Re: OPA695 current feedback amplifier

From: Yannick (yannick_de_wit_at_pandora.be)
Date: 09/26/04 

Date: 26 Sep 2004 16:01:52 -0700

>
> yes i agree, sory i misunderstood slightly, the resistor inevitably adds
> noise so leaving it out means you have less noise,
> wich is what i think i said as well anyway, but i thought you were still
> meaning the higher frequency wich you mentioned in the previous paragraph. i
> wasnt fully awake when i read it lol.
>

hehe no problem

> If i were you i would measure what signal you get when u reflect it off a
> target, my estimate of 10% reflected back towards the lense was pure
> gueswork. the real chalenge comes when u have a smooth dark surface that is
> angled away from the detector.
>

indeed , the good news is that my phase meausurement works, i now can
do distance measurement with direct light path on the photodiode. I am
now gonne make a microcontroller interface wich does the ADC,
calculations(phase-distance algorithm) and drives a DDS frequency
synthesiser. After this i am going to replace my PIN photodiode with
an avalanche one and the difficult part will began : measuring the
reflection on a diffuse surface... i almost cant wait:)

> 177 na at 20 mhz acros 10pf = 140uv wich sounds ok compared to the 18uv
> noise for a 20mhz bandwidth from the amplifier, but this wil stil give u
> quite a bit of jitter, of course this can be averaged out over many
> milliseconds, but ive found it quite dificult to get as good as results as i
> wld expect from this simple calculation. i have quite narow bandwidth too,
> but i find most of the problem lies in noise picked up. in particular my
> high voltage bias generator frequency seems to apear a lot on the signal,
> despite a sheild over the rf input section and a sheild over the hv
> generator and using a 7 stage multiplier so i need a lower voltage of the
> 200khz squarewave to the step up transformer, however you may have les
> problem here as my input is tuned so is very much higher impedance.
>

I see, i will have to be carefull with that, are you sure this
crosstalk is from HF electromagnetic waves and not via the same power
supply?

> Also i mentioned in another post one day i had it resting on my keyboard
> wich is wirless and so was transmiting constant keypresses at 27mhz wich
> swamped the output and had me looking for the cuase for ages. i normaly
> switch off my computer/monitor flourescent lamp etc. when i try to measure
> low noise performance, but hadnt considered the keyboard.
>

Hehe, that's frustrating, another question: how do you measure noise,
because i have a digital 60Mhz scope with high impedance probes,these
high impedance probes give me high voltage noise (4ktR) soo i cant see
my circuit noise.I suppose i need another probe (low impedance) ???

> you sugested using a sinewave wich i gues would make more sense as would not
> puting the thing so dam close to the detector like i did lol, i just thought
> best to have a short a track as posible at 250v.
>
> Of course the rf signal that drives the laser also apears on the signal
> despite this being further away and shelded also. Although I havnt however
> fuly soldered the sheilds in place yet as then it would be hard to make
> changes. it is however only noticable when the gain is turned up max and the
> detector totaly blanked out, also as i think i said before stray reflections
> albeit invisible were also an issue at one point.
>

these thinks are the hard part i suppose, do you use a common mass
plane or did you split it up ?

Yannick

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