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Photodiode case

W

WhiteDog

Jan 1, 1970
0
Hi,

I am working on a wideband photodiode receiver for a laser ranging
system. I have a bootstrapped photodiode plus transimpedance amplifier
that has a BW of about 75MHz. My signals of interest are in the
3MHz-6MHz region and the 35-50MHz regions. The whole thing (PD plus
amp) is inside of a stainless steel box that provides E-field
shielding (but not much H-field, I fear). I know stainless was not the
best choice for HF, but there were other factors involved. I am trying
to get that changed now.

My question is regarding what to do with the case of the photodiode
(an InGaAs PIN). The case of the photodiode is providing some strange
coupling effects. The PD case pokes out of the stainless enclosure
that contains the PD front end amplifiers, etc. The PD case is NOT DC-
coupled to anything inside itself (the TEC cooler, thermistor, or the
PD substrate itself).

Initially, I had the PD case mounted on the enclsoure itself. The
thinking here was that the PD case plus the enclosure would form a
Faraday shield. This was apparently a fairly bad idea, as the
enclosure apparently capacitively coupled to the PD, and it created a
feedback path from the amp output to the PD itself, and I had in-band
ripples in the response at the higher frequencies.

I tried driving the PD case with the bootstrap. No good.

I currently have the PD case attached to the 'ground' reference near
the transimpedance amp with a length of copper braid. This seems to
have the best results, but still not ideal

Has anyone else had experience with doing this? What is the preferred
way of isolating the PD substrate from the E/H field environment?

Does anyone else have a sore forehead from beating it against a
network analyzer ;-)?

Thanks for any help,

-WhiteDog
 
J

Joerg

Jan 1, 1970
0
WhiteDog said:
Hi,

I am working on a wideband photodiode receiver for a laser ranging
system. I have a bootstrapped photodiode plus transimpedance amplifier
that has a BW of about 75MHz. My signals of interest are in the
3MHz-6MHz region and the 35-50MHz regions. The whole thing (PD plus
amp) is inside of a stainless steel box that provides E-field
shielding (but not much H-field, I fear). I know stainless was not the
best choice for HF, but there were other factors involved. I am trying
to get that changed now.

My question is regarding what to do with the case of the photodiode
(an InGaAs PIN). The case of the photodiode is providing some strange
coupling effects. The PD case pokes out of the stainless enclosure
that contains the PD front end amplifiers, etc. The PD case is NOT DC-
coupled to anything inside itself (the TEC cooler, thermistor, or the
PD substrate itself).

Initially, I had the PD case mounted on the enclsoure itself. The
thinking here was that the PD case plus the enclosure would form a
Faraday shield. This was apparently a fairly bad idea, as the
enclosure apparently capacitively coupled to the PD, and it created a
feedback path from the amp output to the PD itself, and I had in-band
ripples in the response at the higher frequencies.

I tried driving the PD case with the bootstrap. No good.

I currently have the PD case attached to the 'ground' reference near
the transimpedance amp with a length of copper braid. This seems to
have the best results, but still not ideal

Has anyone else had experience with doing this? What is the preferred
way of isolating the PD substrate from the E/H field environment?

Does anyone else have a sore forehead from beating it against a
network analyzer ;-)?

I have always grounded the can directly to chassis resp. the ground
plane on the circuit boards. Reverse bias it, to gain a wee bit more BW
and avoid rectification of RF noise. I usually select a (very clean!)
bias of about 80% of spec sheet limit. If BW is still an issue you can
cascode to reduce the capacitive load by the TIA. Phil Hobbs explains
this very nicely:

http://users.bestweb.net/~hobbs/frontends/frontends.pdf
 
J

John Larkin

Jan 1, 1970
0
Hi,

I am working on a wideband photodiode receiver for a laser ranging
system. I have a bootstrapped photodiode plus transimpedance amplifier
that has a BW of about 75MHz. My signals of interest are in the
3MHz-6MHz region and the 35-50MHz regions. The whole thing (PD plus
amp) is inside of a stainless steel box that provides E-field
shielding (but not much H-field, I fear). I know stainless was not the
best choice for HF, but there were other factors involved. I am trying
to get that changed now.

You won't have h-field problems, so any metal box is as good as any
other.
My question is regarding what to do with the case of the photodiode
(an InGaAs PIN). The case of the photodiode is providing some strange
coupling effects. The PD case pokes out of the stainless enclosure
that contains the PD front end amplifiers, etc. The PD case is NOT DC-
coupled to anything inside itself (the TEC cooler, thermistor, or the
PD substrate itself).

