Why do ADCs use 10 bit rather than 8 or 16?
There's no major technical problem with extending a SAR (successive approximation) ADC to convert 16 bits, but the issue is that you start to see the noise floor of the analog front-end. This tends to make customers panic because they see the ADC codes jumping around, and don't always realize that they are looking at tens of microvolts of deviation.


Assuming a 5.00&nbsp;V reference voltage and a 10-bit ADC, the LSB represents a voltage of 4.88&nbsp;mV (5&nbsp;V &times; 2-10). For a 16-bit ADC with a 5.00&nbsp;V reference, the LSB voltage would be 76&nbsp;&micro;V.

But the power supply in a digital system is not exactly 5.00&nbsp;V, it is usually specified in a range of 4.75&nbsp;V to 5.25&nbsp;V. Whenever there is a switching transient event inside the microcontroller, there is a little pulse of current that causes the supply voltages to twitch. If the LSB is around 5&nbsp;mV you might just barely be able to see it, but at the 76&nbsp;&micro;V level it's hard not to see this noise.

In fact, once you get past 12-bit ADC, you really need to have an analog voltage reference instead of just using the digital power supply. So that adds some more cost. For best results this voltage reference should actually be a separate chip, with its own power lines, and ideally it should be away from the hot / noisy digital circuitry.

If you want a 16-bit ADC to give nice stable readings, you need a very very clean reference voltage, and thermal control, and preferably keep it far away from any fast switching digital signals... so integrating a 16-bit SAR onto the same chip as a microcontroller, would actually defeat the purpose of having those extra bits. You'd just be measuring random noise on those extra bits.

There are customer applications that do use higher resolution ADCs. The company I work at makes quite a few of these. Automated test equipment (ATE), medical ultrasound, and certain other specialized types of customers do use high-resolution ADCs, in some cases 18- or even 24-bit.

Production testing a high-resolution ADC is time-consuming (and therefore expensive). Customers that need this kind of performance pay a premium for an external standalone ADC, not the cheap SAR types that are built into many modern microcontrollers.

Then there are high-speed applications such as radar or digital sampling oscilloscope, which need to sample at 100&nbsp;MHz or faster rates -- at these speeds, you're lucky to get 8 meaningful bits.