In the simplest sense an analog instrument is one using analog circuitry. It contains high gain amplifiers that can present a waveform to the CRO tube for presentation where the x axis is time and the y axis is amplitude. Thus, the waveform of the signal being fed into the input is displayed on the CRT screen.
A digital instrument is one in which the circuitry is digital in design. The signal inputs are digitized using an ADC (Analog to Digital Converter) and processed in various ways and when ready by the user displayed on the screen of a CRT in the same manner as above. Interestingly enough, the digital signal after processed and manipulated is then converted back to an analog form (by an DAC, Digital to Analog Converter) where it drives the deflection plates of the display tube thus allowing the visualization of the event.
Well, in the simple explanation above it is just a question of the electronics employed inside the instrument. The digital circuitry has some interesting advantages for measuring and observing electronic phenomenon. For example, when a short pulse or one time burst or series of bursts present an event that must be recorded or viewed the digital circuitry can be arranged to take that measurement, put it in a memory or storage area for later viewing or slow playback. In other words, digital technology allows the digital instrument to do some interesting things that the analog circuitry could not – or at least could not do easily.
Further, like analog oscilloscopes which have more than one channel or input, the digital oscilloscope can likewise have two or several input circuits. This allows taking measurements and observations of several different events simultaneously. In modern digital circuitry there are timing, logic and streaming events going on at the same time and if the engineer or technician needs to observe all of these events at the same time the multiple inputs and storage capability allows this with multiple traces on the screen.
This gives rise, then, to another definition of the “Digital Oscilloscope”. Let’s call it a “scope” for the purpose of examining and testing digital circuitry. In this sense, then, even an analog scope could be used to examine digital circuits! However, due to the greater capabilities of the digital circuitry in an instrument for measuring digital systems when compared with analog circuitry it so happens that all (let’s say 99%) digital oscilloscopes employ digital circuitry in their designs.
Further, to be explored later on, there are many highly specialized functions that digital engineers need for the research, design and testing of their digital systems it gives rise to multi-functional “scopes”, dedicated “scopes” for just specific functionalities and purposes.