Weird photodiode step response.

G

George Herold

Guest
So testing a bunch of photo diode TIA's today. (opa124, not recommended
for new designs.) The low gain (332 ohm) step response
had a ~0.5 us delay, and then a sharper edge than expected.
(higher gains had similar weird stuff.)
(I'm trying to upload dropbox files from home... but it's not working.)
The circuit is TIA, no bias, N-I input grounded, opa124, Z_f is
332 ohms and parallel C ~470 pF, (maybe less.)

Here's the normal response (50 ohm termination.)
https://www.dropbox.com/s/vdnq85luaqa258m/a-normal2.bmp?dl=0

ab-normal, 50 ohm term.
https://www.dropbox.com/s/jcq65c1jxqp53fl/ab-normal2.bmp?dl=0

and ab-normal with no 50 ohm load on the long (~6') thin
(RG-172) coax output... which is how I test it.
Typically the 50 ohms makes little difference.
https://www.dropbox.com/s/wvqf16sddkhug9x/abby%20no%2050%20ohms2.bmp?dl=0

I'll replace the opamp, and hopefully that will fix it...
But it was interesting to me.

George H.
 
On Tue, 28 May 2019 18:02:20 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

So testing a bunch of photo diode TIA's today. (opa124, not recommended
for new designs.) The low gain (332 ohm) step response
had a ~0.5 us delay, and then a sharper edge than expected.
(higher gains had similar weird stuff.)
(I'm trying to upload dropbox files from home... but it's not working.)
The circuit is TIA, no bias, N-I input grounded, opa124, Z_f is
332 ohms and parallel C ~470 pF, (maybe less.)

Here's the normal response (50 ohm termination.)
https://www.dropbox.com/s/vdnq85luaqa258m/a-normal2.bmp?dl=0

ab-normal, 50 ohm term.
https://www.dropbox.com/s/jcq65c1jxqp53fl/ab-normal2.bmp?dl=0

and ab-normal with no 50 ohm load on the long (~6') thin
(RG-172) coax output... which is how I test it.
Typically the 50 ohms makes little difference.
https://www.dropbox.com/s/wvqf16sddkhug9x/abby%20no%2050%20ohms2.bmp?dl=0

I'll replace the opamp, and hopefully that will fix it...
But it was interesting to me.

George H.
Click to expand...

Underdamped step response, not unusual.

If you back-bias the photodiode some, its capacitance would go way
down. Or do one of Phil's fancy cascode+bootstrap things.

Just a cascode can be a remarkable improvement.

What photodiode?


--

John Larkin Highland Technology, Inc

lunatic fringe electronics
 
On Tuesday, May 28, 2019 at 9:13:18 PM UTC-4, John Larkin wrote:
On Tue, 28 May 2019 18:02:20 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

So testing a bunch of photo diode TIA's today. (opa124, not recommended
for new designs.) The low gain (332 ohm) step response
had a ~0.5 us delay, and then a sharper edge than expected.
(higher gains had similar weird stuff.)
(I'm trying to upload dropbox files from home... but it's not working.)
The circuit is TIA, no bias, N-I input grounded, opa124, Z_f is
332 ohms and parallel C ~470 pF, (maybe less.)

Here's the normal response (50 ohm termination.)
https://www.dropbox.com/s/vdnq85luaqa258m/a-normal2.bmp?dl=0

ab-normal, 50 ohm term.
https://www.dropbox.com/s/jcq65c1jxqp53fl/ab-normal2.bmp?dl=0

and ab-normal with no 50 ohm load on the long (~6') thin
(RG-172) coax output... which is how I test it.
Typically the 50 ohms makes little difference.
https://www.dropbox.com/s/wvqf16sddkhug9x/abby%20no%2050%20ohms2.bmp?dl=0

I'll replace the opamp, and hopefully that will fix it...
But it was interesting to me.

George H.


Underdamped step response, not unusual.
Click to expand...
Yeah but I've got a bunch of R//C gain stages on a rotary
switch. All the Rs, Cs, solder joints look fine. I figure the opamp is fried in some way, it mostly works fine.

If you back-bias the photodiode some, its capacitance would go way
down. Or do one of Phil's fancy cascode+bootstrap things.

Just a cascode can be a remarkable improvement.
Click to expand...
Right this is something I did years ago, before I knew as
much.
What photodiode?
Click to expand...
PIN-44D? Big square thing.
(osi opto-electronics?)

George H.
--

John Larkin Highland Technology, Inc

lunatic fringe electronics
Click to expand...
 
