Amplifier design pre-consultation consultation

[[email protected]]

Greetings!

I'm a research scientists with the Johns Hopkins University, and I'm
working on a set of designs for an X-ray detector, and trying to spec
out various methods for obtaining the data we need. One of the designs
is a system based on diode arrays + amplifiers + ADC system. I've
already got a good handle on the detector end, and the data
acquisition system, but I'm stuck on the amplifier system.

We've used commercial amplifiers in the past, but they would likely be
overkill for our situation, and end up quite pricey on a cost/channel
basis. Given our specifications, I'm wondering if the optimal solution
would be to pay for a consultant to develop and test a design
specifically for our application, and then take that design and punch
out the number of boards that we would need.

Our generic needs seem to be fairly modest, 100-250kHz bandwidth with
a gain of 10^7, but as always the devil is in the details. Naturally,
we want the lowest noise possible so that we can measure signals at
the nA or sub-nA level.

So, here's the question. Are the specifications and schematic sketch
shown here:

http://picasaweb.google.com/ktritz/PhotodiodeAmplifierDesign/photo#5218836415390875906

adequate for a professional to provide a consultation estimate? Would
the amplifiers be simple enough that a 2nd year EE student could
manage the design, or are we talking about skirting the bleeding edge?

I'm never contracted a consultant before, so should I expect a
consulting price tag of $1000? $10000? I'm working with a budget
that's higher than a hobbyist, but not quite corporation level.

I would also be happy to discuss specific amplifier design ideas.
Given the capacitance of the detectors in question, I would imagine
that a very low voltage noise opamp is the way to go, or perhaps a
JFET front end. The BF862 looks pretty good, and it's relatively high
capacitance wouldn't matter much compared to the diode.

Thanks,
Kevin

 

[Tim Shoppa]

Greetings!

I'm a research scientists with the Johns Hopkins University, and I'm
working on a set of designs for an X-ray detector, and trying to spec
out various methods for obtaining the data we need. One of the designs
is a system based on diode arrays + amplifiers + ADC system. I've
already got a good handle on the detector end, and the data
acquisition system, but I'm stuck on the amplifier system.

We've used commercial amplifiers in the past, but they would likely be
overkill for our situation, and end up quite pricey on a cost/channel
basis. Given our specifications, I'm wondering if the optimal solution
would be to pay for a consultant to develop and test a design
specifically for our application, and then take that design and punch
out the number of boards that we would need.

Our generic needs seem to be fairly modest, 100-250kHz bandwidth with
a gain of 10^7, but as always the devil is in the details. Naturally,
we want the lowest noise possible so that we can measure signals at
the nA or sub-nA level.

So, here's the question. Are the specifications and schematic sketch
shown here:

http://picasaweb.google.com/ktritz/PhotodiodeAmplifierDesign/photo#52...

adequate for a professional to provide a consultation estimate? Would
the amplifiers be simple enough that a 2nd year EE student could
manage the design, or are we talking about skirting the bleeding edge?

I'm never contracted a consultant before, so should I expect a
consulting price tag of $1000? $10000? I'm working with a budget
that's higher than a hobbyist, but not quite corporation level.

I would also be happy to discuss specific amplifier design ideas.
Given the capacitance of the detectors in question, I would imagine
that a very low voltage noise opamp is the way to go, or perhaps a
JFET front end. The BF862 looks pretty good, and it's relatively high
capacitance wouldn't matter much compared to the diode.

You don't say what part of Johns Hopkins you work at, but go talk to
the particle and nuclear experimental physicists. Diode detectors
followed by amps followed by A/D and triggers are their bread and
butter. Google "silicon strip detectors" and find somebody who has
worked in it more recently than me (1980's!).

Tim.

 

[Kevin]

You don't say what part of Johns Hopkins you work at, but go talk to
the particle and nuclear experimental physicists. Diode detectors
followed by amps followed by A/D and triggers are their bread and
butter. Google "silicon strip detectors" and find somebody who has
worked in it more recently than me (1980's!).

Tim.

That certainly is a good suggestion, though often their designs are
more
suited to high speed pulse shaping/counting. Also, I'm actually
located
offsite at the Princeton Plasma Physics Laboratory, though I do have
a few colleagues on campus that might be able to point me in the
right direction.

Another option is the Applied Physics Lab, but I have a feeling that
unless
you have an actual collaboration with them, getting their engineers
involved
is a 10k or higher proposition. My budget might be a bit constrained
compared
to what they are used to.

