Re: Amplifier design pre-consultation consultation

Kevin Tritz wrote:
Joerg wrote:

Tim Shoppa wrote:
On Jul 3, 1:43 pm, ktr...@xxxxxxxxx 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:

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


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.

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

Correct, thanks.


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?
...

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.


... 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?

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.


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.

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.


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.
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.


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.

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.

I wonder how feasible it would be to chop the photodiode signal in the vacuum, then AC amplify it outside. You should be able to reduce the circuitry in the vacuum to a bunch of diodes, which can be sourced for vacuum use*, at the cost of the chopped version not being the most elegant.

You _would_ get some of your chopper drive signal bleeding through to your return signal, which would come out as a DC bias after amplification and rectification. I have no clue if this would be a killer for you or not.

* I know that 2N2222 dice are regularly used in a vacuum at 77K for temperature sensing.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
.

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