On Tue, 28 May 2019 18:53:02 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Tuesday, May 28, 2019 at 9:13:18 PM UTC-4, John Larkin wrote:
On Tue, 28 May 2019 18:02:20 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

So testing a bunch of photo diode TIA's today. (opa124, not recommended
for new designs.) The low gain (332 ohm) step response
had a ~0.5 us delay, and then a sharper edge than expected.
(higher gains had similar weird stuff.)
(I'm trying to upload dropbox files from home... but it's not working.)
The circuit is TIA, no bias, N-I input grounded, opa124, Z_f is
332 ohms and parallel C ~470 pF, (maybe less.)

Here's the normal response (50 ohm termination.)
https://www.dropbox.com/s/vdnq85luaqa258m/a-normal2.bmp?dl=0

ab-normal, 50 ohm term.
https://www.dropbox.com/s/jcq65c1jxqp53fl/ab-normal2.bmp?dl=0

and ab-normal with no 50 ohm load on the long (~6') thin
(RG-172) coax output... which is how I test it.
Typically the 50 ohms makes little difference.
https://www.dropbox.com/s/wvqf16sddkhug9x/abby%20no%2050%20ohms2.bmp?dl=0

I'll replace the opamp, and hopefully that will fix it...
But it was interesting to me.

George H.


Underdamped step response, not unusual.
Yeah but I've got a bunch of R//C gain stages on a rotary
switch. All the Rs, Cs, solder joints look fine. I figure the opamp is fried in some way, it mostly works fine.


If you back-bias the photodiode some, its capacitance would go way
down. Or do one of Phil's fancy cascode+bootstrap things.

Just a cascode can be a remarkable improvement.
Right this is something I did years ago, before I knew as
much.

What photodiode?
PIN-44D? Big square thing.
(osi opto-electronics?)
Click to expand...

PIN-44D has 700 pF at zero bias. Enough to make any self-respecting
opamp wig out.

You can Spice it as a 700 pF cap in parallel with a current source.

I'd back-bias it 15 or 20 volts.

OPA124 isn't very fast. You might consider something else.

If noise isn't a big deal, you can back-bias the PD and dump the
current into a series R+L to ground, and follow that with a fast
voltage amplifier.

The opposite extreme is a bootstrap cascode into a fast TIA.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics
 
On Tuesday, May 28, 2019 at 11:27:48 PM UTC-4, John Larkin wrote:
On Tue, 28 May 2019 18:53:02 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Tuesday, May 28, 2019 at 9:13:18 PM UTC-4, John Larkin wrote:
On Tue, 28 May 2019 18:02:20 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

So testing a bunch of photo diode TIA's today. (opa124, not recommended
for new designs.) The low gain (332 ohm) step response
had a ~0.5 us delay, and then a sharper edge than expected.
(higher gains had similar weird stuff.)
(I'm trying to upload dropbox files from home... but it's not working.)
The circuit is TIA, no bias, N-I input grounded, opa124, Z_f is
332 ohms and parallel C ~470 pF, (maybe less.)

Here's the normal response (50 ohm termination.)
https://www.dropbox.com/s/vdnq85luaqa258m/a-normal2.bmp?dl=0

ab-normal, 50 ohm term.
https://www.dropbox.com/s/jcq65c1jxqp53fl/ab-normal2.bmp?dl=0

and ab-normal with no 50 ohm load on the long (~6') thin
(RG-172) coax output... which is how I test it.
Typically the 50 ohms makes little difference.
https://www.dropbox.com/s/wvqf16sddkhug9x/abby%20no%2050%20ohms2.bmp?dl=0

I'll replace the opamp, and hopefully that will fix it...
But it was interesting to me.

George H.


Underdamped step response, not unusual.
Yeah but I've got a bunch of R//C gain stages on a rotary
switch. All the Rs, Cs, solder joints look fine. I figure the opamp is fried in some way, it mostly works fine.


If you back-bias the photodiode some, its capacitance would go way
down. Or do one of Phil's fancy cascode+bootstrap things.

Just a cascode can be a remarkable improvement.
Right this is something I did years ago, before I knew as
much.

What photodiode?
PIN-44D? Big square thing.
(osi opto-electronics?)


PIN-44D has 700 pF at zero bias. Enough to make any self-respecting
opamp wig out.
Click to expand...
Right.. but hundreds (of opamps) have answered the call before and
worked fine. (I soldered in a new opamp and all is good.)
You can Spice it as a 700 pF cap in parallel with a current source.

I'd back-bias it 15 or 20 volts.
Right old design.

OPA124 isn't very fast. You might consider something else.
Click to expand...
Yeah it's also got this nasty noise peak around 20 kHz (I'd have to
check my notes. I found that years ago.)

George H.
If noise isn't a big deal, you can back-bias the PD and dump the
current into a series R+L to ground, and follow that with a fast
voltage amplifier.

The opposite extreme is a bootstrap cascode into a fast TIA.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics
Click to expand...
 
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