Kevin

 

[Joerg]

On Jul 3, 1:43 pm, [email protected] wrote:

Ok, Kevin, can't see your post and won't see replies (gmail account?)
but let me comment by tacking on to Tim's post:


I assume that's 100kHz to 250kHz, right?



Basically yes. You'd have to add things like: Production volume? How is
this power-supplied? What environment EMI-wise? $40/ch is quite
realistic but only for large production volumes, of course. Not if you
have to do small boutique runs for circuit boards.

Much of this will have to shake out during the initial design phase, a
fixed bid isn't quite feasible here.

... Would

Not manage, let him/her do it. But a 2nd year student will need
consulting help unless he/she has tons of ham radio or hobby project
experience.


If you want a complete design with layout and all, that will be five
digit Dollars. Since you are at a university why not engage the help of
more students? Good ones will be dying for meaningful hardware projects.
Sure, they'll get stuck here and there and for that case you should line
up a consultant. That's what even many industrial clients do. They sign
up with me and call me only when they get stuck. Then they are only
billed for the hours I helped them but the bulk of the work was done
in-house. An upside is that this way they keep core expertise in-house,
IOW by the end of the project there will be people who know the stuff
inside out.

And there's always this newsgroup


One would have to sit down and scope out what's out there. Chance are,
at this frequency you can beat the JFET with an opamp. That would be
followed by more amps to get the desired gain.

You don't say what part of Johns Hopkins you work at, but go talk to
the particle and nuclear experimental physicists. Diode detectors
followed by amps followed by A/D and triggers are their bread and
butter. Google "silicon strip detectors" and find somebody who has
worked in it more recently than me (1980's!).

Technical comments: The cables lengths are a problem. Mind the
surroundings, there will usually be lots of noise sources. Switch mode
supplies, PFC or variable frequency drives in elevators etc. All this
operates smack dab in your band of interest. 3ft to the diodes is going
to be tough. Same for the 50ft to the ADCs. Why that long? Can you do a
digital link instead? if not you might want to consider fiberoptics or
modulate in onto a carrier somewhere in a quite corner of the RF
spectrum. 100kHz-250kHz will be one hellacious noise bucket unless the
installation is on a remote island or completely shielded.

Can you use a mail domain other than Google? They worked up a bad
reputation because of spam and some folks here have that blocked.

 

[Kevin Tritz]

Ok, Kevin, can't see your post and won't see replies (gmail account?)
but let me comment by tacking on to Tim's post:

Ok, switched to my Verizon account, hopefully this will work for
everyone.

I assume that's 100kHz to 250kHz, right?

Correct, thanks.

Basically yes. You'd have to add things like: Production volume? How
is this power-supplied? What environment EMI-wise? $40/ch is quite
realistic but only for large production volumes, of course. Not if
you have to do small boutique runs for circuit boards.

Much of this will have to shake out during the initial design phase,
a fixed bid isn't quite feasible here.

I think the estimated production run is on the document, anywhere from
~250-800 channels. When I've investigated parts and PCB manufacture, it
seems like I could probably get by with ~$20 in parts, and ~$5-10 for
the PCB. The assembly is where I have no information.

The EMI environment is pretty ferocious actually, so shielding and
grounding will be very important.

Not manage, let him/her do it. But a 2nd year student will need
consulting help unless he/she has tons of ham radio or hobby project
experience.



If you want a complete design with layout and all, that will be five
digit Dollars. Since you are at a university why not engage the help
of more students? Good ones will be dying for meaningful hardware
projects. Sure, they'll get stuck here and there and for that case
you should line up a consultant. That's what even many industrial
clients do. They sign up with me and call me only when they get
stuck. Then they are only billed for the hours I helped them but the
bulk of the work was done in-house. An upside is that this way they
keep core expertise in-house, IOW by the end of the project there
will be people who know the stuff inside out.

And there's always this newsgroup

I'm actually a scientist stationed at a national lab (PPPL), so I don't
have that much of a connection with the engineering department at Johns
Hopkins. I could try and forge a connection. This newsgroup has been
pretty valuable for ideas and component suggestion. In our immediate
group, I probably have the most knowledge and experience, and that is
pretty slim as it is. I have some access to the engineers here at the
national lab, so I might try and have them assist in the design.

One would have to sit down and scope out what's out there. Chance
are, at this frequency you can beat the JFET with an opamp. That
would be followed by more amps to get the desired gain.

Generically, the circuit from the Linear Systems design note:

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C10
26,D16998

looks like it would work for our application. They have a 1M feedback
resistor, but are also speccing a high bandwidth than we need. Of
course, I realize that there's quite a distance between a circuit in an
AppNote, and a realized PCB design that actually works.

