What is Dynamic Range?

Question:  What is Dynamic Range, and why is it important?

Answer:  Dynamic Range is the maximum ratio of the levels of two signals simultaneously present at the input of an instrument. In other words what is the smallest and largest signal levels that can be measured at the same time without errors? Different instruments provide different dynamic range performance due to architectural advantages. Oscilloscopes typically have wide bandwidth but may have less dynamic range relative to other instrumentation like spectrum analyzers.

Spectrum analyzers look at a selected band of frequency in a high dynamic range.  These instruments are very good as resolving close-in frequencies relative to a carrier.  Example:  a sine wave oscillator with low level AM coupling may look like noise or jitter in the time-domain on an oscilloscope.  The spectrum analyzer will help resolve the low level discrete frequency components to enable diagnosis of the coupling agents.

An analog to digital converter (ADC) range is based on the resolution of the ADC. Most real-time oscilloscopes use an ADC with 8 bits of resolution and have a theoretical dynamic range of about 48 dB (6 dB per bit). In order to effectively optimize an oscilloscope for best use of the dynamic range careful control of the volts-per-division control is recommended. The volts-per-division setting (usually written as volts/div) is a scaling factor that varies the size of the waveform on the screen. If the volts/div setting is 5 volts, then each of the ten vertical divisions represents 5 volts and the entire screen can display 50 volts from bottom to top, assuming a reticule with ten major divisions. If the setting is 0.5 volts/div, the screen can display 5 volts from bottom to top, and so on.

Below are two examples of voltage measurements that can be compared with good and poor dynamic range utilization. Note the measurement differences from both setups despite measuring the same signal.

Measurements    Mean    Std Deviation

High (Good)        311mV    0.4mV

High (Poor)          299mV    4mV  

Pk-Pk (Good)      428mV    2.4mV

Pk-Pk (Poor)        528mV    12.5mV

High Dynamic Range

Low Dynamic Range

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