As A/D converters (ADC) and data acquisition boards increase their bandwidth, more and more are including the spurious free dynamic range (SFDR) specification as an indicator of their fidelity. The converter is not the only source of spurious signals; however, because of complex interactions between the ADC and the signal conditioning circuits that invariably precede it. The key to properly interpreting this specification lies in understanding the sources of spurious signals and how SFDR is measured.
The basic definition of the SFDR specification is simple. It is the strength ratio of the fundamental signal to the strongest spurious signal in the output. In many cases, the spurious signal is the result of non-linearity in the A/D conversion, hence the interpretation of SFDR as an indicator of fidelity. But a number of other sources of strong spurious signals may be present in the DAQ system, so the SFDR specification requires a closer look.
Because ADCs are never used as the only element between the input signal and the digital output, the place to begin this closer look is by considering all of the elements in a DAQ system. A DAQ module contains several key functions, including a signal-conditioning filter, a sample-and-hold circuit, and in many cases an analog multiplexer to make one ADC handle multiple input signals. Non-linearity in any of these elements can generate spurious signals that can affect the achievable SFDR.
As a result of these numerous spurious sources, the significance of SFDR in many systems is not the converted signal’s fidelity, but the impact of the spurious signal as a noise source. In effect, SFDR indicates the lowest-energy input signal that can be distinguished from spurious signals. Any signal below the SFDR cannot be reliably identified as a true signal instead of as a spurious one. The practical ramification of this ambiguity is that the spurious signals can mask desired signals.
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