5 ms·

An almost-GHz active differential oscilloscope probe (2016)

segfaultbuserr 7y ago
Part 2: http://blog.weinigel.se/2016/02/28/ghz-differential-probe-2.html http://blog.weinigel.se/2016/02/28/ghz-differential-probe-2.... Part 3: http://blog.weinigel.se/2016/03/16/ghz-differential-probe-3.html http://blog.weinigel.se/2016/03/16/ghz-differential-probe-3....
leetbulb 7y ago
Excellent write up. I'm also a software guy that experiments with hardware, so I love seeing these type of projects. How much time was spent on R&D for this? I'm sure there are plenty of reasons, but it's insane to me that something like this would cost $3k.
- msds 7y ago
  3K in the RF world is cheap... I've watched as more than that in chips fried with a bad reflow profile, and that was with some very very cost-efficent people.
  - madengr 7y ago
    Yep, I have a Keysight 7 GHz diff probe at work that cost $25k, and that’s one of the cheaper ones.
satya71 7y ago
That was almost nostalgic for me particularly. I was part of the group that did the LMH67xx series of parts. I personally didn't design the LMH6702, but I was next cubicle over from the guy who did.
bsder 7y ago
One thing I never saw in the writeup is: Why use discrete components to build the instrumentation amp? Surely, there is an integrated version of a differential amp that has the required specs in one chip somewhere for <$50?
- deleted 7y ago
  [deleted]
- blattimwind 7y ago
  > Surely, there is an integrated version of a differential amp that has the required specs in one chip somewhere for <$50? Not really. This is discrete or an ASIC/hybrid in commercially available probes as well.
- metaphor 7y ago
  A quick DigiKey parametric search yields instrumentation amps with GBW <= 35 MHz.
  - bsder 7y ago
    Sure, if you hunt for "instrumentation amps". Instrumentation amps are meant primarily for amplifying low voltage rather than high-bandwidth. What you're looking for is a "fully differential" amplifier. He apparently actually built a board using an LMH6702 (roughly $3 for 1.7GHz), but then never mentions anything about it. And, what's with the downvotes on my question? If the answer is so obvious, what's the answer oh-so-brilliant-ones?
    - metaphor 7y ago
      Yes, instrumention amps are often used for low-voltage amplification...primarily because the topology enables the elimination of impedance-matched input termination with the distinct benefit of minimizing parasitic load on the UUT. Apparently, readily-available integrated instrumentation amps on the market are designed for relatively low-bandwidth applications as a quick DigiKey parametric search suggests, but the canonical topology itself isn't fundamentally bandwidth-constrained...which is part of the reason why I pointed to max GBW results from the search and left it at that. No, that's not what I'd be looking for, especially in response to your question with the specific constraint of "an integrated version of a differential amp"; given the sensitivity of the in-circuit measurement application being considered, this effectively suggests integrated instrumentation amp. With your suggestion of what I should be looking for, how would you go about input termination without loading the UUT to the detriment of SI? The author does indeed briefly discuss his prototypes and even released his Eagle design on GitHub[1] with still more notes. For the record, I didn't downvote on first response, but your "oh-so-brillant-ones" snark may have earned one. [1] https://github.com/wingel/diff-probe https://github.com/wingel/diff-probe

Part 2: http://blog.weinigel.se/2016/02/28/ghz-differential-probe-2.html http://blog.weinigel.se/2016/02/28/ghz-differential-probe-2.... Part 3: http://blog.weinigel.se/2016/03/16/ghz-differential-probe-3.html http://blog.weinigel.se/2016/03/16/ghz-differential-probe-3....

leetbulb 7y ago

Excellent write up. I'm also a software guy that experiments with hardware, so I love seeing these type of projects. How much time was spent on R&D for this? I'm sure there are plenty of reasons, but it's insane to me that something like this would cost $3k.

msds 7y ago
3K in the RF world is cheap... I've watched as more than that in chips fried with a bad reflow profile, and that was with some very very cost-efficent people.
- madengr 7y ago
  Yep, I have a Keysight 7 GHz diff probe at work that cost $25k, and that’s one of the cheaper ones.

satya71 7y ago

That was almost nostalgic for me particularly. I was part of the group that did the LMH67xx series of parts. I personally didn't design the LMH6702, but I was next cubicle over from the guy who did.

bsder 7y ago

One thing I never saw in the writeup is: Why use discrete components to build the instrumentation amp? Surely, there is an integrated version of a differential amp that has the required specs in one chip somewhere for <$50?

deleted 7y ago
[deleted]
blattimwind 7y ago
> Surely, there is an integrated version of a differential amp that has the required specs in one chip somewhere for <$50? Not really. This is discrete or an ASIC/hybrid in commercially available probes as well.
metaphor 7y ago
A quick DigiKey parametric search yields instrumentation amps with GBW <= 35 MHz.
- bsder 7y ago
  Sure, if you hunt for "instrumentation amps". Instrumentation amps are meant primarily for amplifying low voltage rather than high-bandwidth. What you're looking for is a "fully differential" amplifier. He apparently actually built a board using an LMH6702 (roughly $3 for 1.7GHz), but then never mentions anything about it. And, what's with the downvotes on my question? If the answer is so obvious, what's the answer oh-so-brilliant-ones?
  - metaphor 7y ago
    Yes, instrumention amps are often used for low-voltage amplification...primarily because the topology enables the elimination of impedance-matched input termination with the distinct benefit of minimizing parasitic load on the UUT. Apparently, readily-available integrated instrumentation amps on the market are designed for relatively low-bandwidth applications as a quick DigiKey parametric search suggests, but the canonical topology itself isn't fundamentally bandwidth-constrained...which is part of the reason why I pointed to max GBW results from the search and left it at that. No, that's not what I'd be looking for, especially in response to your question with the specific constraint of "an integrated version of a differential amp"; given the sensitivity of the in-circuit measurement application being considered, this effectively suggests integrated instrumentation amp. With your suggestion of what I should be looking for, how would you go about input termination without loading the UUT to the detriment of SI? The author does indeed briefly discuss his prototypes and even released his Eagle design on GitHub[1] with still more notes. For the record, I didn't downvote on first response, but your "oh-so-brillant-ones" snark may have earned one. [1] https://github.com/wingel/diff-probe https://github.com/wingel/diff-probe