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Modern digitizing oscilloscopes use analog-to-digital converters (ADCs) to convert real-world signals into discrete digitized values. Most oscilloscopes contain 8-bit ADCs. This corresponds to 256 discrete levels of signal representation per channel. Acquisition accuracy is maximized when using the full dynamic range of the ADC. When viewing multiple channels simultaneously, a common practice has been to decrease the vertical scaling of each signal to fit them within the same grid. However, this practice results in a loss of signal integrity for reasons described below. Another method must be used in order to clearly present waveform data while simultaneously maintaining maximum dynamic range.

For example, a common model is to acquire signals across multiple scope channels then display the resultant waveforms into a single grid. As shown in Figure 1, an oscilloscope acquires on all channels while probing four pins of a counter circuit. The four acquired waveforms are overlapped onto a single display grid. With four channels overlapped onto one grid, waveform shapes become difficult to distinguish. The user is then tempted to reduce the vertical scaling of each channel in order to view the waveforms more clearly. However, and often to the user's surprise, critical voltage and timing parameters such as peak-to-peak, amplitude, period, frequency, width, duty cycle, and risetime measurement values have changed dramatically, and are now reporting inaccurate results. By reducing the volt per division setting, signal integrity has been compromised.

How significant could the loss of signal integrity be by simply adjusting the V/div setting? Consider the case of a series of pulses captured on a single channel, with vertical scaling set to 1 V/div. With eight vertical divisions in the graticule, the screen can display 8 V full scale. If using parametric measurements, the peak-to-peak (pk-pk) value could be 6.41 V for this waveform, if the waveform is occupying (6.41 V)/(8 V) = 80.1% of full scale. The noise riding on the top of the pulse can be isolated and monitored by adjusting the measurement gates to measure pk-pk on the top of the pulse only, thereby measuring only the noise riding on the crest of the pulse. With vertical scaling of 1 V/div, the mean value of noise on the pulse is 886.7 mV. Changing vertical scaling to 5 V/div, the mean value of pk-pk noise would now change to 1.783 V.

Further adjusting vertical scaling shows that channel vertical scaling significantly affects the measured noise level. At 550 mV/div, the mean noise level would be 821 mV; at 2 V/div, the noise level is 1.02 V, and at 10 V/div, the measured noise level has increased to 3.06 V. Between 2 V/div and 10 V/div, the measured noise level tripled (factor of 3x) when changing the V/div by a factor of 5x. Note that V/div settings also affect the accuracy of timing measurements. Because horizontal measurements such as period and frequency use vertical top and base measurements to perform thresholding, therefore, the timing measurements have also been compromised by reducing the volt per division setting.

Why does measurement accuracy vary with changes in V/div setting? The dynamic range of the oscilloscope is the range of signal amplitudes that the ADC can process effectively. The minimum of the range occurs where signal power equals noise power. The maximum of the range occurs at or near full scale where maximum counts of the ADC are used while digitizing the waveform, while distortion is minimized. An 8-bit ADC has 2^8 = 256 quantization levels. When using one-quarter of the dynamic range, only a maximum of 64 of the 256 quantization levels are used for acquiring channel 1. Using one-quarter of quantization levels, which occurs when the waveform occupies only one-quarter of the display, then results in a maximum of 6-bit resolution on the acquired channel (2^6 = 64). This loss of resolution causes an increase in quantization noise.

How can the compromise between maximizing dynamic range, and clearly viewing multiple signals, be resolved? Using multiple display grids offers the capability of splitting the display area into separate grids, each containing the full dynamic range of the digitizer. Figure 2 shows eight physically separate grids each containing one input signal at full deflection. Each grid contains full dynamic range and there is no loss of signal integrity by using multiple grids. By enabling the multigrid display, dynamic range can be optimized while all signals are clearly viewed. Multigrid displays eliminate the compromise between clearly viewing multiple channels and maximizing dynamic range.

Mike Hertz is a field applications engineer with LeCroy Corp. in Michigan. Before joining LeCroy, he was an applications engineer with Agilent Technologies. He has three U.S. patents pending in the area of oscilloscope measurement design. Hertz received a BSEE from Iowa State University and an MSEE from the University of Arizona.

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