Aren’t more bits better?

& -March 06, 2013

Over the last two years, there have been new higher resolution oscilloscopes introduced to the market. The idea is to use a 12-bit digitizer instead of an 8-bit digitizer to acquire a waveform. With an 8-bit digitizer, there are 256 voltage levels that define the wave shape. In theory, 12-bits means that there are 4096 distinct voltage levels, a great improvement in resolution. Such an oscilloscope should allow you to see small signals in the presence of big ones, provide greater accuracy of DC gain, and less noise on a signal.

Aren’t more bits always better? Let’s review. Well, let’s look at a simplified example. Below is a high resolution RAW image taken on a digital camera. The resolution is high enough that you can clearly read the sign reads “Howard Vollum Plaza Dedicated 2005” (Howard Vollum was the founder of Tektronix).

Image 1: High Resolution Image taken from Digital Camera

Now, the same photo is taken but with a significant reduction in resolution. There are now fewer pixels in it and less detail.  You can no longer decipher the sign in the garden.

Image 2: Low Resolution image taken from Digital Camera

Using a photo editing tool like Adobe Photoshop, the low resolution picture was “enhanced” back up to full resolution. However, despite the image having full resolution, the letters in the sign are still not visible. The difference between the image below and the top image is the source. In this case, the lower resolution source  (image 2) lacked the detail needed to make out the sign, even with full resolution. So more resolution does not  necessarily mean more detail because the lower resolution starting image introduced sources of error.

Image 3: Low Resolution image enhanced by Photo Editing Tool.

Now let’s take a look at how this applies to 12-bit oscilloscopes.
The assertion with a 12-bit digitizer is that the individual voltage steps are smaller, so the waveform will have greater fidelity. In Figure 4, the staircase is the digitizing level, or quanta. Suppose the signal was 100mV peak to peak, filling the screen of the oscilloscope. In theory, an 8-bit oscilloscope could only display a signal feature as small as 390uV (100mV / 256), but a 12-bit digitizer could show one as small as 24uV (100mV/4096). But how does this theory work in the real world?

Figure 4: Digitizing Levels (Quanta) on an Analog Waveform

The first thing is to remember that while high-resolution digitizers can be effective at low frequencies, they have far fewer effective bits at full bandwidth. Effective number of bits (ENOB) is the true resolution of the A/D once imperfections are included, such as non-linearities, gain errors, distortion, and noise. Just as Image 3 above is a high-resolution representation of a low-resolution source, the same is true of an oscilloscope that has a high-resolution digitizer but other sources of error. If there is noise on the signal, that extra resolution is just extra bits of noise, and the waveform like the text above will remain blurry.
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