I usually ground the case, in my case to the pc board ground plane.
Seems fine.
Initially, I had the PD case mounted on the enclsoure itself. The
thinking here was that the PD case plus the enclosure would form a
Faraday shield.

It should.

This was apparently a fairly bad idea, as the
enclosure apparently capacitively coupled to the PD, and it created a
feedback path from the amp output to the PD itself, and I had in-band
ripples in the response at the higher frequencies.

If the pc board that contains the tia is securely grounded to the
case, and the pd can is grounded, and the leads from the pd to the tia
are short, it should work.
I tried driving the PD case with the bootstrap. No good.

What's the pd capacitance? I'm surprised you need to bootstrap. The
bootstrap drive may be causing some of the weirdness you're seeing.
I currently have the PD case attached to the 'ground' reference near
the transimpedance amp with a length of copper braid. This seems to
have the best results, but still not ideal

Has anyone else had experience with doing this? What is the preferred
way of isolating the PD substrate from the E/H field environment?

Ground everything!

It would help if you could post a schematic and some pictures.

John
 
J

John Larkin

Jan 1, 1970
0
I have always grounded the can directly to chassis resp. the ground
plane on the circuit boards. Reverse bias it, to gain a wee bit more BW
and avoid rectification of RF noise.


Reverse bias helps a lot. It not only reduces capacitance, mostly in
the first few volts, but higher voltages sweep charges out of the
junction faster. So use all the volts you can.

I usually select a (very clean!)

RC filter it close to the pd.

bias of about 80% of spec sheet limit. If BW is still an issue you can
cascode to reduce the capacitive load by the TIA. Phil Hobbs explains
this very nicely:

http://users.bestweb.net/~hobbs/frontends/frontends.pdf

His book is worth having if you're doing electro-optical stuff. It's
fun to read anyhow.

John
 
J

Joerg

Jan 1, 1970
0
John said:
Reverse bias helps a lot. It not only reduces capacitance, mostly in
the first few volts, but higher voltages sweep charges out of the
junction faster. So use all the volts you can.

Yes. But I don't dare to come closer than 80% to the abs max. Which can
be as low as 5V on modern ones so I usually go for 4V.
RC filter it close to the pd.

Absolutamente. Costs next to nothing.
His book is worth having if you're doing electro-optical stuff. It's
fun to read anyhow.

I've got to buy one. Not having read Phil's book before my last design
almost felt like driving without a license.
 
W

WhiteDog

Jan 1, 1970
0
Wow. So many other people withh nothing better to do on a Saturday ;-)
Thanks for the comments, I will head into the lab later this weekend.
Reverse bias helps a lot. It not only reduces capacitance, mostly in
the first few volts, but higher voltages sweep charges out of the
junction faster. So use all the volts you can.

I had tried just the cascode and reverse biasing, but I only had
available -5V reverse bias.
That did not get me the BW I needed. The Cd on the part is about 2.5pF
at that bias. I could
not resolve this, as I should have had an r_e on the cascode of about
725 ohms (should have been
a BW of about 87MHz).
RC filter it close to the pd.

Yep, I would do this, plus use a ferrite. Or maybe a capacitance
multiplier.
His book is worth having if you're doing electro-optical stuff. It's
fun to read anyhow.

I have this, it is quite a good book. The circuit is actually the
bootstrap plus cascode he covers in the book and the paper, so there's
the schematics (different op-amp in the transimpedance stage). I've
also corresponded with him several times for clarification (not that
he was unclear, I was just being dense). Quite a nice fellow.
 
W

Winfield Hill

Jan 1, 1970
0
John said:
It should.


If the pc board that contains the tia is securely grounded to the
case, and the pd can is grounded, and the leads from the pd to the
tia are short, it should work.


Ground everything!

It would help if you could post a schematic and some pictures.

Yes, perhaps.

There's sometimes an issue arising in high-gain preamps, with
the output stage's coax-driving current return path, and whether
any of that couples back to the input signals. For example, I
have made wideband high-gain amplifiers (e.g. for channeltrons,
etc., with 50MHz bandwidths and G = 5000), and I've found that
sometimes a single ground stud from the PCB to the box was a
problem, but ringing (or oscillations) stopped once I had two
ground studs, one near the input and one near the output.

