Low drift, low noise, fast opamp

I haven't had to deal with high voltages like that for quite a long time now...

OPA847 is a nice low-noise part. As is typical with high speed parts, the noise 1/f corner is rather high, but it's still typically only around 8nV/rtHz at 10Hz. It does need to run with a high noise gain for stability, though. I suppose the 0.6mV input offset voltage and

1.5uV/C input offset drift, max over temperature, would do for you. Figure 3 in the data sheet shows it connected to a 1pF photodiode. As a bonus, it's also quite low distortion ('specially after I got them to make a die change to fix a minor problem).

(Brian L., you owe me one for this...)

Cheers, Tom

Hello Tom,

Thanks. The OPA847 like a great candidate here. I'll have to run the offset budget but it looks close to being acceptable. The 1/f doesn't bother me much because noise isn't critical under a MHz or so in this application.

Cool. I never had anyone changing the die for me except for my custom chip designs. But there was an instant where a mfg had to correct a data sheet after we found a gotcha.

--
Regards, Joerg

http://www.analogconsultants.com

Hello Tom,

P.S.: The only crimp in the OPA847 is the min stable gain of 12. That's too much in my case.

--
Regards, Joerg

http://www.analogconsultants.com

Joerg schrieb:

Hi Joerg

Why don't you use a wideband op amp together with a precision op amp providing the dc performance? There are a lot of app notes available with such a dc servo loop.

HTH

Reinhard

Hello Reinhard,

Sewing them together later might be a challenge but that is certainly an idea to keep in mind, especially if I can't get the desired noise performance. Thank you.

--
Regards, Joerg

http://www.analogconsultants.com

Well, I did warn you about that. ;-) But be sure to look at the whole data sheet; there are some compensation tricks in there. Also, check out others in the family. They are more noisy, but not outrageously so. OPA84x...

Curious that you are so worried about input offset drift, but won't have room for 20dB of gain...

Cheers, Tom

Hello Tom,

I have done some compensation tricks with the old HA2540 amps. Hey, anyone remember these? From the days when electrity cost next to nothing and every chip got freaking hot?

Anyhow, at 100MHz compensation tricks can become iffy.

Well, I need precision at the low end of the spectrum but can't have too much gain up in the VHF region because things would saturate and intermodulate. The DC level needs to be measured very accurately even though the actual amplitude coming in can be high.

OTOH I might just split the band up right at the photodiode. Kind of an AC path and a DC path from there. This might even be easier than the servo loop Reinhard had suggested. Have done it before but never with a photodiode.

--
Regards, Joerg

http://www.analogconsultants.com

What's the photodiode current level like? And how much equivalent input noise current can you tolerate?

I like to use really fast (400 MHz and up) current-mode opamps for DC-coupled photodiode tia's, but their noise performance probably isn't ideal. I usually have a lot of light to work with, hundreds of uW, whereas the telecom tia's are intended to work in the low uW range. Most telecom tia's have AGC, which wrecks them for DC applications.

John

Hello John,

That's still in the air at this point. Most likely a few mA max. The noise current needs to be as low as possible because this is an analog measuring setup.

Today I am going to design the PD front end. Looks like I am going to split it into a sub-MHZ and an RF section. The OPA847 Tom suggested looks like a good chip for the RF part and there I'd also need more gain so it's x12 limit won't hurt. For the sub-MHz range the AD8628 appears to be a nice chip although I'd have to set the crossover frequency a lot lower than a MHz.

We also have a lot of light to work with (thousands of uW...) but must detect extremely minute changes.

Then comes a whole 'nother little problem. This client doesn't do electronics much so they have no tech who could build a neat lab prototype on a large Veroboard with ground plane. I can do it but that wouldn't be cost effective. Oh well, got to cross that bridge in a couple of weeks.

--
Regards, Joerg

http://www.analogconsultants.com

I could let you use one of my J730 boards for experimenting...

This has a DC-180 MHz tia/amp chain, currently outputting 1 v/mW at the analog output connector, using a fairly pokey silicon PIN diode. It would be easy to replace the silicon pin with something else; I recently spun a 1310 nm version with a GaAs pin diode, just dropped it in and changed one gain-set resistor. Email me if interested.

John

Hello John,

Thanks for the offer. That is very kind. However, this app is analog. Got a discrete solution on paper, still have to compare it with the OPA847 from a noise point of view. But my wife just put our favorite casserole in the oven and I can almost smell it so I don't know if I'll get that done tonight.

The BFR505 looks really nice for the discrete fiber amp and is fast enough for this case. Of course these are European transistors and that leaves a bit of worry. Got a black I with EU semiconductors before when they sudenly became unobtanium in the US for the prototype phase. Somehow my favorite hotrod RF BJT from the days of Motorola have vanished. They offered stunning noise figures.

--
Regards, Joerg

http://www.analogconsultants.com

Hello John,

Just re-read your post and the spec and realized it does provide analog out. Unless it's a trade secret which device did you use in the TIA?

--
Regards, Joerg

http://www.analogconsultants.com

AD8014, current-mode opamp.

John

Boy, do I remember those Harris amps, and the way they did a die shrink every couple of years which made everything go unstable.....

Another candidate might be the AD8099.

Regards Ian

Is this 100mhz just a carrier wave, or is the light itself just modulated upto 100mhz ? If not could you modulate it with a higher frequency ?

What sort of current do you get out of the PD? for real low drift and low freq noise floor maybe you need to use a zero drift op amp as a servo, then you have wider choice for the main device.

Jfets types look like they promise good speed/noise. or phemt would be good, if/when they make them.

Colin =^.^=

Hello John,

Maybe you could try out the lower noise OPA847 that Tom suggested if the board in there ever needs a re-spin. In the past I always found discrete solutions better in terms of noise but this was mostly for all kinds of transducers and not photodiodes.

BTW, it's interesting that you use kilometers in the advertising text. The only other places I saw doing that in the US were NASA and the IEEE.

--
Happy Thanksgiving, Joerg

http://www.analogconsultants.com

Hello Ian,

Thanks. At around 1nv/rtHz that one comes quite close to what's possible with RF transistors. I'll give the AD8099 a good look.

--
Happy Thanksgiving, Joerg

http://www.analogconsultants.com

Hello Colin,

It's light modulated at up to 100MHz. Plain old AM style.

Well, that's not really my call. I just have to make the receive end work as good as possible.

Probably around a mA or so. The challenge is to detect wee changes in that.

Drift isn't an issue by now anymore since I already split out the lower MHz range down to DC. That part works ok (on paper...).

I am also looking at a discrete amp for the higher frequency portion. That would help the design longevity because with chips there is always a chance that the last order bell clangeth one day.

--
Happy Thanksgiving, Joerg

http://www.analogconsultants.com

I'd like to do a faster version some day, so a better amp would help. Noise isn't a big deal in my apps, as I usually have a lot of fiber-coupled light... my lasers easily launch a milliwatt. Maybe a common-base transistor stage would decouple the pd capacitance from the opamp input, but by the time you put all that junk in the emitter, it's gonna want to oscillate, and by the time you damp that the bw may go away. Life sucks.

We use SI units for anything that matters. Fiber attenuation is always spec'd in dB/km and dispersion in ps/km. Dumb stuff, like box dims and patch cable lengths, we still tend to do in inches and feet.

Gotta go make bread pudding. Popular demand.

John