Technical comments: The cables lengths are a problem. Mind the
surroundings, there will usually be lots of noise sources. Switch
mode supplies, PFC or variable frequency drives in elevators etc. All
this operates smack dab in your band of interest. 3ft to the diodes
is going to be tough. Same for the 50ft to the ADCs. Why that long?
Can you do a digital link instead? if not you might want to consider
fiberoptics or modulate in onto a carrier somewhere in a quite corner
of the RF spectrum. 100kHz-250kHz will be one hellacious noise bucket
unless the installation is on a remote island or completely shielded.

Can you use a mail domain other than Google? They worked up a bad
reputation because of spam and some folks here have that blocked.

Unfortunately, there is not much to be done about the cables. The
detectors have to be inside of a vacuum chamber, and the electronics
are not generally vacuum compatible. One of the options I'm considering
is a vacuum compatible front-end, but that would severly restrict the
available components.

The ADCs are located further from the machine to get the computers and
other associated hardware out of the radiation environment. EMI
shielding will be of utmost priority, and we do have a fair amount of
flexibility with the chassis, so I could build it out of 1/4" copper if
need be.

I had toyed with the idea of trying to do this with a vacuum compatible
ASIC which would incorporate an amplifier, multiplexed ADC, and digital
output right near the detector head, but my guess is that would break
our budget. I'm also not sure if we could get the required bandwidth
out of such a system.

 

[Joerg]

Ok, switched to my Verizon account, hopefully this will work for
everyone.

Yes, thanks, that works great.

Correct, thanks.

Makes life easier. A lot. Last time I had to deal with noise down to
about 5Hz and that is no fun at all because of not well defined 1/f
noise-knees.

I think the estimated production run is on the document, anywhere from
~250-800 channels. When I've investigated parts and PCB manufacture, it
seems like I could probably get by with ~$20 in parts, and ~$5-10 for
the PCB. The assembly is where I have no information.

Ok, I thought that was channels per system. The last really small
prototype run I did was 40 channels (four per board, so 10 boards) and
it came in just under $3k total for assembly (non-RoHS). But the next
one would be under $2k since the stencils and programming will be
re-used. I think $40/channel can be done at 800.

The EMI environment is pretty ferocious actually, so shielding and
grounding will be very important.

Then I'd really consider moving at least part of the amp right up to the
diodes. Or shield/diff the heck out of it but that will not be easy.
Often EMI efforts cost more time than the actual design.

I'm actually a scientist stationed at a national lab (PPPL), so I don't
have that much of a connection with the engineering department at Johns
Hopkins. I could try and forge a connection. This newsgroup has been
pretty valuable for ideas and component suggestion. In our immediate
group, I probably have the most knowledge and experience, and that is
pretty slim as it is. I have some access to the engineers here at the
national lab, so I might try and have them assist in the design.

Ah, Princeton. Even back in the 80's when I was studying for my masters
we were always looking for outside projects. Sometimes as course
projects where we had to complete two mandatory ones or just as paid
work. I built a lot of RF stuff back then to augment my
beer/food/parachuting budget. Later HW projects became scarce at our own
institutes and students would almost start fist fights over who'd get
in. Many went outside academia for that, even for their big masters
project. I don't think finding someone should be a problem. The tough
part will be to find a student with at least some practical know-how. A
ham radio license is usually a pretty good indicator, if the student has
built some stuff from scratch for their hobby or for others.

Generically, the circuit from the Linear Systems design note:

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C10
26,D16998

looks like it would work for our application. They have a 1M feedback
resistor, but are also speccing a high bandwidth than we need. Of
course, I realize that there's quite a distance between a circuit in an
AppNote, and a realized PCB design that actually works.

That's the way it is usually done, plus follower amps for more gain.
Phil Hobbs wrote a great article about the topic:
http://www.electrooptical.net/www/frontends/frontends.pdf

He can be found here in the newsgroup quite often.

A word of caution: The LTC6244 is non-stock at Digikey for all versions.
Usually not a good sign. But there are others.

Unfortunately, there is not much to be done about the cables. The
detectors have to be inside of a vacuum chamber, and the electronics
are not generally vacuum compatible. One of the options I'm considering
is a vacuum compatible front-end, but that would severly restrict the
available components.

