DIY oscilloscope front end queries

[JoeN]

Hmmm ... schematic checked, documentation checked, gerber files checked, then clicked the buy button on the ADC, and TI is telling me to pay $3,611.18  ... instantaneously emptied the "My cart" , and closed the tab at the browser.

My point isn't the ADC.  I pointed out that it is expensive.  I am interested in the front end up to the ADC.  If the front end is good, it answers some of the questions in this thread, which is the analog magic part.  You can substitute an an ADC more to your ability to pay or convince somebody to send you a sample.  However, even the front end has some pretty expensive parts.  I guess that is what is required to get a quality signal chain with 2Ghz BW up to the ADC inputs.

Page change already.  I am going to repost some stuff.

After this thread occurred, TI published a reference design for a 2 Ghz scope front end last year.  I am going to link it here.

What do you think of their design?  All high-spec parts, for sure.  The one thing that adds a bunch of cost to this is the RF relays they are using for their attenuator are $50 each and they use 5 of them.   Most of the TI parts, despite really high end performance, are downright cheap compared to those relays (not inclusive of the ADC!).  I assume there is no way to achieve that using some sort of RF IC switches?

Page: http://www.ti.com/tool/TIDA-00826
Schematic: http://www.ti.com/lit/df/tidrjw1/tidrjw1.pdf   
User guide: http://www.ti.com/lit/ug/tiduba4/tiduba4.pdf

[Cerebus]

What do you think of their design?

Not a lot. Just looking at the 'Features' section one sees:


Features

    ...
    System ENOB of 6 to 8 bits is achieved with this signal chain
    ...
 


So an ENOB of 6. Add that to 'just throw a handful of $50 coax relays at it' style of design and you don't need to look at the detailed design, you've already got enough information to tell you what you need to know.

Almost forgot, I was so horrified at TIs 'reference' design. For switching, go back through the thread and find the "Signal Conditioning in Oscilloscopes" paper - there's a switching scheme in there using reverse biased PIN diodes that's elegant and simple.

[T3sl4co1l]

What a steal, Digi-Key has them for $2301.40 in singles.  Or only $2195.98 if you buy a tray at a time! 

Worth noting, such parts are subject to ITAR, unless they've bumped the rules recently.

Tim

[Kalvin]

Hmmm ... schematic checked, documentation checked, gerber files checked, then clicked the buy button on the ADC, and TI is telling me to pay $3,611.18  ... instantaneously emptied the "My cart" , and closed the tab at the browser.

You didn't ask samples?

[linux-dude]

What a steal, Digi-Key has them for $2301.40 in singles.  Or only $2195.98 if you buy a tray at a time! 

Worth noting, such parts are subject to ITAR, unless they've bumped the rules recently.

Tim

This is a steal though: http://www.digikey.com/product-search/en/development-boards-kits-programmers/evaluation-boards-analog-to-digital-converters-adcs/2622527?k=ADC12J4000

[JoeN]

What a steal, Digi-Key has them for $2301.40 in singles.  Or only $2195.98 if you buy a tray at a time! 

Worth noting, such parts are subject to ITAR, unless they've bumped the rules recently.

Tim

You are from the U.S.  How does ITAR affect you?

[not1xor1]

  My query relates to the front end design, more specifically the use of attenuators and amplifiers. I am aware of what they do (reduce and increase a signal's strength), but I am unsure of their use in an oscilloscope, especially since many of the block diagrams I have viewed vary in this respect; some have either, neither or both. Could anyone tell me more about their application specific uses please, and where they would fit into my 'schematic' as attached below.

You might have a look at Digilent's Analog Discovery.
They provide the schematic diagram.
You can download it here:
http://store.digilentinc.com/analog-discovery-2-100msps-usb-oscilloscope-logic-analyzer-and-variable-power-supply/

BTW Analog Discovery inputs are differential.

[Cerebus]

What a steal, Digi-Key has them for $2301.40 in singles.  Or only $2195.98 if you buy a tray at a time! 

Worth noting, such parts are subject to ITAR, unless they've bumped the rules recently.

Tim

You are from the U.S.  How does ITAR affect you?

Perhaps Tim is just capable of seeing it from the other guy's perspective without prompting; an attitude I commend to everybody.

[T3sl4co1l]

As a small company, I want to avoid touching anything ITAR in anything that leaves my hands.  A $100k fine, even if very unlikely, isn't appealing.

Tim

[JoeN]

Perhaps Tim is just capable of seeing it from the other guy's perspective without prompting; an attitude I commend to everybody.

Or there might actually be a logical explanation.

As a small company, I want to avoid touching anything ITAR in anything that leaves my hands.  A $100k fine, even if very unlikely, isn't appealing.

Tim

There it is.  Thank you.  Makes sense to me, I guess.  I guess I am not quite as risk-adverse as you, I wouldn't have guessed that.  I have a night-vision scope that is subject to ITAR, at least it was when I got it.  Bought it mainly as a cool toy.  Never occurred to me someone would steal it from me, export it somehow, and the government would show up and kick my ass for it.