Re: Photodiode wich is fast enough to detect +50Mhz analog (sinus) signal??
From: colin (no.spam.for.me_at_ntlworld.com)
Date: 08/11/04
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Date: Wed, 11 Aug 2004 14:51:40 GMT
"Winfield Hill" <Winfield_member@newsguy.com> wrote in message
news:cfcshm0nl9@drn.newsguy.com...
> Yannick wrote...
> >
> > Winfield wrote...
> >> Careful, using a sufficiently-wideband opamp can insure the summing-
> >> junction impedance will be low compared to the total shunt
capacitance.
> >> Resistors have 0.05pF to 0.1pF of self capacitance, this should be
your
> >> total feedback capacitance. With 3k resistor you'd have a -3dB
rolloff
> >> at 530MHz. You want high R for low noise, so we'll try 100k, yielding
> >> a 16MHz rolloff. Then we can apply the standard R-C-R trick (this is
> >> more than 30 years old) to get a flat frequency response to 75MHz, or
> >> whatever you decide your bandwidth should be.
> >
> > R-C-R trick to get a flat frequency responce, i never heard of this
> > before, can you explain how it works?
>
> I posted the circuit previously in this thread (20 July) and described
> in detail how it works. Here's the circuit again:
>
> | Rf R2 adjustable
> | ,---/\/\---+---/\/\--/\/\----,
> | | '--||--' | C2 R3 | nA-sensitivity wideband
> | | Cf '--||--/\/\-- gnd | transresistance amplifier
> | | |
> | | __ ,-||--/\/\--+ correction network details
> | input O--+---|+ \ | __ | R2 C2 = Rf Cf
> | | >-+-/\/\-+-|- \ | R3 C2 sets bandwidth
> | ,-|-_/ | | >-----+---
> | | | gnd --|+_/
> | gnd --/\/\--+-/\/\--' composite amplifier
>
> The undesired Rf capacitance Cf is canceled by the R2 C2 network.
> R3 is used to limit the upper frequency of this cancellation.
>
> R2 C2 and R3 constitute the standard R-C-R trick. I thought of this
> about 18 years ago, and have used it with great success since then.
> Later I learned that it had been described in an old Keithley manual,
> and probably in many other places years before that.
>
> BTW, the circuit above will outperform (sensitivity, bandwidth, SNR,
> phase accuracy) any of the resonant schemes you've been contemplating
> here. It's not true that feedback makes things more noisy. In this
> circuit feedback (and a high-performance composite amplifier) insures
> that all of the signal current is used by the amplifier, rather than
> becoming uselessly drained away by the input-node capacitance. Thus
> feedback actually improves the SNR. Using a resonant input doesn't
> solve the capacitance problem because if a high enough Q is used for
> a solution, it simply creates insurmountable phase-error problems.
>
>
>
> --
> Thanks,
> - Win
>
> (email: use hill_at_rowland-dot-org for now)
i like your idea and admit that phase variance is a big issue for his
method, and i now see the significance of the R-C-R, but i cant agree with
you on this one issue of SNR is beter even than a tuned ciruit.
the capacitance will rob most of the signal before it gets to the amplifier,
the amplifier will add its own input noise
to this small signal. whatever feedback topology u consider the noise has
already been added by the time the feedback gets back to the inputs.
the feedback cant improve the SNR. it cant just selectivly amplify the
signal and not the noise. it merley recovers the signal strength lost into
the capacitor by extra amplification of the signal and noise.
the photodiode is like a curent source and the capacitor is an impedance. if
you convert this to the thevenin equivelent at the frequency of interest you
end up with a voltage source and src impedance and a normal inverting
amplifier with gain of Rfeedback/Rsrc.
the strenght of this thevenin voltage against amplifier input noise is what
sets the SNR. as the frequency rises the voltage will fall but so will the
src impedance hence the gain wil rise in proportion. the response remains
flat therefore but the SNR deterioates with rising frequency.
as you pointed out the dominant noise is set by the amplifier input
noise/ZCin*rfeedback.
ie this is the extra amplification thats going on as the frequency rises
wich is amplifing the input noise along with the reduced signal strenght.
tuning negates the efect of this capacitance without ading noise, hence
making the signal stronger at the input to the amplifier therefore the SNR
is better for the tuned frequency.
Colin =^.^=
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