Choosing voltage opamp

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Hello,

I'm looking to add the 2nd stage to photodiode transimpedance
amplifier. All I want to achieve by 2nd stage is voltage gain and
capability to drive 50 Ohm loads. The first stage is OPA657.

1) a) Given power supply for opamp +-5V, should I select the opamp that
has Io,max=>200mA. I do expect Vpeak-to-peak,output to be 10V.
   b) Is that output current achievable only at maximum supply voltage,
or as long as

2) I need to get an opamp with maximum input impedance (well, anything
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amplifier will do the job?

3) What is Iq per channel(Max)(mA)?

4) Would SOIC packaging be better in general? Given that pcb layout is
optimized.

Thanks in advance,
Vitaliy


Re: Choosing voltage opamp


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amplifier will do the job?
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All of the usual suspects (National, TI [including Burr-Brown], Linear
tech, AD and others) make devices specifically designed to drive 50 ohm
loads.

Check out the websites and use the parametric search function and
specify 50 ohm load and then choose a suitable device. We don't know
what type of load that is beyond 50 ohm, so the above is the only
advice I can give.

Cheers

PeteS


Re: Choosing voltage opamp


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
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Re: Choosing voltage opamp


I'm not sure if I'm answering my own question, but I can put 1MOhm at
the input of opamp to the ground.

Vitaliy

Vitaliy wrote:
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Re: Choosing voltage opamp



Vitaliy wrote:
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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


Re: Choosing voltage opamp



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  put an old stand by TL082 type op-amp infront of it.



Re: Choosing voltage opamp


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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:
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But that would kill my BW (the BW on that is only 4MHz, and I need
40MHz), right?

redbelly wrote:
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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


Re: Choosing voltage opamp


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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


Re: Choosing voltage opamp


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


Re: Choosing voltage opamp


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:
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Re: Choosing voltage opamp



Vitaliy wrote:
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You're welcome.  Good luck. -- Mark


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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:
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But that would kill my BW (the BW on that is only 4MHz, and I need
40MHz), right?

redbelly wrote:
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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


Re: Choosing voltage opamp



Jamie wrote:
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But that would kill my BW (the BW on that is only 4MHz, and I need
40MHz), right?


Re: Choosing voltage opamp


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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:
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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


Re: Choosing voltage opamp



Vitaliy wrote:
...
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...

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


Re: Choosing voltage opamp


Hi Kevin,

On OPA657 output voltage swing with normal operating conditions (+25C)
is +-3.9V.

I think the following opamps have similar specs to MAX477 Mark
suggested.


http://focus.ti.com/docs/prod/folders/print/sn10501.html
http://focus.ti.com/docs/prod/folders/print/opa690.html
http://focus.ti.com/docs/prod/folders/print/opa820.html

Vitaliy

Kevin White wrote:
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Re: Choosing voltage opamp



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amplifier will do the job?
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google for "High current Op-amp"
there are lots of them out there..


--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5


Re: Choosing voltage opamp


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You won't get 10Vp-p output from an opamp with +/-5V rails, you need a
larger supply voltage.

Dave :)


Re: Choosing voltage opamp


Hi David,

Is the maximum Vp-p I can get from an opamp with +/-5V rails 5V then?
I know I measured the output of transimpedance amplifier and I was
getting maximum ~8Vp-p :confused:

Vitaliy

David L. Jones wrote:
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Re: Choosing voltage opamp


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that
voltage,

Are you sure about that.
Assume you have a  1k resistor on the inverting input in series with the
incoming signal. On the feedback line from the output back to the input you
have a 10k res, and on the non inverting input you have a 1k (909ohm
resister if you do the math).
With an input  sinewave of 1 volt at 1ma you will have a 10v p-p output
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