This question is somewhat related to this other EE.SE question, and some hints are given there, but are not completely satisfactory for me.

I've understood that, besides safety considerations, the biggest obstacle to making a fully differential, i.e. floating, input stage is that it is far more difficult (hence costly) to design a wide band DC-coupled differential amplifier than to design a single-ended amp having the same bandwidth (or the same performance, in general).

The usual answer could be "OK, you need a differential input, so buy a (costly) differential probe like this and be done with that".

On the other hand, oscilloscopes are constantly dropping in price, with companies offering ever more functionalities and performance at lower prices (heck! In the low-end market the venerable Rigol DS1054Z has had 4 channels and lots of features for years and now is getting competition from Siglent and others Chinese manufacturers, too).

So, my question really involves answering to the following points: why embedding the differential probe circuitry in a scope is not deemed a right step to do to gain market share? Wouldn't it be beneficial to the average user to be able to use a scope input like a floating multimeter input? Surely the circuitry of the diff probe would become less expensive once embedded in a bigger device and mass market production dynamics kicks-in. Am I wrong in assuming this? Is there something in a differential probe circuitry inherently so difficult to design/build that makes it not quite amenable to cost reduction by volume production?

Note that I know that there are some rare models that have differential inputs, but my question is really about why, with the continual production cost reduction of oscilloscopes, this has not become the rule among manufacturers.

  • \$\begingroup\$ Don't forget that new functionality is all in displays and computers, where bigger players compete and drop costs. At the same time analog circuitry, fast adcs, remain a niche, so there is not as much competition. \$\endgroup\$ – Gregory Kornblum Dec 19 '17 at 7:36
  • \$\begingroup\$ @GregoryKornblum Yep, I understand that the analog front-end and other high-speed sections can be harder to be dropped in costs, but they do try. IIRC in the reverse engineering EEVblog video of the DS1054Z front-end, Dave Jones actually showed how they used discrete JFETs to cut costs down relative to their higher level models. Therefore I often felt they don't try hard to add differential inputs just because the market doesn't require that, which seems funny. \$\endgroup\$ – Lorenzo Donati -- Codidact.com Dec 19 '17 at 7:57
  • \$\begingroup\$ Trust me, uf basic device becomes a little more expensive and you can't sell it to all your customers that don't need the new feature, it's a huge problem. \$\endgroup\$ – Gregory Kornblum Dec 19 '17 at 8:00
  • \$\begingroup\$ My guess is that many of the potential customers (for a cheap scope) would not even understand why they would need a diff. input. or how to benefit from it. Then the probes / inputs: with a diff. input you cannot use BNC anymore. Which means new probes. What will these look like. The red/black testleads on a diff. probe. do not give me confidence of having a high bandwidth. I mean: a diff. input can be a good idea but there are significant implications/changes to be made to a scope's design as it is now. \$\endgroup\$ – Bimpelrekkie Dec 19 '17 at 8:12
  • \$\begingroup\$ @Bimpelrekkie I understand that low-end scope users wouldn't understand all the implications, but I cited that example only to show how cheap the market has gone. Even more high level scopes from high-end manufacturer are getting cheaper. That's why I was interested in why companies are not investing in such a "paradigm" switch. \$\endgroup\$ – Lorenzo Donati -- Codidact.com Dec 19 '17 at 8:44

Differential probes usually used to monitor very fast signals with relatively small amplitude. In such case diff amp must be placed as close to the source as possible. Proximity is a key feature.

Also, I'd like to point out that single-ended isolated input usually is not equal to differential input. It's easy to make 1500 V isolation of the whole single-ended circuit, but it's utterly not so easy to create fast diff amp with such allowed common-mode voltage.


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