On a DSO that uses an 8-bit A/D converter, would it be possible to have an option where at slower scan rates you could have the LSB of the A/D dithered so as to gain several extra bits of resolution? And by dithered, I mean add a high frequency triangular signal of exactly 1 LSB peak to peak to the incoming waveform and then measure the duty cycle of the resulting LSB as it jiggles between two adjacent values to see where it lies between those two LSB values.
Obviously this approach wouldn’t work at higher signal frequencies and scan rates because the A/D would be too busy, but at the lower end, particularly at audio frequencies it might have half a chance. So, would it be a practical thing to do? Or do some scopes already do this?
Basically, there's no scope that "measures the duty cycle of the LSB", yet on the other hand any serious DSO should be able to do this - as long as they have deep measurements (use the entire record without decimation) and provide measurement statistics, that is. Since even "serious" DSOs like Keysight Megazoom (DSOX, MSOX) series do not meet all these requirements, one has to be careful here.
With an appropriate dither, the LSB will toggle at a ratio according to the intermediate value of the sampled voltage. Collecting a few hundred measurements in the statistics and looking at the mean value usually gives a reliable and fairly
accurate (on scopes with deep measurements) result (taking the duty cycle of the LSB into account). Basically, this could already be what you're looking for. It's also not limited to low frequencies, but you need a stable signal over a certain time interval (depending on the frequency) to gather all the data.
If you're not after measurements but simply the graphical representation on the screen, then HiRes, ERES or Average Acquisition modes and/or math functions can do what you want. Many midrange scopes provide a resolution enhancement up to 16 bits based on oversampling. This reduces the input bandwidth though. You can get this function even in the top entry level Siglent SDS2000X Plus, where there is a 10 bit Acquisition mode that limits the bandwidth to 100 MHz and halves the sample memory (because it now has to be 16 bits wide), but has no drawbacks otherwise. And it is almost indistinguishable from the "real thing", except that the noise, hence also dynamic range (ENOB) is only improved by a single bit, as always, when math is used to increase the resolution instead of real hardware. But then again, even real hardware 12 bit scopes rarely ever reach more than 9 bits ENOB at best...
You might need an additional dither on very low noise scopes like the SDS2000X Plus. At least at lower sensitivities, say >10 mV/div. At high sensitivities (down to 500 µV/div) there is enough noise from the frontend to serve as a dither. The other cases are managable as well. Usually the measurements with passive high impedance probes capture enough environmental noise - even low levels of mains hum or signals from nearby radio stations or other transmitters will do. If it is a lab standard 50 ohms test setup, we can always add a dither signal via a resistive power combiner. In any case, it has never been a problem for me in practice as the linked demonstrations show.
My scope has both hi res and averaging modes. Naturally there is a limit to the resolution of the signal displayed on the screen because of the pixel pitch, but if the individual samples are logged and brought out externally to be examined, in hi res or average mode have they been truncated to 8 bits or do they have greater resolution?
The screen of a modern DSO could at least display 9 bits vertically, and with vertical zoom you can inspect even higher resolution captures quite closely. On most advanced scopes like the Siglent SDS2000X Plus, the full sample memory will be exported. That means you get 16 bit data as soon as you're working with a resolution >8 bits, 8 bit data otherwise.
Of course we can easily get 16 bit resolution even with the moderate SDS2000X Plus, if we use the 10 bit mode combined with ERES 3.0 (Extended Resolution 3.0 bits). ERES 3.0 provides 3 bits of increase in ENOB and 6 bits of resolution enhancement. This way, we get 16 bits, can store it and export it.
Of course, the ERES operation further limits the bandwidth of the scope. In case of ERES 3.0 and full channel mode with 1 GSa/s sample rate, it would be only 7 MHz - but you get 4 x 16 bits...
ExamplesHere is a demonstration of the 10 bit acquisition mode:
https://www.eevblog.com/forum/testgear/siglent-sds2000x-plus-coming/msg2786952/#msg2786952This is a demnstration of ERES and Average (3 consecutive posts):
https://www.eevblog.com/forum/testgear/siglent-sds2000x-plus-coming/msg2840282/#msg2840282And finally the demonstration of 10 bit acquisition mode combined with ERES 3.0:
https://www.eevblog.com/forum/testgear/siglent-sds2000x-plus-coming/msg2857298/#msg2857298