The oscilloscope’s acquisition modes

The acquisition modes of an oscilloscope control how waveform points are generated from sample points, which refer to the digital values taken directly from the analog-to-digital converter (ADC). The difference in time between the sample points is known as the sample interval. The stored digital values in the oscilloscope’s memory, waveform points are displayed to construct the waveform. Waveform interval refers to the time between these waveform points.

Waveform interval and sample interval may or may not be similar. This is attributed to the fact that there are various acquisition modes in which a waveform point features a number of sequentially acquired sample points.

The most commonly utilized acquisition modes are sample mode, peak detect mode, hi-res mode, average mode, envelope mode, waveform database mode and random equivalent time (ET) mode.

The simplest acquisition mode, sample mode creates a waveform point in the oscilloscope by saving one sample point during every waveform interval.

In peak detect mode, the oscilloscope utilizes the maximum and minimum sample points saved during two waveform intervals, as its corresponding waveform points. This mode is specifically useful for viewing narrow pulses spaced far apart in time. Although operating at a slow time base settings, digital oscilloscopes with peak detect mode operates the ADC at a fast sample rate and are capable of capturing fast signal changes occurring between the waveform points that are in sample mode.

Just like peak detect mode, hi-res is a manner of obtaining more information when the ADC samples faster than that required by the time base settings. Thus, multiple samples within one waveform interval are averaged to produce a single waveform point. This effectively reduces the noise and improves the resolution in low-speed signals. Unlike average, hi-res mode can be utilized even on a single shot event.

The oscilloscope, in average mode, saves a sample point taken during a waveform interval. The final displayed waveform is produced by averaging the waveform points from consecutive acquisitions. Although this mode requires a repeating signal, average mode minimizes noise without compromising bandwidth.

Like peak detect mode, envelope mode also combines the minimum and maximum waveform points from multiple acquisitions. However, envelope mode forms the waveform to determine the min/max accumulation over time. Peak detect mode is commonly utilized to acquire the combined records that form the envelope waveform.

The oscilloscope, in waveform database mode, accumulates a waveform database that delivers a three-dimensional array of time, amplitude and counts.

In random equivalent time (ET) mode, the oscilloscope gathers a waveform record from acquisitions in various trigger events. Samples will, then, fill parts of the waveform record as they randomly occur in any one trigger with respect to samples from other trigger. Although this mode can capture a number of samples in a single acquisition, this cannot be utilized on single-shot signals as it demands a repetitive waveform that is consistent from trigger to trigger.