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:
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.
> 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:
> 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!).
> > 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:
> > 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.
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.
>> 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?
>> So, here's the question. Are the specifications and schematic sketch >> shown here:
>> 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.
> ... 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 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.
> 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.
> > > 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.
> > > 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:
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'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.
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.
>>>> 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.
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.
>>> ... 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.
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.
>>>> 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:
> 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.
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
...
> Joerg wrote: > > Tim Shoppa wrote: > > > On Jul 3, 1:43 pm, ktr...@gmail.com 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:
> > > > 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:
> 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
> > > 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.
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.
