Hello Folks,
It would be nice to tie a TIA into the cathode to get positive polarity output in one of my circuits. That would mean the (critical) RF path also needs to be connected to cathode. The anode would then get the bias.
On all PDs I looked at so far neither anode nor cathode are connected to the housing. Is this customary or should I assume that cathode could be grounded or have higher capacitance to the enclosure on some models?
Interesting issue. I have here an Appointech InGaAs pin diode. It's in a 4-lead TO-18 can, one lead obviously the can itself, one lead snipped off, and the remaining two leads the pin diode, floating from the can.
Capacitance a-k is 2.34 pF
A-can is 1.12
K-can is 1.19
so at least this one is pretty symmetric.
But nobody is consistant here, not even on pinout. Lots of laser diodes are soldered to the can inside, for heat sinking I suppose.
John
Strange, the ones I have used, that have a metal case, have the case connected to the cathode. (This is often a pain, since you then need a negative supply to reverse-bias the diode, or else float the case).
Interesting. Can you share a part number there? With the cathode connected to case you'd be ok since the classic way of biasing is AFAIU connecting the cathode to a positive supply and running the anode into the TIA input. So the anode needs to be really low capacitance. Unfortunately that results in a negative voltage swing.
Of course, now the case would be on a DC level (but RF ground) and that source thus needs to be protected against shorts. So it's not the fiery scenario that results from wedging your wedding band between GND and a car battery terminal.
That would be nice but John Devereux mentioned in the other post that he has some where cathode and case are joined.
I assume it's the cathode that's grounded on those. Otherwise driving them would be a bear.
Sure, e.g. Hamamatsu S5821, S2386, Centronic OSD15 etc.
Ah yes, of course this must be why they do it that way around. My applications have been relatively low frequency ( Unfortunately that results in a negative voltage swing.
Still inconvenient though!
Thanks, John. Hamamatsu is a main contender so if they connect the cathode to the case then I better not use that as my signal tap.
For RF that positive bias is fine because you can run into an RF amp that is biased properly. But it's a drag if you have to measure DC also. Either bias the TIA upwards which for a broadband version is a white knuckle ride or provide a negative supply. Oh drat, so it's negative supply then for me.
Yes, especially for stuff that has to go down to DC.
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