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Tips for increasing oscilloscope vertical resolution – Part I

Tips for increasing oscilloscope vertical resolution – Part I

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By eeNews Europe



Do you need your oscilloscope to view increasingly smaller electronic signal detail? You are not alone. Interest is growing in small signal visibility – both for current and voltage. Specifically, scope users want better ability to see small signal changes on a large signal (high dynamic range measurements), or seeing small signals where high dynamic range isn’t required. Often these signals change over just a few millivolt or milliamps. Industry examples include testing of high-quality power rails, medical technology developed to interacts with human physiology, high-energy physics one-time experiments that produce small pulses, and mobile devices where current and overall power consumption in sleep mode is critical.

Measurement of very small signals can be challenging as the ability to view small signals is impacted not only by the noise of the scope, but also by scope settings and probing. The following seven techniques can help your scope see smaller signals than you’ve previously seen with it.

Tip 1: Start with a scope that has low noise

While the other techniques described to see small signals are scope vendor agnostic, having a scope with low noise is critical if you really want visibility to small currents and voltages. You won’t be able to see signal details smaller than the noise level of the scope.

What’s a quick way to check how much noise a specific scope has? Most oscilloscope vendors will characterize noise for a specific model numbers and include these values on the product datasheet. If not, you can ask for the information, or find out yourself. It’s easy to measure in a few minutes. Disconnect all inputs from the front of the scope and set the scope to 50  input path. You can also run the test for the 1M path. Turn on a decent amount of acquisition memory, 100Kpts to 1Mpts will suffice, run the scope with infinite persistence and see how thick the resulting waveform is. The thicker the waveform, the more noise the scope is producing internally.

Each scope channel will have unique noise qualities at each vertical setting. You can view the noise visually just by looking at wave shape thickness, or you can be more analytical and take a Vrms AC measurement to quantify. Create a chart as the one shown in figure 1. These measurements will allow you to know how much noise each scope produces. Don’t expect to measure signals that are less than the noise of the scope.

The industry now offers several scopes with more than 8-bits of resolution. How valuable are the additional bits? Provided there is a sufficient signal-to-noise ratio (SNR), more ADC bits allow finer details of the signal to be seen. Noise typically plays a greater role in limiting some of the effectiveness of the additional bits of resolution.

Figure 1: Noise is typically characterized with typical values listed in vendor datasheets. Alternatively, you can characterize noise on your scope in a few minutes
Click on image to enlarge


Tip 2: Scale waveforms for maximum ADC resolution

Resolution is the smallest quantization level determined by the oscilloscope. An 8-bit ADC can encode an analog input to one in 256 different levels, since 28=256. The ADC operates on the scope’s full scale vertical value. Thus, the Q-level steps are associated with the full-scale vertical scope setting. If the user adjusts the vertical setting to 100mV per division, full screen equals 800 mV (8 divisions * 100 mV/div) and Q-level resolution is equal to 3.125 mV/level (800mV divided by 256 levels).

Scaling the waveform to take the whole display of the scope enables you to use more of the scope’s analog-to-digital (ADC) converter. If a signal is scaled to take up only ½ of the vertical display, you’ve just decreased the number of ADC bits being used from 8 to 7. Scale the waveform to ¼ 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.


31 mV resolution


16.5 mV resolution

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

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.

Do more scope ADC bits enable you to see small signals? Theoretically, yes. In practice, scopes with 12-bit ADCs have noise levels that are far able the smallest quantization levels. Hence, not all 4096 levels can be used as the least significant digits are just quantizing noise. 8-bit ADCs using high-resolution mode achieve the same noise levels as scopes with 12-bit ADCs. This is because the quantization 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 8-bit ADC is a step in the right direction. Why not 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.

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.

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 center in order to double the resolution they got when the entire waveform was displayed on screen
Click on image to enlarge


Editor’s note

In Part Two, Agilent will present four other techniques that can help you observe smaller signals than you have ever seen with your scope.

About the author

Joel Woodward is senior product manager, Oscilloscope Products, at Agilent Technologies.

Joel 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.

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