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JohnG:
I'm wondering if anyone has a working design or knows of one for a DIY wideband (DC to > 1 GHz) DIY differential probe. I've search for this on and off, and found some attempts, but have not found a successful design. I have come across references to a 2 GHz design published in Elektor July/Aug 2015, but I don't have the article so I don't know if this is a working design or not. I'd buy the issue if I had some indication that this was indeed a working design.

I've used these types of probes before, so I understand the voltage and impedance limitations they present.

There are a few reasons I'd like to have a DIY design. First, to learn from. Second, to have a design that is not tied to a particular brand scope  >:(. Third, to extend or modify for different applications. In theory, one could have a few different probes for different voltage ranges, or similar.

Any help appreciated!

John
joeqsmith:
I attempted to design a DC coupled wide band differential probe last year.  Got as far as coming up with requirements.  In the end I could not find parts suitable for the job.   

I had seen a few homemade probes people had came up with but nothing DC coupled and nothing as fast as I was looking to build.     

Beyond the DC-1GHz diff, what are your requirements? 

https://xellers.wordpress.com/electronics/1ghz-active-differential-probe/
https://xellers.wordpress.com/2014/09/28/diy-active-differential-probe-characterization-round-2/
JohnG:
Since you asked about requirements:

Bandwidth: > 1GHZ (risetime < ~300 ps) or so, faster is better
Input: >= 1k resistive, <= 2pF capacitive (higher resistance would be preferred
Attentuation: 10:1
INput voltage range: >10 V DM, >5V CM
CMRR: >20 db at 1 GHz, if possible

If I could do a 100:1 with a variation on the same circuit, that would be great. I could take some hit on speed at the higher voltage, but the input resistance would have to go up to at least 10k. It would not need to meet 100V continuous operation, but the ability to get capture short pulses (up to 1-2us wide) at 100V or close would be a big plus. Not many commercial probes can do this at this kind of BW.

Here is what I really want, but it will only work with a Tek Scope, which I don't have. It will probably cost as much as the scope, or close: http://www.tek.com/dl/51W-60485-0%2520IsoVu%2520White%2520Paper%2520%2520TN%25203-25-16.pdf


All the CMRR you could ever want. I have seen it in action, and it works. But, I won't be getting one anytime soon :(.

John
Marco:

--- Quote from: JohnG on August 14, 2016, 02:53:41 pm ---Here is what I really want

--- End quote ---

Not really. That's a high frequency 50 Ohm input isolation amplifier, not a high input impedance differential probe. You can simply add two resistors for a differential version of the passive attenuated probe, but just as with the passive attenuated probe that's not optimal as far as noise is concerned.

The Elektor design uses a high frequency fully differential amplifier (ADA4927-1) as a differential to single ended converter, witch matched resistors to provide attenuation. Boring :) Also they didn't actually measure CMRR, you can't rely on datasheet CMRR because that's for differential->differential and common mode errors for the two outputs could very well cancel out in differential mode.
joeqsmith:
When I saw 1GHz to DC I was thinking digital, not something high voltage dual purpose.  I was hoping to keep loading above a couple hundred ohms with  maybe +/-2.5V differential and same for common mode.  Maybe 0.2pf at the tip sort of thing, couple of GHz BW.  I was thinking that with modern parts I could pull it off but I spent a few weeks looking and gave up.  I was also having problems trying to figure out how I was going to tackle the DC part.   My plan was to use 2 AC paths and two DC to something paths.  Then somehow combine the whole mess and not ruin signal.  The circuitry would have been fairly complex, at least for me.   

Keep us posted if you try it.   

On the bright side, at least for digital work the used probe market seems to be growing.  Prices are all over the place.   Thousands for broken probes with missing parts, to under a K for this baby!

http://www.ebay.com/itm/Teledyne-LeCroy-D13000PS-13GHz-Differential-Probe-System-/131895698595?hash=item1eb598a4a3:g:4WEAAOSw~otWce6k

I could see this for my hobby use.
http://www.ebay.com/itm/LeCroy-D600-7-5-GHz-WaveLink-Probe/162154103823?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20131003132420%26meid%3D1b91768d4ae64145a92f0a3a455d4605%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D311133558055

 
That Tektronix probe looks slick.  Having a 2KV CM standoff or galvanic isolation is not something I would need at home.  I did find another article on it.  Sounds like it may not yet be available. 

http://electronicdesign.com/blog/1-ghz-isovu-leads-tektronix-probe-performanceusability-charge-apec

And a video on their facebook page
https://www.facebook.com/tektronix/videos/vb.50990632804/10153395327767805/?type=2&theater
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