Electronic – Why modern oscilloscopes inputs are not fully differential (i.e. floating)


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.

Best Answer

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.