What's the difference between Top and Base, vs Maximum and Minimum?
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Introduction
Top and Base are measures of the high and low levels of a signal, excluding any noise or ringing present on those levels. The difference between the top and the base is the amplitude.
Maximum and Minimum identify the highest and lowest points of a waveform, including any overshoot or undershoot, noise and ringing. The difference between the Maximum and Minimum is the Peak to Peak.
Background
The image here gives an example of maximum compared to top.
The top of this rising edge has an overshoot. The maximum measurement finds the very highest value of the overshoot, while the top measurement ignores the overshoot.
Maximum measurements are useful when ensuring a voltage does not exceed a certain level under any circumstances, while the top measurement is useful for ensuring, for example, that a digital signal’s high and low voltages meet the specification.
All of these terms can be inverted to discuss minimum vs base. The minimum measurement finds the very lowest value of the signal, while the base will neglect overshoot.
The peak to peak measurement is the difference between the maximum and the minimum (as opposed to the amplitude, which is the difference between the top and the base).
The image below shows some example measurements. In the measurements panel, the maximum and top measurements can be compared – the positive overshoot is included in the maximum measurement, but ignored in the top measurement.
Calculation and algorithm
The algorithm for max and min is very simple: It just finds the largest and smallest values in the buffer:
Maximum Measurement = max (x₁, x₂, ..., xₙ).
Minimum Measurement = min (x₁, x₂, ..., xₙ).
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Where x₁, x₂, ..., xₙ are the sample values along the waveform.
Top and base measurements are calculated using the histogram method. In this case, the histogram measures the vertical axis. The histogram contains a number of bins which span across the measured signal range: for example, from -5 V to +5 V.
Each bin’s width is a range of sample values set by the measured sample range. For example, a 9-bit scope would use the calculation , so each bin would be approximately 20 mV wide.
Each bin contains a count of how many samples falls within its (for example) 20 mV range. If a bin contains a large number of samples then that bin is likely to represent a relatively flat portion of the waveform. In contrast, a bin that contains very few samples would occupy a rapidly changing portion of the waveform such as an edge.
To obtain Top and Base measurements, only bins that are at the high and low portions of the histogram are considered. From those portions, the bins with the highest counts are taken. The mean value of those bins then gives the actual value.
The amplitude is calculated as: Amplitude = Top - Base
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