Ok, depends on how much of a vacuum and whether contamination by the
electronic box is a concern. Potting and/or local pressurizing might be
an option but I am not an expert for vacuum situations. 3ft of cable in
a noisy environment is no small feat. The photodiode is only a weak
current source, almost like a whisper at a rock concert.

The ADCs are located further from the machine to get the computers and
other associated hardware out of the radiation environment. EMI
shielding will be of utmost priority, and we do have a fair amount of
flexibility with the chassis, so I could build it out of 1/4" copper if
need be.

There are ways to do it. The low-tech way with shields will make for a
bulky and pretty stiff cable. You'll likely end up with as many twin-ax
cables as there are channels in a system, plus maybe a large metal
conduit for them. Basically similar to aircraft wiring.

Modulation or FO would both increase the BOM budget and R&D expenses
while reducing cables costs and providing better noise margins. It's
just one of those compromises that have to be weighed and pondered.

Another option is to place the ADCs on board and pipe the data over
serially. You'll reach Ethernet speeds but 50ft are easy for that. Lots
of work though. What receives the data? Does that card already exist in
a shape where a change is not feasible anymore?

I had toyed with the idea of trying to do this with a vacuum compatible
ASIC which would incorporate an amplifier, multiplexed ADC, and digital
output right near the detector head, but my guess is that would break
our budget. I'm also not sure if we could get the required bandwidth
out of such a system.

I am sure it could but unless you can get almost free IC design help
plus a MOSIS MPW run or something like that it'll break the budget, big
time. Longterm this might be the best avenue. And the most expensive in
design.

 

[[email protected]]

Ok, switched to my Verizon account, hopefully this will work for
everyone.







Correct, thanks.








I think the estimated production run is on the document, anywhere from
~250-800 channels. When I've investigated parts and PCB manufacture, it
seems like I could probably get by with ~$20 in parts, and ~$5-10 for
the PCB. The assembly is where I have no information.

The EMI environment is pretty ferocious actually, so shielding and
grounding will be very important.









I'm actually a scientist stationed at a national lab (PPPL), so I don't
have that much of a connection with the engineering department at Johns
Hopkins. I could try and forge a connection. This newsgroup has been
pretty valuable for ideas and component suggestion. In our immediate
group, I probably have the most knowledge and experience, and that is
pretty slim as it is. I have some access to the engineers here at the
national lab, so I might try and have them assist in the design.





Generically, the circuit from the Linear Systems design note:

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C10
26,D16998

Everybody wants to get a bit of business that the Burr-Brown (now
Texas Instrument) FET-input OPA656 hogs.

http://focus.ti.com/lit/ds/symlink/opa656.pdf

looks like it would work for our application. They have a 1M feedback
resistor, but are also speccing a high bandwidth than we need. Of
course, I realize that there's quite a distance between a circuit in an
AppNote, and a realized PCB design that actually works.

Linear Systems has published some great application notes but that may
not be one of them

Unfortunately, there is not much to be done about the cables. The
detectors have to be inside of a vacuum chamber, and the electronics
are not generally vacuum compatible. One of the options I'm considering
is a vacuum compatible front-end, but that would severly restrict the
available components.

I did quite a lot of work at Cambridge Instruments (UK) on their
electron microscopes. They operate with a "chemical vacuum" - o-ring
seals and no baking-out - and the only real issue for electronics in
the vacuum chamber was heat dissipation in the absence of convection.
Good wide thermal conduction paths to structural metal-work mostly
worked pretty well. If you need a physical vacuum close to the
detector you might get away with baffles and differential pumping ...

The ADCs are located further from the machine to get the computers and
other associated hardware out of the radiation environment. EMI
shielding will be of utmost priority, and we do have a fair amount of
flexibility with the chassis, so I could build it out of 1/4" copper if
need be.

Four (or more) layer boards with buried ground and power planes are
surprisingly insensitive to external fields. Joerg has publicly
advocated burying signal lines as strip-lines in inner layers (between
ground planes) though this does make it difficult to get
characteristic impedances over 50 ohms. Fanatics have been known to
use semi-rigid coaxial cable (or conformable coaxial cable which
relies on soaking the outer braid with solder) for maximal screening
on cable links. The bonus is that the coaxial connections are good up
to a few GHz (18GHz with SMA connectors. when I last looked).

I had toyed with the idea of trying to do this with a vacuum compatible
ASIC which would incorporate an amplifier, multiplexed ADC, and digital
output right near the detector head, but my guess is that would break
our budget. I'm also not sure if we could get the required bandwidth
out of such a system.

Overkill if you don't need a really good physical vacuum right up
against the detector.

