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High-speed digitizer selection depends on more than banner specifications

Selecting the best digitizer for any application is a complex task. At a minimum, it involves an in-depth evaluation of what must be measured, how it can be measured and the required degree of accuracy. These factors should then be used in the comparison the myriad of available acquisition schemes and specifications for any individual low-cost, mid-performance or high-speed digitizer.

Digitizer manufacturers typically present “banner specifications” such as bandwidth, resolution and sampling rate for each device. Although these are offered as an indication of instrument quality, they often have little or no impact on the ultimate measurement fidelity in many applications.

Consider five key specifications
There is no simple answer to the question, “Which digitizer should I use?” Instead, each application and device must be reviewed on a case-by-case basis. Fortunately, a shortlist of key specifications can help distinguish the best low-cost, mid-performance or high-speed digitizer for an application:

  • SINAD ratio: The signal-to-noise-and-distortion (SINAD) ratio is needed to calculate the effective number of bits (ENOB) for a digitizer.
  • ENOB: The ENOB value provides information about amplitude-measurement accuracy that isn’t captured by a simple resolution specification such as “16 bits.”
  • Linearity: This addresses the fact that the discrete measurement levels described by the bit resolution are not necessarily equally spaced, leading to amplitude-measurement error.
  • Sampling error: Factors such as clock accuracy and sampling jitter are better indicators of frequency-measurement accuracy than a simple sample rate. This specification is especially important for high-speed digitizer modules.
  • SFDR: Spurious-free dynamic range affects the usable dynamic range in a measurement.