Re: OPA695 current feedback amplifier

From: colin (no.spam.for.me_at_ntlworld.com)
Date: 09/24/04 

Date: Fri, 24 Sep 2004 17:06:17 GMT

"Yannick" <yannick_de_wit@pandora.be> wrote in message
news:cc50d220.0409240628.dfe758c@posting.google.com...

> ofcourse, but if you take everything for one fixed frequency, say
> 20Mhz then you can calculate the signal noise ratio in function of Rf.
> then it seems that when rf is infinite only voltage noise of the
> amplifier is in the game, and because the feedback impedance is then
> only from the feedback capacitance you get highest S/N ratio. If i
> calculate the optimal Rf for the largest transimpedance gain
> (D(Af)/Drf = 0 => Rf = 62k) i get 62K but for S/N ratio Rf has to be
> as high as possible. The only reason you need a lower value of Rf is
> to get like you say a flat frequency response.

>>

> No this isnt what i am missing, i calculated the Signal as Sout =
> ID*Af(Rf) with ID = 177na (calculated this for 10meter distance with
> avalanche photodiode...)
>
> Then i calculated the noise as Nout = Ieq(Rf) *Af(Rf)
>
> then i calculated the SIgnal noise ratio as S/N = 20*log(Sout/Nout)
>
> i plotted this in a X-Y plot with S/N on the y as and Rf on the x as
> and it this gives a graph wich keeps increasing (although flatter and
> flatter) for an increase in Rf , and the formula for Af(Rf) is
> correct, it matches perfect with the Pspice simulation , soo i cant
> see any errors...
>

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.

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.

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.

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.

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.

Colin =^.^=

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