 

[Joerg]

On Thu, 3 Jul 2008 18:11:05 -0700 (PDT), the renowned


I suspect he's more worried about outgassing than the effect on the
electronics.

Do you have any info on that? I wouldn't think that ceramic hermetic
parts or chip passives would be much of a problem. It's probably
possible to get big "hybrid" packages with solder-on lids.

Problem is that defense went COTS to a large extent and not many parts
are available in ceramic anymore. Those that are usually break the bank
in terms of BOM budget.

Four (or more) layer boards with buried ground and power planes are
surprisingly insensitive to external fields. Joerg has publicly
advocated burying signal lines as strip-lines in inner layers (between
ground planes) though this does make it difficult to get
characteristic impedances over 50 ohms. Fanatics have been known to
use semi-rigid coaxial cable (or conformable coaxial cable which
relies on soaking the outer braid with solder) for maximal screening
on cable links. The bonus is that the coaxial connections are good up
to a few GHz (18GHz with SMA connectors. when I last looked).

Speaking of connectors- the housing, cable and connectors could
*easily* dominate the cost of this system-- part cost, lead time and
even assembly cost. Even without the 1/4" copper (shudder) the OP
suggested. [Lead might be better (if you can keep it cold enough), but
it's not RoHS. ;-) ]

That's where creativeness comes in. Looking into potting compounds and
such. If you can combine amp and detectors the problem may be reduced to
one connector at the system side and that can be a cheap shielded
multi-pin type. It doesn't have to be a Lemo.

[...]

 

[[email protected]]

I suspect he's more worried about outgassing than the effect on the
electronics.

Epoxy packages and FR4 boards are fine in a chemical vacuum. I'm sure
that they do outgas to some extent, but we had moving parts in our
vacuum chamber and they had to be minimally lubricated with some kind
of fluorcarbon grease, which also outgassed.Every metal surface in the
vacuum chamber carried a monolayer of what turned out to be some
kindof carbon compound - anyplace that got hit by a lot of electrons
would eventually develop a visible black spot of insulating gunge. In
my stroboscopic electron microscope the "off" electron beam hit a spot
about 0.1mm from the 0,1 mm diameter hole, and initially we had to
pull out and clean the apperture every day because the gunge would
charge up and push the beam off-axis.

We solved the problem by using a thin film apperture (where the metal
doing the blocking was only a few microns thick). We still got our
spot of gunge, but the parked beam kept the metal hot enough that
gunge was mainly graphite, and conductive.

Speaking of connectors- the housing, cable and connectors could
*easily* dominate the cost of this system-- part cost, lead time and
even assembly cost.  Even without the 1/4" copper (shudder) the OP
suggested. [Lead might be better (if you can keep it cold enough), but
it's not RoHS. ;-) ]

Vacuum tight electrical connectors are available - not all that
easily - and they certainly aren't cheap. In the late 1980's you
could buy vacuum rated BNC sockets (with an O-ring seal), but SMA/SMB
sized stuff tended to be improvised.

 

[[email protected]]

I suspect he's more worried about outgassing than the effect on the
electronics.

Epoxy packages and FR4 boards are fine in a chemical vacuum. I'm sure
that they do outgas to some extent, but we had moving parts in our
vacuum chamber and they had to be minimally lubricated with some kind
of fluorcarbon grease, which also outgassed.Every metal surface in the
vacuum chamber carried a monolayer of what turned out to be some
kindof carbon compound - anyplace that got hit by a lot of electrons
would eventually develop a visible black spot of insulating gunge. In
my stroboscopic electron microscope the "off" electron beam hit a spot
about 0.1mm from the 0,1 mm diameter hole, and initially we had to
pull out and clean the apperture every day because the gunge would
charge up and push the beam off-axis.

We solved the problem by using a thin film apperture (where the metal
doing the blocking was only a few microns thick). We still got our
spot of gunge, but the parked beam kept the metal hot enough that
gunge was mainly graphite, and conductive.

Speaking of connectors- the housing, cable and connectors could
*easily* dominate the cost of this system-- part cost, lead time and
even assembly cost. �Even without the 1/4" copper (shudder) theOP
suggested. [Lead might be better (if you can keep it cold enough), but
it's not RoHS. ;-) ]

Vacuum tight electrical connectors are available - not all that
easily - and they certainly aren't cheap. In the late 1980's you
could buy vacuum rated BNC sockets (with an O-ring seal), but SMA/SMB
sized stuff tended to be improvised.

The long cables are likely to be a real problem, and expensive.

