Homebrew active probe

panson_hk · 26th November 2012, 04:11 AM

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.

Samuel Groner · 26th November 2012, 08:53 AM

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

panson_hk · 26th November 2012, 11:18 AM

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.

Samuel Groner · 26th November 2012, 01:09 PM

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

26th November 2012, 04:11 AM	#1
panson_hk diyAudio Member Join Date: Aug 2005 Blog Entries: 10	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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26th November 2012, 08:53 AM	#2
Samuel Groner diyAudio Member Join Date: Jun 2005 Location: Zürich	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

26th November 2012, 11:18 AM	#3
panson_hk diyAudio Member Join Date: Aug 2005 Blog Entries: 10	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.

26th November 2012, 01:09 PM	#4
Samuel Groner diyAudio Member Join Date: Jun 2005 Location: Zürich	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