Why are active probes shaped like this? - Page 1
Forum section: electronics / projects

author: MattHollands
author_rank: Frequent Contributor
timestamp: September 09, 2016, 06:41:41 am
content: At work we have a bunch of active probes for Tek and Agilent/Keysight scopes, but they are all "shrimp heads". ie the ground is offset by a ~15mm from the signal. The Agilent N2796A probes are an example of this:http://www.keysight.com/en/pd-1912753-pn-N2796A/active-probe-2-ghz?cc=US&lc=engSo why are they this shape? It makes it a pain to connect to your 0.1" header, and I can't think of anything that it connects to easily. Wouldn't it make more sense to have the two connections next to each other.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 09, 2016, 07:04:44 am
content: not everything is 0.1" header.. sometime the other signal is too far away. in your link, they show you how to make 0.1" header adapter...

author: TheSteve
author_rank: Unknown
timestamp: September 09, 2016, 07:24:12 am
content: I guess they could have the signal and ground pins side by side but then trying to probe anything not a standard 0.1 header etc would be a real pain. If you experiment with the accessory kit included with the N2796A you might find several probe/ground combos work quite well. And as already mentioned as long as it isn't exceeding the frequency limit from additional lead length(detailed in the manual) you can use the included header adapter. The bonus is you don't even have to hold the probe, you can just plug it in. I really like the smallest probe tip and the Z shaped ground wire combo on my N2795A.

author: MattHollands
author_rank: Frequent Contributor
timestamp: September 09, 2016, 07:29:14 am
content: Yes, of course not everything is a 0.1" header. I don't see much use in not having them side by side. In all of my use cases I have tried to keep ground and signal as close together as possible. Only if I can't keep them that close together do I add unnecessary wires to my probe set up. Why force me to use extra wire if I don't need it.

author: TheSteve
author_rank: Unknown
timestamp: September 09, 2016, 08:37:12 am
content: Of course you don't want extra wire, thankfully a slightly longer ground lead on an active probe is often not a problem. I think it would be awful tough to probe uneven connections and such if they were side by side/same height. Such as a ground pad on top of an IC and then the IC pin. Many times what you're probing will not be equal height etc. Having the ground connection as a flexible spring seems pretty handy.

author: tggzzz
author_rank: Super Contributor
timestamp: September 09, 2016, 08:42:56 am
content: Quote from: MattHollands on September 09, 2016, 06:41:41 amAt GHz you can't tolerate 0.1" test headers on signals; such stubs will cause signal integrity problems. Instead you have to connect the probe's ground to the adjacent ground plane.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 09, 2016, 09:11:52 am
content: my guess is. The height on 2nd probe is so that it can cover larger radius and probing angle variant. That they had taken into consideration many application possibilities for that model.i know i've seen agilent diff probe like what you desired,very close probe distance, with smaller pitched adapter and all.6ghz model iirc.edit: here...http://www.keysight.com/en/pd-2156299-pn-N2751A/infiniimode-35-ghz-active-differential-probe?cc=US&lc=eng

author: Berni
author_rank: Super Contributor
timestamp: September 09, 2016, 12:30:22 pm
content: In every day scenarios quite often ground is not where you need it so having some length in it can be useful.Active proves are fine with having a ground link that long anyway. Having the amplifier right at the tip reduces the probes parasitic capacitance well under 1 pF and this reduces the ground return current a lot. Not needing a super short ground is one of the biggest benefits of active probes that most people don't even know about.On a passive probe having that 5cm crocodile ground clip will screw up anything above 10MHz, but an active probe will still keep perfectly good signal integrity past 300MHz on such a long clip. The proper short little ground spring becomes vital once you get to 1 GHz and above on active probes.Anyone out there that has not tried this have a go at using a 15cm alligator clip to connect ground on a active probe while looking at a high speed digital signal and then use a passive probe with a proper short ground spring to probe the same signal. Chances are the active probe will show a cleaner signal despite having that giant long ground path.

author: Sceptre
author_rank: Regular Contributor
timestamp: September 10, 2016, 05:15:25 am
content: Quote from: Mechatrommer on September 09, 2016, 09:11:52 amAlso seehttp://www.tek.com/datasheet/tdp1000-tdp0500-p6251-differential-probes.But if you want single-ended active probes, the offset ground configuration may be more common.

author: VK5RC
author_rank: Unknown
timestamp: September 10, 2016, 07:45:20 am
content: +1 re inter-lead capacitance , even 1pF at 2GHz is about 80ohms impedance.Edit calculatorhttp://www.qsl.net/pa2ohh/jslcimp.htm