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Spurious-free dynamic range (SFDR) is the ratio of the rms signal amplitude to the rms value of the peak spurious spectral component. Two fundamental limitations to maximizing SFDR in a high-speed ADC are the distortion produced by the front-end amplifier and the sample-and-hold circuit; and that produced by nonlinearity in the transfer function of the encoder portion of the ADC. The key to achieving high SFDR is to minimize both sources of nonlinearity. Nothing can be done externally to the ADC to significantly reduce the inherent distortion caused by its front end. However, the differential nonlinearity in the ADC’s encoder transfer function can be reduced by the proper use of dither (external noise that is intentionally summed with the analog input signal). Dithering can be used to improve SFDR of an ADC under certain conditions (see Further Reading 2–5). For example, even in a perfect ADC, some correlation exists between the quantization noise and the input signal. This correlation can reduce the SFDR of the ADC, especially if the input signal is an exact sub-multiple of the sampling frequency. Summing about 1/2-LSB rms of broadband noise with the input signal tends to randomize the quantization noise and minimize this effect (see Figure 5a). In most systems, however, the noise already riding on top of the signal (including the input-referred noise of the ADC) obviates the need for additional dither noise. Increasing the wideband rms noise level beyond approximately one LSB will proportionally reduce the SNR and result in no additional improvement. |
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发表于 2008-6-5 14:02:25