Define and Measure Dynamic Range

One of the breakthrough features of the CoCo-80, Spider-80X, and Spider-81 is their outstanding dynamic range performance of 150 dBFS. This kind of performance has never before been achieved with such small, portable, and low cost devices. The main advantage of this high dynamic range is that it eliminates the need to adjust the input gain/range settings on the front end, thus simplifying the setup and measurement process. Both full scale 10 volt signals and very small microvolt signals are measured at the same time with no adjustments. This capability makes setup and testing more simplified in comparison with an instrument capable of only a lower dynamic range.

Dynamic range is one of the critical performance specifications of a dynamic measurement system. If the dynamic range is too low, large signals will typically be clipped and distorted, while small signals are buried in system noise. This noise is inherent in the electronics associated with transducer elements and transducer signal conditionings. Maximizing dynamic range is paramount in instrumentation design, so that both small and the large signals are measured accurately.

Understanding dynamic range is complicated by the fact that different vendors describe and measure it differently. This is understandable because there is no universally accepted technical definition. Instead, dynamic range is loosely defined as the ratio between the largest and smallest signal a system can accurately measure at the same time. There are several issues with this vague definition:

  1. What does the “same time” mean? Do we compare the small signal on top of a large signal to the large signal, or a small signal to a large signal that is present at the same time record but at different periods?
  2. What does the “largest signal” mean? There are three commonly used measures of the largest signal and they each have a place.
    1. Peak-to-Peak or Full-Scale Range. This value is the difference between the two most extreme signals.
    2.  Zero-to-Peak. This value is the difference between the mid-value and the maximum value;
    3. RMS Full-scale. This is the root-mean-square value of the largest possible sine wave signal (=0.707 x its peak amplitude)
  3. What does the “smallest signal” mean? Does this refer to noise? Does it only include a fixed band-limited range, or the full analysis band? This is important because the measured RMS noise level will depend on the bandwidth over which it is measured.

In order to understand a dynamic range specification you must understand these issues and know how the manufacturer measures the dynamic range so that you can compare one device to another. This document describes the concept of dynamic range, the different methods used to assess it, and how Crystal Instruments assesses dynamic range of the instruments: Define and Measure Dynamic Range