The OP will need at least one vacuum leadthrough per photodiode
channel if the amplifiers are located outside the vacuum chamber with
long cables. Let's consider that a "sunk cost" that will be incurred
no matter what, unless vacuum compatible electonics can be found (and
that implicitly contains lots of vacuum leadthroughs, it's just built
into the component price).

Why not build all of the electronics inside a pressurised (1
atmosphere *) welded or soldered metal can that goes inside the vacuum
chamber with vacuum seals to connect to the photodiodes. Amplify and
digitise the signal inside this can, and send the data either over a
single high speed digital link, or an optical link as others have
suggested. The cost of these circuits will easily be paid for by the
cheaper cable. If the chamber needs to be baked, then put thermal
insulation around the components inside the sealed metal can, so that
the can gets hot but the components do not. If you need really good
insulation, use a Dewar (stainless steel thermos flask). If the
electronics dissipates too much heat and needs to be kept cool, or if
you want to bake the system for days, then you could run a couple of
small diameter metal water pipes from the lab through the necessary
vacuum seals, through the electronics can and back out again, to water
cool the PCB.

* Having an air pipe to vent the inside of the can to atmosphere might
also help in diagnosing leaks - if you are using a helium mass
spectrometer to find the leaks then you could squirt helium into the
can when you want to know if the can is the source of your slow
leak...

Chris

 

[axolotl]

The ADCs are located further from the machine to get the computers and
other associated hardware out of the radiation environment. EMI
shielding will be of utmost priority, and we do have a fair amount of
flexibility with the chassis, so I could build it out of 1/4" copper if
need be.

I had toyed with the idea of trying to do this with a vacuum compatible
ASIC which would incorporate an amplifier, multiplexed ADC, and digital
output right near the detector head, but my guess is that would break
our budget. I'm also not sure if we could get the required bandwidth
out of such a system.

Olin College of Engineering's SCOPE program might be a good fit.

http://scope.olin.edu/index.cfm?itemid=PRA

Kevin Gallimore

 

[Tim Shoppa]

That certainly is a good suggestion, though often their designs are
more
suited to high speed pulse shaping/counting. Also, I'm actually
located
offsite at the Princeton Plasma Physics Laboratory, though I do have
a few colleagues on campus that might be able to point me in the
right direction.

Another option is the Applied Physics Lab, but I have a feeling that
unless
you have an actual collaboration with them, getting their engineers
involved
is a 10k or higher proposition. My budget might be a bit constrained
compared
to what they are used to.

Kevin -

Having looked at other requirements you've mentioned (Vacuum
compatibility, cabling, noise) you don't need just an EE to design a
pre-amp for you. At the rate I see requirements appearing, your
proposed scheme with all its cabling will make the preamp engineering
and construction costs less than 1 percent of the total price of
instrumentation. Most of your cost is going to go into cables and
connectors!

You need someone with broader experience in experimental design,
construction, integration, and data acquisition. An EE could certainly
be part of this team, but it would be one with experience in
integrating such experiments into your particular environmental
requirements.

When I was in the strip detector business, the preamps were mounted
at the detector, and then there were massive bundles of differentially
driven twisted pairs in ribbon cables ("Twist-N-Flat") running
hundreds of feet to the big electronics. This was pretty "hot stuff"
and in fact the trigger and A/D converters were massive racks cooled
by chilled water. (Now, I also know chilled water is also something
common to plasma experiment cooling!)

But that was being done with essentially late 70's technology -
electronics had evolved rapidly over the 80's and far more integrated
solutions were coming available. By the early 90's, designs had
realized that all that cabling wasn't optimal - instead integrated
solutions with preamp, trigger, A/D all mounted within inches of the
detector were being done. Again, google "silicon strip detector" to
see the compromises and solutions.

Tim.

 

[Joerg]

Kevin -

Having looked at other requirements you've mentioned (Vacuum
compatibility, cabling, noise) you don't need just an EE to design a
pre-amp for you. At the rate I see requirements appearing, your
proposed scheme with all its cabling will make the preamp engineering
and construction costs less than 1 percent of the total price of
instrumentation. Most of your cost is going to go into cables and
connectors!

You need someone with broader experience in experimental design,
construction, integration, and data acquisition. An EE could certainly
be part of this team, but it would be one with experience in
integrating such experiments into your particular environmental
requirements.

When I was in the strip detector business, the preamps were mounted
at the detector, and then there were massive bundles of differentially
driven twisted pairs in ribbon cables ("Twist-N-Flat") running
hundreds of feet to the big electronics. This was pretty "hot stuff"
and in fact the trigger and A/D converters were massive racks cooled
by chilled water. (Now, I also know chilled water is also something
common to plasma experiment cooling!)

