Experiments to convert analog 10ns pulse to digital 10ns pulse

Question

My objective :

converting laser pulses from photo diode ranging from 10nA - 100mA (60dB dynamic range of incident power) to digital format, for measuring pulse width exactly

pulse width 10ns and repeating at a rate of 20us

EDIT: Modifications: photo diode current ranging 100nA -10mA

My problem statement

what i tried is a conventional approach using a TIA and limiting excess current at TIA input and using 3 additional opamp stages as i have to see the 10nA as 50mV , so i can fix a threshold of 25mV , i can limit the excess current to avoid saturation of opamps,the problem is the more number of opamp stages, they came because of GBWP as i require atleast 120dB of gain at 100MHz(1/10ns), this is done(simulated model) and i was able to achieve calculation of pulse width, but i did not like this !

My Idea !

what i am thinking is to boost the 10nA current to atleast 1uA and any way current limiting is taken care now i can detect pulse at low gains itself ! and i can overcome the stability problems at high gain

am i thinking wrong ? is boosting such a low current is possible ? any way i should be ready keeping the fact that "this will boost my dark and noise current also " in mind

EDIT:

sorry that i have made some serious claims, may be due to underestimating the design problem

here below i have the schematic and results obtained,and the results were not at all up to the mark as expected in the comments

schematic:

10nA input perfectly output is preserved

1uA input

1mA input

my simulator shows me some noise analysis results which looked impressive, but i have to find out how they are calculated

noise analysis results

At the end the pulse width is not all maintained, my current implementaion failed due to saturation of opamp i believe, any how doors open for the comments and criticism

Answer 1

To put a little perspective on Andy's concerns, 10nA is 10nC/second, or 6.2 * 10^10 electrons/second. Or 62 electrons/nanosecond.

Or 620 electrons per pulse.

I suspect you need to be looking at avalanche techniques - photomultipliers or avalanche diodes for optical detection, which will pose problems of their own in terms of overload characteristics, recovery time.

Answer 2

1 - 10 nA to 100 mA is 70 dB, not 60.

2 - I am astounded that you're getting acceptable results over this dynamic range. Running a TIA to give 50 mV for 10 nA will produce 500 kV for 100 mA, and frankly I'm having problems believing that you have designed and built a TIA with that sort of clipping/overload performance. You also have an effective transimpedance of 5 Mohm. Granted that you're using a much lower transimpedance followed by voltage amplification, getting adequate speed in this setup is not something I'd want to try, and the dynamic range of the output chain remains at wildly improbable levels. Assuming, just as a SWAG, 1 nsec rise and fall times, a perfectly compensated TIA with a single pole response will need 350 MHz bandwidth, rather than 100 MHz, and the subsequent amplifiers will also need that bandwidth or better. I suggest that you take a long hard look at your test setup, from your test emitter all the way down, and make sure you're not fooling yourself about your results.

3 - I also don't think I believe your results on the basis of noise. Assume, just for instance, a 10k feedback resistor on your TIA. This will boost a 10 nA pulse to 100 uV. At the same time, Johnson noise from the feedback resistor combined with a 350 MHz bandwidth will give you 240 uV rms noise, which is not what I call a good SNR for measuring pulse width. Note that going to a 1k resistor gives 10 uV signal and 75 uV rms noise, so you want your transimpedance as high as possible. This, of course, will bang head-on with your amplifier speed and stability.

Like I say, I'm very skeptical about your claimed performance. Unless you post a detailed circuit showing how you got it, I think you're kidding yourself.