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Sci.Electronics.Basics -> Choosing voltage opamp
There are 29 messages in this thread.
You are currently looking at messages 20 to 29.
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Author: VitaliyDate: 13:00 28-09-06
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Johnny Boy wrote:
>
> You should listen to David, he knows what he's saying - even the very best
> of rail-to-rail op-amps won't go close to the supply rails, except if
> they're very lightly loaded, 1M etc. They still usually fall short of the
> negative rail by 5-50mV and 0.5-1.5V short of the positive rail.
> ... Johnny
OK, I doublechecked the requirements, I don't have to get the exact
10Vp-p. Maybe I was getting such high output voltages (for OPA657)
because of 1M load
Jamie wrote:
> 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?
> >
> > Vitaliy
>
> put an old stand by TL082 type op-amp infront of it.
But that would kill my BW (the BW on that is only 4MHz, and I need
40MHz), right?
redbelly wrote:
> 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)?
I have broadband light source w/ rapid scanning optical delay line
(RSOD) and phase modulator. Are you suggesting this would affect the
required slew rate?
Thanks for all advices,
Vitaliy
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Author: VitaliyDate: 16:35 28-09-06
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Jamie wrote:
> 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?
> >
> > Vitaliy
>
> put an old stand by TL082 type op-amp infront of it.
But that would kill my BW (the BW on that is only 4MHz, and I need
40MHz), right?
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Author: VitaliyDate: 16:36 28-09-06
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redbelly wrote:
> 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)?
I have broadband light source w/ rapid scanning optical delay line
(RSOD) and phase modulator. Are you suggesting this would affect the
required slew rate?
Johnny Boy wrote:
>
> You should listen to David, he knows what he's saying - even the very best
> of rail-to-rail op-amps won't go close to the supply rails, except if
> they're very lightly loaded, 1M etc. They still usually fall short of the
> negative rail by 5-50mV and 0.5-1.5V short of the positive rail.
> ... Johnny
OK, I doublechecked the requirements, I don't have to get the exact
10Vp-p. Maybe I was getting such high output voltages (for OPA657)
because of 1M load
Thanks for all advices,
Vitaliy
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Author: redbellyDate: 09:59 29-09-06
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> Jamie wrote:
> > put an old stand by TL082 type op-amp infront of it.
Vitaliy wrote:
> But that would kill my BW (the BW on that is only 4MHz, and I need
> 40MHz), right?
> redbelly wrote:
> > 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)?
Vitaliy wrote:
> I have broadband light source w/ rapid scanning optical delay line
> (RSOD) and phase modulator. Are you suggesting this would affect the
> required slew rate?
I was thinking in terms of bandwidth, but yes, the slew rate is
relevant also.
You'll almost certainly need a 50 ohm load at 40 MHz speed, so forget
my earlier suggestion of using a higher load to reduce the current
requirement.
Mark
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Author: redbellyDate: 11:19 29-09-06
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Author: Kevin WhiteDate: 11:49 29-09-06
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Vitaliy wrote:
...
>
> Johnny Boy wrote:
> >
> > You should listen to David, he knows what he's saying - even the very best
> > of rail-to-rail op-amps won't go close to the supply rails, except if
> > they're very lightly loaded, 1M etc. They still usually fall short of the
> > negative rail by 5-50mV and 0.5-1.5V short of the positive rail.
> > ... Johnny
>
> OK, I doublechecked the requirements, I don't have to get the exact
> 10Vp-p. Maybe I was getting such high output voltages (for OPA657)
> because of 1M load
...
You keep mentioning 10V p-p, but if you have a conventional
transimpedance amplifier you will get a unipolar signal so you will
only get a maximum of 5vp-p (0v to +5V) unless you offset the output or
AC couple the output amplifier.
kevin
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Author: VitaliyDate: 13:40 30-09-06
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Thanks, Mark. I've ordered the samples for now to see how it works in
reality.
I will get a few samples of opamps with similar specs from TI as well.
I'll put their part numbers tomorrow.
Vitaliy
redbelly wrote:
> I found this, it might be just what you need:
>
> http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1096
>
> 300 MHz and 100 mA output current.
>
> Available at Digi-key for 6.36 $US each, or get 100 for $2.82 each.
> Search on MAX477 at http://www.digikey.com
>
> Mark
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Author: redbellyDate: 21:02 30-09-06
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Vitaliy wrote:
> Thanks, Mark. I've ordered the samples for now to see how it works in
> reality.
You're welcome. Good luck. -- Mark
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Author: VitaliyDate: 13:53 01-10-06
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