Global Sources
EE Times-Asia
Stay in touch with EE Times Asia
EE Times-Asia > Amplifiers/Converters

Increase vertical resolution of scopes (Part 1)

Posted: 17 May 2013 ?? ?Print Version ?Bookmark and Share

Keywords:oscilloscope? small signal visibility? dynamic range?

Scaling the waveform to take the whole display of the scope enables you to use more of the scope's analogue-to-digital (ADC) converter. If a signal is scaled to take up only ? of the vertical display, you've just decreased the number of ADC bits being used from 8 to 7. Scale the waveform to ? of the vertical display and you've reduced the number of ADC bits used from 8 to 6. Scale the waveform to take close to consume full vertical scale and now you are using all 8 bits of the oscilloscope's ADC. Use the most sensitive vertical scaling setting while keeping the waveform on the display.

Many scope vendors allow users to populate the scope with multiple grids. This is done simply to allow users who want to see individual waveforms instead of overlaying the waveforms. One or more waveforms can be placed and scaled in each graticule making the scope display easier to view. Each waveform can be scaled for full-scale vertical value within a grid as shown in figure 2.

Figure 2: Many scopes allow multiple grids such as Agilent's Infiniium where users can have up to four simultaneous grids. This allows for easier viewing while scaling the waveform to use the entire ADC dynamic range.

Do more scope ADC bits enable you to see small signals? Theoretically, yes. In practice, scopes with 12bit ADCs have noise levels that are far able the smallest quantisation levels. Hence, not all 4096 levels can be used as the least significant digits are just quantising noise. 8bit ADCs using high-resolution mode achieve the same noise levels as scopes with 12bit ADCs. This is because the quantisation noise is overshadowed by the front-end noise of the scope.

Tip 3: Exploit the scope's (off screen) dynamic range spec

Scaling a waveform for full-scale usage of the scope's full 8bit ADC is a step in the right direction. Whynot increase the vertical zoom even more?If too much of the signal is above the top or below the bottom of the scope's on-screen vertical limits, the scope's ADC will be driven into saturation. When in saturation, the ADC will not produce valid results. The scope requires an unspecified amount of time to recover from saturation and during this recovery time, no measurements are valid.

Figure 3: In this example with an Infiniium DSO9104A which has a 8 division dynamic range setting, the user moved the signals 4 divisions off centre in order to double the resolution they got when the entire waveform was displayed on screen.

Scope vendors specify an off screen dynamic range. This value is typically given in number of divisions that a signal can be driven off the top or bottom of the display without causing the ADC to be driven into saturation. This technique allows users some additional vertical zooms in order to apply more of the scopes vertical resolution to the portion of the signal on the display. Figure 3 shows an example where the user was about to double the vertical resolution by moving the signal to 4 divisions off screen, then adjust the vertical setting to achieve a 2X increase in resolution.

About the author
Joel Woodward is senior product manager, Oscilloscope Products, at Agilent Technologies. He joined Agilent Technologies (formerly Hewlett-Packard) 24 years ago and is a senior product manager and planner for Agilent's Infiniium oscilloscopes.

Joel holds a degree in Electrical and Computer Engineering from Brigham Young University, an MBA from Regis University, has completed coursework from Harvard Business School, and holds an FPGA debug patent.˜His outside interests include digital photography and hiking in national parks.

To download the PDF version of this article, click here.

?First Page?Previous Page 1???2

Article Comments - Increase vertical resolution of scop...
*? You can enter [0] more charecters.
*Verify code:


Visit Asia Webinars to learn about the latest in technology and get practical design tips.

Back to Top