Products > Test Equipment
> 1 GHz DIY differential probes
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Carrington:
I just wanted to say that this is getting more and more interesting, and I think that something pretty good is going to come out of here.  :-+
Marco:

--- Quote from: nctnico on October 21, 2018, 10:37:40 am ---That is interesting! Are these FETs available as a matched pair? I think that will greatly improve the CMMR and DC offset matching.

--- End quote ---
How close can you match the capacitance of the resistors though? Seems to me the solder blobs would throw it off.
0xdeadbeef:
I'm aware it's neither differential nor >1GHz, and well, it's only available in German, but the most elaborate take on a DIY active probe I stumbled over in the last years was this one:
http://welecw2000a.sourceforge.net/docs/Hardware/Aktiver_Tastkopf_mit_OPA659.pdf
The probe uses an OPA659 with a trimmable capacitive divider. The main developer and author of this PDF "branadic" is also active in this forum btw.
Building some of these is one of multiple projects on my "stack".

As a side note: I also bought some of the probes by Alfred Rosenkraenzer (PCB version). When I bought the first one, I was well aware that the bandwidth sounded a bit optimistic. Still, my main intention was to measure LVDS signals in the 20-40MHz range and the probes worked excellently for that. I.e. where the passive 500MHz probes just showed a sine shaped signal, I got a clear rectangular signal with steep edges. I already used them for 5V signals as well though (e.g. 100ns CS pulses) where they are also far superior to 500MHz passive probes. Admittedly, the scoped used was a 600MHz Lecroy so I don't really care if the probes really have a much higher bandwidth or not. And yes, looking at the differential amplitude, the probes show a 20:1 attenuation, I guess the 10:1 value was meant for each input. In the meantime, the description in the eBay offers was also changed as far as I can tell.
dietert1:
Today i received a THS4302 evaluation module. Compared to the OPA659 the THS4302 has more bandwidth and some gain.
The EVM gain curve  measured by our HP 8560A shows 7.8 dB at low frequencies (G=5 gives 14 dB - 6 dB output line attenuation). 3 dB bandwidth is roughly 1.8 GHz.  Up to 2.9 GHz it levels off smoothly, so this will result in low ringing.
The evaluation module has a terminated strip line as input. A probe needs a different design. THS4302 input capacitance is specified as < 1 pF, so if you put a 10:1 divider in front, you may get a 0.2 pF high impedance probe.
Hydron:
Looks like there's also a gain-of-10 version of the THS4302 too (the THS4303) if it's of interest.

Thanks for all the work being done in this thread - sounds like the final result could be pretty handy.
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