Hi there - I am looking at methods of sensing current in an Avalanche PhotoDiode (APD). Typical current in them is along the lines of 100 nA to 1 mA. I'm trying to figure out how to sense current in the buggers. Here is what I think I need:
-Very low added resistance/inductance/capacitance on APD
-If using some sort of inline current sensing, I'd like to see a voltage drop across the shunt/transformer/etc. of less than a volt, ideally mV range (the APD will have a very large reverse bias (~100-200V), so its gain will be very sensitive to changes in supply voltage)
-bandwidth in the GHz region. I'm looking for a rise time from the circuit of ~10ps, which I believe puts the necesarry bandwidth at about 35GHz
When I saw the 35GHz number, I just about gave up - but figured I'd ask before doing that. What little analog stuff I've done has had bandwidth in the single digit MHz... So this is very new for me. I'm prepared to spend a lot of time learning, though.
Now, as for the output of this circuit, let me explain a bit about what I need: eventually I'll want it to go to an input of a comparator. I may end up only using a comparator, so in that case linearity would not be a big concern, as the other input to the comparator will be a constant, adjustable voltage. Ideally though, I'd have a nice linear output with a sufficiently high enough voltage so as to read from an ADC and do other fun stuff with. But the comparator function is much more important, so if having a highly non-linear output makes this more achieveable, than I will survive.
So - is this at all possible? Can anybody point me down the right path? I'm not even sure what sort of technique I should be using with this - resistive shunt, transformer, hall effect, etc.
Thanks!
-Michael