The driving force behind DSO's has been digital electronics. As clock speeds in digital circuits increase, manufacturers have responded by designing scopes with faster sampling rates and higher bandwidths.
Unfortunately analogue designers have been left behind: the quest for higher speed has been at the expense of accuracy, dynamic range and precision.
|Oscilloscope resolution||No of steps||Smallest change that can be detected||Dynamic range|
|6 bit oscilloscopes||64||1.6% (16000ppm)||36dB|
|8 bit oscilloscopes||256||0.39% (4000ppm)||54dB|
|12 bit oscilloscopes||4096||0.024% (244ppm)||72dB|
|16 bit oscilloscopes||65536||0.0015% (15ppm)||96dB|
As well as resolution noise is an issue. The amplifiers that make up the 'front end' of a conventional DSO are designed to have a high bandwidth. Low noise is not a priority and so 1% noise figures are typical. The designer of a 16 bit oscilloscope has a tough job, noise must be kept below 0.0015% (15ppm), otherwise the extra resolution will be swallowed up by noise.
Accuracy is not usually regarded as important for most oscilloscopes. You can make measurements within a few percent (most DSO's quote 3% to 5% DC accuracy) but for accurate measurements you have to reach for a multimeter. With precision oscilloscopes, accurate measurements are possible at full speed. Most hand held meters are 12 bit resolution and a 16 bit oscilloscope is equivalent in resolution to a 4.5 digit 'benchtop' meter.
The oscilloscope displays below all show the output of a high quality audio pre amp being fed with a pure sinewave. The output is fairly clean, but has a picked up a small amount of 50Hz mains noise. The first display is an 8 bit oscilloscope (almost all DSO's are 8 bit), the second shows a 12 bit oscilloscope (Pico ADC-212) and the third a 16 bit oscilloscope (Pico ADC-216).
An 8 bit oscilloscope gives a good visual representation of the wave. From the top left hand view you can clearly see that it is a sine wave and work out its approximate frequency and amplitude. If however, you wish to 'zoom' in on the signal to magnify an area of interest, this soon shows up the limitations of an 8 bit oscilloscope. The bottom left trace shows the top trace magnified by x50.
The view on the right shows a spectrum analyser plot (FFT) of the signal. The peak shows the frequency to be exactly 200Hz. The other signals (approx 60dB down) represent the 'noise floor' of the 8 bit scope. To put this into context a typical tape recorder has about 50 to 60dB of dynamic range. The signal contains 50Hz noise due to mains pickup, but the 8 bit scope does not have enough resolution to detect it.