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Boost oscilloscope vertical resolution (Part 1)

Posted: 25 Apr 2013     Print Version  Bookmark and Share

Keywords:oscilloscope  small signal visibility  dynamic range 

Do you require 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.

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

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 characterise 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 Ω input path. You can also run the test for the 1MΩ 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 8bits 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.

Tip 2: Scale waveforms for maximum ADC resolution
Resolution is the smallest quantisation level determined by the oscilloscope. An 8bit ADC can encode an analogue 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).

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