Subject: Re: wide range optical power measurement
Date: Wed, 30 Oct 2002 10:30:20 +0800
Organization: IMS Netvigator
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Sorry for missing so many info. I am just a rookie and I have already
learned so much in these 1-2 message
What I am doing is to measure the optical power in a fiber using a
photodiode with 0.8 responsivity, then measure the voltage and finally get a
digital output of the power level.
I am using the LM722 for current to voltage conversion(PD cathode connecting
to inverting input of LM722, non-inverting input grounded, feedback using
250k 1% resistor)
However, the current result is not linear enough when fall below -45dBm
power. and Voltage output saturate around -20dBm
I am looking for some other means to measure wider range of optical power
and give a correct digital output
> 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
> >be 0.7A/W.
> Or .5 or .3 or ? Good enough, given no information at all from
> the poster.
> >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.
> (Optical power.)
> >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.
> An option.