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Old 26th November 2012, 04:11 AM   #1
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Default Homebrew active probe

I want to add an active probe to my test gears. Downloaded D. Feucht "Buffer Amplifier Design" (http://www.analogzone.com/col_01302004.pdf). Is the JFET circuit shown in this article adequate? LTspice AC analysis shows f_3dB at about 225 MHz. At 100 MHz, gain is about -1 dB.
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Old 26th November 2012, 08:53 AM   #2
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How should we know if a certain circuit is adequate if we don't know your requirements? Key parameters are:

* Input resistance and capacitance
* attenuation (perhaps even gain?)
* bandwidth
* AC coupling
* input signal range
* offset control

Some links:

http://elektrotanya.com/files/forum/2009/10/e04a036.pdf
http://welecw2000a.sourceforge.net/d...mit_OPA659.pdf (German only)
http://igor.chudov.com/manuals/Tektr...obe-Manual.pdf

BTW, Tektronix P6201 are now getting so cheap that, besides the fun and learning experience, there's probably few reasons to DIY.

Samuel
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Old 26th November 2012, 11:18 AM   #3
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I need the probe be able to measure down to 10 Hz. Upper frequency cut-off should be at least 100 MHz. 500 MHz would be better to match with the scope BW.

Input resistance 1 MOhm, capacitance less than or equal to 1 pF.
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Old 26th November 2012, 01:09 PM   #4
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That low input C at this bandwidth is surely pretty demanding. Your best bet is probably a 10x design. Layout will be very critical--there's quickly a few 100 fF sneaking in.

At low frequencies you can use bootstrapping (see http://www.ti.com/lit/an/snoa620/snoa620.pdf, page2), but I'm not sure if this can be extended to such high bandwidths using ordinary PCB level techniques.

Also see Pease, Troubleshooting Analog Circuits, page 16/figure 2.3 (available at Google Books).

Samuel
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Old 15th September 2013, 12:33 PM   #5
jcx is offline jcx  United States
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N2795A Active Probe, 1 GHz | Agilent

so the questions are how is it done and how much does making your own cost - and how much you're wiling to give up in performance, usability, reliability, accuracy...

Last edited by jcx; 15th September 2013 at 12:36 PM.
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Old 29th September 2013, 09:12 AM   #6
alayn91 is offline alayn91  France
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Hello,

May be this one ??

It's from Elektuur 04/2004.
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