From: "Ian Buckner"
Subject: Re: wide range optical power measurement
Date: Wed, 30 Oct 2002 09:33:57 -0000
Organization: Agilent Technologies
References: <firstname.lastname@example.org> <email@example.com> <5dmLR+Aywvv9EwDn@jmwa.demon.co.uk>
NNTP-Posting-Date: Wed, 30 Oct 2002 09:33:59 +0000 (UTC)
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"John Woodgate" wrote in message
> I read in sci.electronics.design that Jonathan Kirwan
> wrote (in
> ax.com>) about 'wide range optical power measurement', on Tue, 29
> >On Tue, 29 Oct 2002 13:52:31 +0000, John Woodgate
> > wrote:
> >>I read in sci.electronics.design that Ian Buckner
> >> wrote (in
> >>t.com>) about 'wide range optical power measurement', on Tue, 29
> >>>You may be using an LMV722, in which case if you use a resistor
> >>>to get a max 4V swing at 20 uA, your output at 200pA is only 40
> >>>How does this compare with the op amp offset voltage and drift?
> >>>How about the bias current and drift?
> >>He needs to look at a logarithmic amplifier to cover 100 dB range.
> >If the application can tolerate the logarithmic errors (ripple)
> >over the range, I suppose.
> Ripple? Do you have progressive saturation in mind? there are other,
> ripple-free, ways of making log amps.
> I have a little magnetic field meter that has a 60 dB scale range on
> moving-coil meter. I suspect it uses just one silicon diode to do
> logging. (Silicon diodes give a 60 dB scale without an option.) But
> not sure how the temperature dependence is compensated, or even if
> Regards, John Woodgate, OOO - Own Opinions Only.
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Then go to
> PLEASE do NOT copy news posts to me by E-MAIL!
I was intending to suggest a log approach, as long as the OP doesn't
the response speed which will vary quite a lot over the 100dB range.
Using a pair of well matched transistors as the logging elements helps
with temperature, look at www.analog.com for logging application
I have used a diode for this over a 70dB+ range with >10MHz bandwidth,
but I was not all that concerned about log fidelity, this was for a
(and the diode was something very special).
I was also trying to suggest that the LM722 is nowhere near good
for this range.
(Waiting for responses with a wooden timbre...;-)