wrote:
> I've been trying to do that.
>
> In a lot of examples on the datasheets of amplifiers I'm looking for, I
> see they are using 50Ohm input. However, I know for sure (test results)
> that output of the first stage (photodiode amp) does not work with
> 50Ohm loads, but works with 1MOhm loads. Should I be concerned about
> that?

It depends on how fast a response time you need.  Photodiodes often use
50 ohm loads for impendance matching, so that a pulse or step signal
does not reflect back and forth in the cabling and cause "echo"
signals.

If you're okay with, say 1 usec or slower response time, I wouldn't
worry about using a 50 ohm load.  You might even be okay with the
higher impendance at somewhat faster response times, you might need to
experiment a little.  If you could get by with a 1k or 10k load then
you'd have an easier time finding an opamp.

You didn't say much yet about the nature of your signal, other than
it's optical.  Are you looking at light pulses (and at what pulse
duration), modulated light (and at what frequency), or a more-or-less
cw signal that changes slowly in intensity (and on what sort of time
scale does the intensity change)?

Mark

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From: "redbelly" <redbelly98@yahoo.com>
Newsgroups: sci.electronics.design,sci.electronics.basics,aus.electronics
Subject: Re: Choosing voltage opamp
Date: 23 Sep 2006 21:30:46 -0700
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