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Photodiodes: Anode, cathode always uncommitted?

J

Joerg

Jan 1, 1970
0
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?
 
J

John Larkin

Jan 1, 1970
0
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
 
J

John Devereux

Jan 1, 1970
0
Joerg said:
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?

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).
 
J

Joerg

Jan 1, 1970
0
John said:
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.
 
J

Joerg

Jan 1, 1970
0
John said:
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.

That would be nice but John Devereux mentioned in the other post that he
has some where cathode and case are joined.

But nobody is consistant here, not even on pinout. Lots of laser
diodes are soldered to the can inside, for heat sinking I suppose.

I assume it's the cathode that's grounded on those. Otherwise driving
them would be a bear.
 
J

John Devereux

Jan 1, 1970
0
Joerg said:
Interesting. Can you share a part number there?

Sure, e.g. Hamamatsu S5821, S2386, Centronic OSD15 etc.

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.

Ah yes, of course this must be why they do it that way around. My
applications have been relatively low frequency (<1MHz).
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.

Still inconvenient though!
 
J

Joerg

Jan 1, 1970
0
John said:
Sure, e.g. Hamamatsu S5821, S2386, Centronic OSD15 etc.

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.
Ah yes, of course this must be why they do it that way around. My
applications have been relatively low frequency (<1MHz).

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.
Still inconvenient though!

Yes, especially for stuff that has to go down to DC.
 
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