Memory Depth in Digital Storage Oscilloscopes (DSO)

Digital Storage Oscilloscopes (DSOs) are available with varying amounts of memory depth. This parameter is often misunderstood and misinterpreted when purchasing a new DSO.

Since these scopes are based on digital technology, the analog signal that is applied to the oscilloscopes input terminal is converted to a digital value which is then stored in memory so it may be processed and displayed.

This amount of memory becomes important as you capture a longer period of time (more samples). The more memory available, the higher you can keep the sample rate of the scope. This means a higher “effective” bandwidth. As an example, we will look at memory depth as it applies to a typical DSO with a 1 GSa/sec sample rate at various time base settings.

Sample rate = memory depth/(time per division*10)

So using an example of a sweep rate of 100 us/div

with a 1 Mpts memory depth we are at the maximum sample rate possible of 1 GSa/sec. But if we slow the sweep rate down to 1 ms/div we now get an equivalent sample rate of 100 Msa/sec. Modern DSOs will automatically adjust the Memory Depth to maximize the sample rate. Note that scope with a 2 Mpts memory depth would double the equivalent sample rate at this slower time base setting.

Memory depth can be an important parameter when viewing low frequency of slowly changing signals but may not be as critical at faster time base settings.


George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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