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Enprovement of a DIY fiber optic isolated voltage probe.

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FriedMule:
Hi I have found a galvanic isolated probe that look like it's a great start but maybe a bit noisy, max 70MHz and bw of 35MHz.
I was thinking if it would be possible to reduce noise, get higher max MHz and maybe a wider bandwidth, by replacing some of the "old" chips.


The author wrote about the possibility to improve the design, but to me do it look like the project is abandoned.

Full article with original schematic here: https://hackaday.io/project/12231-fiber-optic-isolated-voltage-probe#menu-description

Here are some of the most important exemption from the authors notes:

Ideally, the probe will have 50-80MHz bandwidth, without horrible distortion and with less noise that an oscilloscope.
Goal, an isolated oscilloscope probe with 10x attenuation, +-50 volt input voltage range, less than 2mVrms noise, and more than 30MHz bandwidth.

The core of the probe are the IF-E91D and IF-D91 plastic optical fiber transmitter max 70Mhz.

Managed to get 9-10nS rise and fall times. Works out to an approximate bandwidth of 35MHz. Roll-off past 35MHz is first order as best I can tell with a noise source and scope FFT. Might be able to compensate with the post-amplifier and get more bandwidth.

Had to remove trimmer C5 due to the unbalanced capacitive dividers.

First problem is that the OPA 847 is oscillating a bit near 100MHz. The output is also rather noisy. So since the OPA847 has more bandwidth than required due to the 35MHz front end, Boosting it to 15K was enough to quiet the oscillation, but I went all the way to 39K to help with noise. Also dropped the LT1819 gain because of the increased sensitivity of the OPA847 stage... (from 10k)

Noise is now about 15mVrms at the output or 150mVrms at the input.... The noise level is a lot higher than I'd like and limits the useful signal amplitude range of the probe.

 I did attempt to extend the bandwidth of the probe by adding a 22pf capacitor in parallel with R31. While this did extend the bandwidth, it also resulted in a blunt peek at 60-70MHz. Didn't keep the capacitor because the distortion was just too obvious.

I get 636uVrms of noise. (3900MHz gain bandwidth, 0.85nV/Hz voltage noise, 2.5pa/Hz current noise, 30Kohm Rf, Cf around 0.2pf, Cin around 7pf) That's consistent with my 7-10mV peak to peak noise I'm seeing after the 3-6x gain from the LT1819.

A noise-free op-amp would drop my noise floor about 5x. Looks like it's worth spending time to develop a better amplifier

Found a few links online I want to keep track of. First is paired plastic optical fiber. This would allow a V2 probe to use differential optical signalling to eliminate offset drift and allow gain compensation. POF pair http://www.afwoptics.com.au/index.php?route=product/product&path=119&product_id=495

END OF NOTES

I have tried to find some alternative to some of the chips but have a hard time to select one that is more usable: https://www.analog.com/en/parametricsearch/11091#/

Kleinstein:
The OPA847 is the wrong type of OP for the photo-detector. It just has way to much current noise. The noise matching source impedance is more like .9 nV / 2.5 pA = 360 Ohms. A really fast TIA is a tricky thing, but this OP is more like the wrong one. The more obvious choice would be either a fast FET based OP (e.g. OPA659 or similar) or a discrete FET (JFET or maybe dual gate MOSFET) based front end.
There may be ready made TIAs for the job. Usually the FB resistor is smaller than 30 K for a BW.

FriedMule:

--- Quote from: Kleinstein on January 30, 2020, 03:51:48 pm ---The OPA847 is the wrong type of OP for the photo-detector. It just has way to much current noise. The noise matching source impedance is more like .9 nV / 2.5 pA = 360 Ohms. A really fast TIA is a tricky thing, but this OP is more like the wrong one. The more obvious choice would be either a fast FET based OP (e.g. OPA659 or similar) or a discrete FET (JFET or maybe dual gate MOSFET) based front end.
There may be ready made TIAs for the job. Usually the FB resistor is smaller than 30 K for a BW.

--- End quote ---

Thanks a lot! I did fight a lot to find something like that chip, but as you say, it just seemed wrong. But I did think it were me who were wrong.
Am I wrong in also assuming that the LM317 is a sort of a bad choice here, I am thinking on it's mediocre specification?
The LT1819 do also confuse me a bit, is that really the "best" chip, would a faster and more silent chip not be of advantage here?
Sorry for the many questions but I am still a totally noob! :-)

StillTrying:
I've looked at that hackaday a few times over the years, I don't think these IF-E91D and IF-D91 ideas are worth much, but I'd still like to test the bare devices.:)

You wouldn't need all that circuitry to test their analogue bandwidth and linearity, which would probably be the first step, I don't think analogue light bandwidth can get anywhere near 70MHz or even 20MHz.

There's some analogue light experiments by JS and me here:
www.eevblog.com/forum/beginners/floating-probe!-for-$2-50/

Another mention of the IF-E91D that goes nowhere!
www.eevblog.com/forum/testgear/cheap-diy-fiber-optic-isolated-voltage-probe-with-bandwidth-up-to-70-mhz/

Marco:
Lets first guesstimate if you need a transimpedance amplifier, lets say you simply use a 330 Ohm resistor to ground, that shouldn't hurt bandwidth. Lets say you use modest modulation depth to keep things linear and you're left with 10 uW at the receiver, so that gives 6 uA, so that gives 2 mV. Peak to peak resistor noise is 100 uV ... well okay, you need the transimpedance amplifier.

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