With two ground connections to the box it's also helpful to split
the ground planes, mid-circuit (couple with a differential stage)
and use the box for the required dc-path connection. This scheme
prevents any of the high coax drive currents from flowing near the
input sensor and amplifier.

You can also add an electrostatic shield soldered to the input
ground plane, made from thin copper sheet stock, etc.

Another helpful trick you can use when only one ground plane, etc.,
is available and such modifications would be painful, is to use a
floating BNC for the output signal, carefully wiring the ground
return right back to the output stage's bypass cap and its ground
reference point. There may be a distant ground return, via the AC
power ground to a vacuum chamber, or whatever, but the distance may
eliminate any feedback problems. In some cases the high-frequency
isolation can be assured with a ferrite clamp around the coax cable.
 
J

Joerg

Jan 1, 1970
0
WhiteDog said:
Wow. So many other people withh nothing better to do on a Saturday ;-)
Thanks for the comments, I will head into the lab later this weekend.


I had tried just the cascode and reverse biasing, but I only had
available -5V reverse bias.
That did not get me the BW I needed. The Cd on the part is about 2.5pF
at that bias. I could
not resolve this, as I should have had an r_e on the cascode of about
725 ohms (should have been
a BW of about 87MHz).


Yep, I would do this, plus use a ferrite. Or maybe a capacitance
multiplier.


I have this, it is quite a good book. The circuit is actually the
bootstrap plus cascode he covers in the book and the paper, so there's
the schematics (different op-amp in the transimpedance stage). I've
also corresponded with him several times for clarification (not that
he was unclear, I was just being dense). Quite a nice fellow.

What is the P/N of your diode, and what opamp are you using? My last
design only needed 50MHz or so but I got well in excess of 100MHz, sans
cascode and with -4V reverse bias.

Pay close attention to the capacitances around your TIA. A stray pF here
and there can really ruin things.
 
J

Joerg

Jan 1, 1970
0
Winfield said:
Yes, perhaps.

There's sometimes an issue arising in high-gain preamps, with
the output stage's coax-driving current return path, and whether
any of that couples back to the input signals. For example, I
have made wideband high-gain amplifiers (e.g. for channeltrons,
etc., with 50MHz bandwidths and G = 5000), and I've found that
sometimes a single ground stud from the PCB to the box was a
problem, but ringing (or oscillations) stopped once I had two
ground studs, one near the input and one near the output.

Use all mounting holes for grounding to chassis. Plate them correctly,
i.e. nickel (do _not_ tin-plate). For long term performance make sure
mounting HW and studs are plated compatibly. For proto series unit I
usually order that stuff from McMaster.

With two ground connections to the box it's also helpful to split
the ground planes, mid-circuit (couple with a differential stage)
and use the box for the required dc-path connection. This scheme
prevents any of the high coax drive currents from flowing near the
input sensor and amplifier.

You can also add an electrostatic shield soldered to the input
ground plane, made from thin copper sheet stock, etc.

Another helpful trick you can use when only one ground plane, etc.,
is available and such modifications would be painful, is to use a
floating BNC for the output signal, carefully wiring the ground
return right back to the output stage's bypass cap and its ground
reference point. There may be a distant ground return, via the AC
power ground to a vacuum chamber, or whatever, but the distance may
eliminate any feedback problems. In some cases the high-frequency
isolation can be assured with a ferrite clamp around the coax cable.

Split or floating grounds are IMHO a recipe for disaster but it's still
taught at universities. OTOH that does provide a nice income for me
because I get to reverse all that. The last one was a week ago, but on
the way home I got stuck in traffic, big time :-(
 
J

John Larkin

Jan 1, 1970
0
Yes. But I don't dare to come closer than 80% to the abs max.


Why not? If it's designed and spec'd for X volts, why not run it at X
volts? Confession: sometimes we use schottky diodes and phemts at 2X.

Which can
be as low as 5V on modern ones so I usually go for 4V.

That will still not be completely "depleted", namely the capacitance
is still on the curvy part of the curve.

John
 
J

John Larkin

Jan 1, 1970
0
Wow. So many other people withh nothing better to do on a Saturday ;-)
Thanks for the comments, I will head into the lab later this weekend.


I had tried just the cascode and reverse biasing, but I only had
available -5V reverse bias.
That did not get me the BW I needed. The Cd on the part is about 2.5pF
at that bias. I could
not resolve this, as I should have had an r_e on the cascode of about
725 ohms (should have been
a BW of about 87MHz).

