Choosing voltage opamp

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

=1MOhm will do it). Should I get the one with JFET-input? Or any voltage

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

Reply to
Vitaliy
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amplifier will do the job?

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

Reply to
PeteS

amplifier will do the job?

google for "High current Op-amp" there are lots of them out there..

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Reply to
Jamie

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

Dave :)

Reply to
David L. Jones

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

Reply to
Vitaliy

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. J> > Hello,

Reply to
Vitaliy

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:

Reply to
Vitaliy

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

>
Reply to
Dana

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

Reply to
redbelly

It depends on the particular device. As a general rule of thumb an opamp can get to within at least 2V of either rail, so a +/-5V supply will give you at a minimum +/-3V. Your opamp could aboviously get within 1V of the rail to give you 8Vp-p. There are "rail-to-rail" opamps that claim to get very close to the full output voltage rail, but usually not at any significant output current. You need a higher rail voltage regardless of the device you choose.

Dave :)

Reply to
David L. Jones

Not with +/-5V rails he won't.

Dave :)

Reply to
David L. Jones

a

you

So help me out here. If he has 5 and -5 rails, , and an input sinewave comes in, so lonag as the op amp is properly biased, will not the output swing between 5 and -5 (not exactly 5 do to loss), in essence giving a 10vpp signal depending on how the output is referenced to ground.

Reply to
Dana

Only rail-to-rail-output opamps come close to the rail at the expense of some other parameter. Meaning RR-Output is not exactly free.

Rene

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Reply to
Rene Tschaggelar

comes

True. But I think Dave was saying that with a +/- 5v supply, the output would only be able to get a 5v p-p signal. I would think it would get close to both the 5 and -5 supply giving close to a 10v p-p output. Like the original poster mentioned when he observed an approx 8v p-p output. And back to the original poster, yes if he required an absolute minimum of

10v p-p out, yes his rail supplies would have to be in line with that requirement.
Reply to
Dana

I was not saying a +/-5V rail will give you 5Vp-p, go have another read. A regular op-amp will only give about 8Vp-p as the OP found out. Only (output) rail-to-rail opamps will get close to the rail voltage. Normal rule of thumb for ordinary op-amps is you design for a 2V margin on the rails, unless you inspect the datasheet and can take it further.

Dave :)

Reply to
David L. Jones

Most op-amps clip the signal when the output is within 2 volts (or so) of the + or - supply. Hence the need for special "rail to rail" op amps, which can get much closer to the supply voltage before clipping (tens or hundreds of mV?)

The problem is worse the higher the output current, and the OP's desire for 200 mA is a high current for an opamp.

Mark

Reply to
redbelly

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

Reply to
Johnny Boy

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

Reply to
Jamie

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:

But that would kill my BW (the BW on that is only 4MHz, and I need

40MHz), right?

redbelly 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?

Thanks for all advices,

Vitaliy

Reply to
Vitaliy

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:

But that would kill my BW (the BW on that is only 4MHz, and I need

40MHz), right?

redbelly 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?

Thanks for all advices,

Vitaliy

Reply to
Vitaliy

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