From: Jonathan Kirwan
Subject: Re: wide range optical power measurement
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NNTP-Posting-Date: Tue, 29 Oct 2002 13:40:06 GMT
Organization: AT&T Broadband
Date: Tue, 29 Oct 2002 13:40:06 GMT
On Tue, 29 Oct 2002 12:46:21 -0000, "Ian Buckner"
>"timtt" wrote in message
>> How to do that ??
>> I have tried using Op Amp 722 to convert current into voltage
>> however, the result is not good, the change is almost undetectable
>> as power start going down from -45dBm.
>> I need a wide range, such as -15dBm to 65 dBm
>> Any suggestion??
>3 kilowatts sounds a bit large for optical power....
>I'll assume the range is -15dBm to -65dBm, or 30 uW to 300pW.
That's also consistent with the poster's comment about -45dBm.
>A further wild guess for your photodetector responsivity might
Or .5 or .3 or ? Good enough, given no information at all from
>This suggests your signal current range is 20 uA to 200 pA.
>Note that the ratio of the two means that you are trying for a dynamic
>range of 100dB.
>You don't say what bandwidth you need, or if you need a linear
>response, accuracy, drift, noise, etc.
Excellent points. I'd add; what about temperature drift effects
in the optics, detector sensitivity, detector resistance vs
offset voltage at the lower currents, and so on. In particular,
how much accuracy and repeatability is required on the low end
of the scale...
>You may be using an LMV722, in which case if you use a resistor
>feedback to get a max 4V swing at 20 uA, your output at 200pA is
>only 40 uV. How does this compare with the op amp offset voltage
>and drift? How about the bias current and drift?
Sounds like a single gain on the front end won't cut it. And
that some mechanism may be needed for adjusting against drift in
offset for the lower end of things vs temp and time.
For the poster, an integrator may be okay. I'd recommend
looking at the Burr Brown (TI) DDC-112, which includes all the
required machinery to get to digital, up through an ADC. If you
operate it in continuous mode with the maximum spec'd external
integration cap, you can reach 8uA or so. Non-continuous, you
can hit 20uA with some comfort. But the fet switch resistance
will start to force a small bias voltage across the detector
with those higher currents. Livable. On the low end, it'll
manage 200pA full scale in about 1 second. With maybe 14 bits
deep into it before the noise floor at that rate.