This gets 180 MHz from a 2.5 pF silicon pin diode at, I seem to
recall, -9 volts or some such. I just dumped it into a medium-fast
opamp as the tia, no cascode or bootstrapping.

http://www.highlandtechnology.com/DSS/J730DS.html


Bootstrapping is likely to cause more troubles than it's worth.

Yep, I would do this, plus use a ferrite. Or maybe a capacitance
multiplier.

Overkill. Just a cap would be a better hf ground than any active
c-multiplier.

John
 
J

John Larkin

Jan 1, 1970
0
Yes, perhaps.

There's sometimes an issue arising in high-gain preamps, with
the output stage's coax-driving current return path, and whether
any of that couples back to the input signals. For example, I
have made wideband high-gain amplifiers (e.g. for channeltrons,
etc., with 50MHz bandwidths and G = 5000), and I've found that
sometimes a single ground stud from the PCB to the box was a
problem, but ringing (or oscillations) stopped once I had two
ground studs, one near the input and one near the output.

A photodiode is the ideal floating signal source, so doesn't have
these problems.

My little e/o boxes have 4 pems in the bottom of the box, with the pcb
ground plane solidly screwed to all. We're doing a custom one now
where the board is screwed to several machined metal bars, with lots
of screws, and the bar are in turned screwed to the box. These are
serious electrical and thermal barriers.

The more you ground things, the happier you'll be.
With two ground connections to the box it's also helpful to split
the ground planes, mid-circuit (couple with a differential stage)
and use the box for the required dc-path connection. This scheme
prevents any of the high coax drive currents from flowing near the
input sensor and amplifier.

NEVER split ground planes. NEVER use multiple ground planes.
You can also add an electrostatic shield soldered to the input
ground plane, made from thin copper sheet stock, etc.

Another helpful trick you can use when only one ground plane, etc.,
is available and such modifications would be painful, is to use a
floating BNC for the output signal, carefully wiring the ground
return right back to the output stage's bypass cap and its ground
reference point. There may be a distant ground return, via the AC
power ground to a vacuum chamber, or whatever, but the distance may
eliminate any feedback problems. In some cases the high-frequency
isolation can be assured with a ferrite clamp around the coax cable.

We seem to be philosophically incompatible.

John
 
W

Winfield Hill

Jan 1, 1970
0
Joerg said:
Use all mounting holes for grounding to chassis. Plate them correctly,
i.e. nickel (do _not_ tin-plate). For long term performance make sure
mounting HW and studs are plated compatibly. For proto series unit I
usually order that stuff from McMaster.




Split or floating grounds are IMHO a recipe for disaster but it's still
taught at universities. OTOH that does provide a nice income for me
because I get to reverse all that. The last one was a week ago, but on
the way home I got stuck in traffic, big time :-(

The split ground is _inside_ the small box and is a totally
appropriate way to control where the cable-driving currents go,
thank you. Without it, even your nickel-plated contact may one
day become insufficient, with it, the longterm contact quality
is _much_ less important, if at all. You don't think extremely
careful control of the RF current pathways is important in
high-gain wideband amplifiers? I seriously doubt that.

As for whether it's taught at universities, I wouldn't know -
that's across town from me. I'm an engineer, not a professor.

My suggestion for a floating output BNC with ferrite clamp was
stated to be for a poor bloke who has to live with what's in
front of him, and needs a way to get it working. In any case
it's a more reliable technique than you might imagine. Keep in
mind it would be employed by the user of the boxes in question.
 
J

Joerg

Jan 1, 1970
0
John said:
Why not? If it's designed and spec'd for X volts, why not run it at X
volts? Confession: sometimes we use schottky diodes and phemts at 2X.

Well, the client has to pay for another 50 or so photodiodes per system
if the PS hicks up ;-)
That will still not be completely "depleted", namely the capacitance
is still on the curvy part of the curve.

I've looked at that with a Hamamatsu and a JDS diode and didn't find too
much change. IOW we didn't need the last few percent in BW. Initially I
thought about making that programmable but then thought about the
consequences if someone accidentally steps onto that register.
 
J

Joerg

Jan 1, 1970
0
Winfield said:
The split ground is _inside_ the small box and is a totally
appropriate way to control where the cable-driving currents go,
thank you. Without it, even your nickel-plated contact may one
day become insufficient, with it, the longterm contact quality
is _much_ less important, if at all. You don't think extremely
careful control of the RF current pathways is important in
high-gain wideband amplifiers? I seriously doubt that.

