
An almost-GHz active differential oscilloscope probe (2016) - segfaultbuserr
http://blog.weinigel.se/2016/02/26/ghz-differential-probe.html
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satya71
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.

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segfaultbuserr
Part 2: [http://blog.weinigel.se/2016/02/28/ghz-differential-
probe-2....](http://blog.weinigel.se/2016/02/28/ghz-differential-probe-2.html)

Part 3: [http://blog.weinigel.se/2016/03/16/ghz-differential-
probe-3....](http://blog.weinigel.se/2016/03/16/ghz-differential-probe-3.html)

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leetbulb
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.

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msds
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.

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madengr
Yep, I have a Keysight 7 GHz diff probe at work that cost $25k, and that’s one
of the cheaper ones.

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bsder
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?

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metaphor
A quick DigiKey parametric search yields instrumentation amps with GBW <= 35
MHz.

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bsder
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?

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metaphor
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)

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bsder
I actually did originally go back to the GitHub repo, but I missed this in the
README:

"So far I have only built the B variant of the probe, that is, the probe with
separate OP amps for the inputs."

So, that's why he doesn't discuss the others. He never built them.

> 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?

I would almost certainly want to grab the LMH6552 which is a fully-
differential. It's input bias current isn't dramatically worse (110uA (about
10K Ohm) vs 34uA (about 30K Ohm)) than the LMH6702 that he used and likely
wins by being matched/same temperature because same die/no PCB routing
mismatch/etc.

