Author Topic: [Rant] Why does this ever exist?  (Read 2175 times)

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Offline blueskull

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[Rant] Why does this ever exist?
« on: November 09, 2016, 01:41:16 am »


Why would I want to measure power rail transient noise issues with DC coupling in the first place? A cap along isolates bias voltage well, and even in case I want to measure slow voltage drop, just use another channel with 10:1 in DC mode!
Why would anyone care about loading effect of a 50 Ohm probe on a 1.2V 10A FPGA Vcore regulator? Why would people pay $3k on a DC offset-superimposed tracking amplifier plus an AC coupling cap?
And finally who cares about GHz digital noise on a 24V rail? There is nothing commonly seen that can generate GHz digital noise at anywhere above 3.3V voltage rail.

And take a look at Bode plot from their user manual! DC response is not included, let along its gain drops by 9dB at higher input impedance! That makes this probe PI measurement only, no possibility for other uses at all!
Can anyone tell me what the hell has happened to engineers nowadays? Collecting $3k toys? Geez, for the same $3k I can buy a 6GHz probe with +-15V offset range and higher input impedance, plus that's actually useful in RF measurements, rather than a single purpose PI measurement tool.

Yes, I know it is a rage thread, but why this thing ever exists?
 

Offline David Hess

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Re: [Rant] Why does this ever exist?
« Reply #1 on: November 10, 2016, 03:44:07 am »
A DC blocking capacitor is not a suitable replacement for offset capability where frequency response has to extend to both almost DC and high frequencies and it is even more of a problem at low impedances where the physical size limits performance unless you want to include a GR-874 sized transmission line and I am not sure it can be done even then.

If an attenuator is used to raise the input impedance allowing a smaller capacitor, then sensitivity is lost unless amplification is added which will result in more noise.  For a good example of this, check out the Tektronix 7A13 which has a "x10 mode" where a x10 attenuator *and* x10 gain stage are added to produce the same sensitivity but with 10 times the offset range.  It is a useful feature but it increases noise significantly.  This points to exactly what makes this probe different which is the first feature listed; it is much lower noise than the typical active probe with offset capability.

Given the above, it is just easier to design and build an offset 50 ohm DC coupled probe or in this case a 50k probe.

I do not think they were concerned with the 50 ohm input resistance affecting the power supply; instead they used a 50k input resistance to allow a much greater non-destructive overload range.  It is difficult to protect 50 ohm inputs although I know a way they might have overlooked; check out the design of the Tektronix 7A29 and 485 50 ohm input stages.

The Bode plot from their user manual does not extend below 100 Hz because the response is flat below that.  I do not know why they even showed the response up to 10 kHz.
 

Offline blueskull

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Re: [Rant] Why does this ever exist?
« Reply #2 on: November 10, 2016, 03:50:53 am »
The Bode plot from their user manual does not extend below 100 Hz because the response is flat below that.  I do not know why they even showed the response up to 10 kHz.

The teardown shows a DC blocking cap for AC and a tracking DC amplifier for DC, so there must be a cross over point, where bode plot will show a bump.
Since it was not shown here, I assume they hide that somewhere between 10Hz to DC.
 

Offline David Hess

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Re: [Rant] Why does this ever exist?
« Reply #3 on: November 10, 2016, 06:03:39 am »
The Bode plot from their user manual does not extend below 100 Hz because the response is flat below that.  I do not know why they even showed the response up to 10 kHz.

The teardown shows a DC blocking cap for AC and a tracking DC amplifier for DC, so there must be a cross over point, where bode plot will show a bump.
Since it was not shown here, I assume they hide that somewhere between 10Hz to DC.

Most of these designs use a configuration which requires medium frequency compensation between the low and high frequency paths and when done correctly, there is nothing to see on the bode plot.
 


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