I've seen it go wrong almost every single time. In theory a split works,
of course. In reality stuff has to be connected to the individual sides
and suddenly you've got loop antennas. Even in case of opto electronics
because the stuff on the PD side of the split must be supplied with
juice. Now you could do that with fully isolated forward converters and
stuff but with more than one voltage that gets old. Then again I may be
a bit biased here because many of my designs must work in rather nasty
environments. Defibrillators, diathermia gear, elevators and so on.

As for whether it's taught at universities, I wouldn't know -
that's across town from me. I'm an engineer, not a professor.

IMHO the real education happens at institutes. That's where I learned
some of the ropes (RF Institute at the RWTH Aachen in Germany). Seems
not to happen much anymore, most of the young grads I interviewed could
not even solder (!).

My suggestion for a floating output BNC with ferrite clamp was
stated to be for a poor bloke who has to live with what's in
front of him, and needs a way to get it working. In any case
it's a more reliable technique than you might imagine. Keep in
mind it would be employed by the user of the boxes in question.

In a pinch a ferrite can "make it work". I do that, too, but only when
there is no other option. It's like pain pills, makes the symptoms go
away ;-)

The last system with isolated coax connection and ferrites was
ultrasound and before throwing out the split grounds the guys were
wondering why the image background "lights up" if someone touches the
transducer jacket.
 
J

Joerg

Jan 1, 1970
0
John said:
A photodiode is the ideal floating signal source, so doesn't have
these problems.

My little e/o boxes have 4 pems in the bottom of the box, with the pcb
ground plane solidly screwed to all. We're doing a custom one now
where the board is screwed to several machined metal bars, with lots
of screws, and the bar are in turned screwed to the box. These are
serious electrical and thermal barriers.

The more you ground things, the happier you'll be.


NEVER split ground planes. NEVER use multiple ground planes.

Shhht! Not so loud. This is 30% of my revenue source. Just imagine a
body shop when nobody crashes their cars anymore.

We seem to be philosophically incompatible.

So, on to politics then?
 
W

Winfield Hill

Jan 1, 1970
0
I've seen it go wrong almost every single time. In theory a split works,
of course. In reality stuff has to be connected to the individual sides
and suddenly you've got loop antennas. Even in case of opto electronics
because the stuff on the PD side of the split must be supplied with
juice. Now you could do that with fully isolated forward converters and
stuff but with more than one voltage that gets old. Then again I may be
a bit biased here because many of my designs must work in rather nasty
environments. Defibrillators, diathermia gear, elevators and so on.

Not to beat a dead horse, but to be very clear, the split ground
plane
is for the purpose of eliminating any loops, or more importantly, of
preventing two possible paths for the output return current.
Remember,
both planes are grounded to the insides of the small box (the input
is
an isolated PD sensor). And I will strongly state, you will NOT ever
find it "going wrong." Quite the contrary. Ahem, may I assume we're
talking about the same thing here?
IMHO the real education happens at institutes. That's where I learned
some of the ropes (RF Institute at the RWTH Aachen in Germany). Seems
not to happen much anymore, most of the young grads I interviewed could
not even solder (!).

OK, I'll take that as a compliment for the Rowland Institute,
if I may. (I'll take whatever I can get.)
 
J

Joerg

Jan 1, 1970
0
Winfield said:
Not to beat a dead horse, but to be very clear, the split ground
plane
is for the purpose of eliminating any loops, or more importantly, of
preventing two possible paths for the output return current.
Remember,
both planes are grounded to the insides of the small box (the input
is
an isolated PD sensor). And I will strongly state, you will NOT ever
find it "going wrong." Quite the contrary. Ahem, may I assume we're
talking about the same thing here?

Maybe we aren't. If you bolt it all to chassis inside the box you don't
really have a split ground anymore. Might as well make it one plane then :)

OK, I'll take that as a compliment for the Rowland Institute,
if I may. (I'll take whatever I can get.)

Yes, institutes can play a major role in teaching young students about
the real stuff. They might cause grief and even destroy this, that and
the other thing but the value to society you guys are adding by taking
them in is huge. AoE is the other huge contribution. I guess I may be
responsible for sales of a few dozen copies, telling young grads that no
matter what their professors say "you've got to have this book".

Academia alone seems unable to do that hands-on anymore. One client
tried out three grads and finally the forth turned out to be a good fit.
Imported from Canada...
 
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