But that was being done with essentially late 70's technology -
electronics had evolved rapidly over the 80's and far more integrated
solutions were coming available. By the early 90's, designs had
realized that all that cabling wasn't optimal - instead integrated
solutions with preamp, trigger, A/D all mounted within inches of the
detector were being done. Again, google "silicon strip detector" to
see the compromises and solutions.

Although, back in the 70's I remember jumping up and down when I
discovered the uA733. Tons of bandwidth, modest consumption. I used to
do all this stuff with Harris HA2540 and similar chips but they got hot
and were very expensive. However, later the ceramic DIP version and the
LCC disappeared from the marketplace :-(

 

[Joerg]

Yup, the 733 used to be hot stuff. Nowadays you can get a real opamp
with g=10 at 1.8 GHz, or a sub-dollar 8 GHz MMIC. Makes it almost too
easy. ...

MMIC are great but I often can't use them because of channel to channel
gain tolerances. Most imaging systems must remain within +/-0.5dB.
Factory trim is frowned upon and pretty much off-limits in this here
office. Opamps are really cool but they can hardly touch the uA733 in
terms of cost. 35-40c a pop.

... Hell, a coke at Zeitgeist costs 2 bucks.

Don't they have Russian River IPA for around $10/pitcher? Much better
deal and hugely better tasting

 

[JosephKK]

Ok, switched to my Verizon account, hopefully this will work for
everyone.

Interesting, really high gain is best accomplished in stages, then you
can put very low power front ends in vacuum. The current levels are a
bit challenging as well. My normal reaction to seeing that high of a
gain specification is "What went wrong in the design?" I am not sure
that it applies to this situation. Vacuum without conductive heat
relief is difficult, the water pipe suggestion should be considered.

 

[neon]

for A=10x7 measuring na you are looking for some very special amplifier. At this time i don't know if there are any such animal around off the shelf.

 

[Kevin Tritz]

account?) >> but let me comment by tacking on to Tim's post:
and >> > > I'm working on a set of designs for an X-ray detector, and
trying >> > > to spec out various methods for obtaining the data we
need. One >> > > of the designs is a system based on diode arrays +
amplifiers + >> > > ADC system. I've already got a good handle on the
detector end, >> > > and the data acquisition system, but I'm stuck
on the amplifier >> > > system.
on >> > > a cost/channel basis. Given our specifications, I'm
wondering if >> > > the optimal solution would be to pay for a
consultant to develop >> > > and test a design specifically for our
application, and then take >> > > that design and punch out the
number of boards that we would need. >> > >
bandwidth >> > > with a gain of 10^7, but as always the devil is in
the details. >> > > Naturally, we want the lowest noise possible so
that we can >> > > measure signals at the nA or sub-nA level.

Interesting, really high gain is best accomplished in stages, then you
can put very low power front ends in vacuum. The current levels are a
bit challenging as well. My normal reaction to seeing that high of a
gain specification is "What went wrong in the design?" I am not sure
that it applies to this situation. Vacuum without conductive heat
relief is difficult, the water pipe suggestion should be considered.

There are specific constraints that set the level of current.
Basically, we are trying to image the edge of a fusion-grade plasma
with high spatial (< 1cm) and time resolution (100kHz-250kHz). The
diodes measure the X-rays from the plasma, but the x-rays of this
energy only travel through a vacuum, so the diodes need to be in the
vacuum with the plasma. It's a high vacuum environment (~10-8 torr), so
the components need to be "clean" (little to no outgassing). There
could be some conductive heat relief by heatsinking to the vessel wall,
though I would have to be careful of the grounds.

Given the constraints on the device, things like in-vessel water
cooling, or a sealed can at atmospheric pressure just aren't possible,
or rather they would not be allowed unless the managers of the device
believed that this diagnostic was absolutely critical. There would be
too much of a risk that a leak would cost run-time on the device.

The lower the current we can measure, the better we will be able to see
the real "edge" of the plasma, where the signal goes to zero. And the
spatial resolution and distance to the plasma limits the size of the
aperature and detector that we can use.

It seems like the task is a bit more daunting than I had initially
hoped, but the suggestions and information in this thread have been
quite valuable.

These are the vacuum feedthroughs I was hoping to use:

http://accuglassproducts.com/product.php?productid=16235&cat=328&page=1

which could handle 80 single-ended signals (with a good shield). If I
had to go differential, the cabling and feedthroughs would likely be
too much to manage.

