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Oscilloscope Sample Rates versus Sampling Fidelity
How to make the most accurate digital measurements
Digital storage oscilloscopes (DSO) are the primary tools used today by digital designers to perform signal integrity measurements such as setup/hold times, eye margin, and rise/fall times. The two key banner specifications that affect an oscilloscope’s signal integrity measurement accuracy, are bandwidth and sample rate. Most engineers have a good idea of how much bandwidth they need for their digital measurements. However, there is often a lot of confusion about required sample rates—and engineers often assume that scopes with the highest sample rates produce the most accurate digital measurements. But is this true?
When you select an oscilloscope for accurate, high-speed digital measurements, sampling fidelity can often be more important than the maximum sample rate. Using side-by-side measurements on oscilloscopes with various bandwidths and sample rates, this application note demonstrates a counterintuitive concept: scopes with higher sample rates can exhibit poorer signal fidelity because of poorly aligned analog-to-digital converters (ADCs) that are time-interleaved to produce a higher net real-time maximum sample rate. This application note also will show how to easily characterize and compare scope ADC sampling fidelity using both time-domain and frequency-domain analysis techniques.
Table of contents
Nyquist’s Sampling Theorem
Interleaved Real-Time Sampling
Testing for Interleave Distortion
- Effective number of bits analysis using sine waves
- Visual sine-wave comparison tests
- Spectrum analysis tests
- Digital clock Measurement stability tests
Interleaved Real-Time Sampling
When ADC technology has been stretched to its limit in terms of maximum sample rate, how do oscilloscope vendors create scopes with even higher sample rates? The drive for higher sample rates may be simply to satisfy scope users’ perception that “more is better” or higher sample rates may actually be required to produce higher-bandwidth real-time oscilloscope measurements. But producing higher sample rates in oscilloscopes is not as easy as simply selecting a higher sample rate off-the-shelf analog-to-digital converter.
A common technique adopted by all major scope vendors is to interleave multiple real-time ADCs. But don’t confuse this sampling technique with interleaving samples from repetitive acquisitions, which we call “equivalent-time” sampling.
Scopes with real-time interleaved sampling must adhere to two requirements. For accurate distortion-free interleaving, each ADC’s vertical gain, offset and frequency response must be closely matched. Secondly, the phase-delayed clocks must be aligned with high precision in order to satisfy Nyquist’s rule #2 which dictates equally-spaced samples. In other words, the sample clock for ADC #2 must be delayed precisely 180 degrees after the clock that samples ADC #1. Both of these criteria are important for accurate interleaving. However, for a more intuitive understanding of the possible errors that can occur due to poor interleaving, the rest of this paper will focus on errors just due to poor phase-delayed clocking.