The vendor that provides the diodes has an in-vacuum amplifier for a
different model diode array, but the gain is a bit low, and the noise
specs don't seem very impressive:

http://www.ird-inc.com/AMP16.html

How >> is this power-supplied? What environment EMI-wise? $40/ch is
quite >> realistic but only for large production volumes, of course.
Not if >> you have to do small boutique runs for circuit boards.
phase, >> a fixed bid isn't quite feasible here.
could >> > > manage the design, or are we talking about skirting the
bleeding >> > > edge?
budget >> > > that's higher than a hobbyist, but not quite
corporation level. >> > >
five >> digit Dollars. Since you are at a university why not engage
the help >> of more students? Good ones will be dying for meaningful
hardware >> projects. Sure, they'll get stuck here and there and for
that case >> you should line up a consultant. That's what even many
industrial >> clients do. They sign up with me and call me only when
they get >> stuck. Then they are only billed for the hours I helped
them but the >> bulk of the work was done in-house. An upside is that
this way they >> keep core expertise in-house, IOW by the end of the
project there >> will be people who know the stuff inside out.
ideas. >> > > Given the capacitance of the detectors in question, I
would >> > > imagine that a very low voltage noise opamp is the way
to go, or >> > > perhaps a JFET front end. The BF862 looks pretty
good, and it's >> > > relatively high capacitance wouldn't matter
much compared to the >> > > diode.
talk to >> > the particle and nuclear experimental physicists. Diode
detectors >> > followed by amps followed by A/D and triggers are
their bread and >> > butter. Google "silicon strip detectors" and
find somebody who has >> > worked in it more recently than me
(1980's!). >> >
All >> this operates smack dab in your band of interest. 3ft to the
diodes >> is going to be tough. Same for the 50ft to the ADCs. Why
that long? >> Can you do a digital link instead? if not you might
want to consider >> fiberoptics or modulate in onto a carrier
somewhere in a quite corner >> of the RF spectrum. 100kHz-250kHz will
be one hellacious noise bucket >> unless the installation is on a
remote island or completely shielded. >>

--

 

[[email protected]]

Greetings!

I'm a research scientists with the Johns Hopkins University, and I'm
working on a set of designs for an X-ray detector, and trying to spec
out various methods for obtaining the data we need. One of the designs
is a system based on diode arrays + amplifiers + ADC system. I've
already got a good handle on the detector end, and the data
acquisition system, but I'm stuck on the amplifier system.

We've used commercial amplifiers in the past, but they would likely be
overkill for our situation, and end up quite pricey on a cost/channel
basis. Given our specifications, I'm wondering if the optimal solution
would be to pay for a consultant to develop and test a design
specifically for our application, and then take that design and punch
out the number of boards that we would need.

Our generic needs seem to be fairly modest, 100-250kHz bandwidth with
a gain of 10^7, but as always the devil is in the details. Naturally,
we want the lowest noise possible so that we can measure signals at
the nA or sub-nA level.

So, here's the question. Are the specifications and schematic sketch
shown here:

http://picasaweb.google.com/ktritz/PhotodiodeAmplifierDesign/photo#52...

adequate for a professional to provide a consultation estimate? Would
the amplifiers be simple enough that a 2nd year EE student could
manage the design, or are we talking about skirting the bleeding edge?

I'm never contracted a consultant before, so should I expect a
consulting price tag of $1000? $10000? I'm working with a budget
that's higher than a hobbyist, but not quite corporation level.

I would also be happy to discuss specific amplifier design ideas.
Given the capacitance of the detectors in question, I would imagine
that a very low voltage noise opamp is the way to go, or perhaps a
JFET front end. The BF862 looks pretty good, and it's relatively high
capacitance wouldn't matter much compared to the diode.

Thanks,
Kevin

The high gain op amps will not have a low impedance output. Be sure
that the ADC doesn't couple back into the gain stage, i.e. effect the
op amp by presenting a time varying load as it converts. In simple
English, make sure the ADC uses a buffered design.

I have an amp design I got from the Yale physics department via a
website that looks pretty good. What they did was use some diode
clamping to insure that if the sensor gets whacked, the amplifier can
recover quickly.

Large-Area, Low-Noise, High Speed, Photodiode-Based Fluorescence
Detectors with Fast Overdrive Recovery
S. Bickman, D. DeMille
Yale Univeristy, Physics Department, PO Box @08120, SPL 23, New Haven,
CT 06520