Most sampled signals are not simply stored and reconstructed. But the fidelity of a theoretical reconstruction is a customary measure of the effectiveness of sampling. That fidelity is reduced when s( t) contains frequency components whose periodicity is smaller than two samples or equivalently the ratio of cycles to samples exceeds ½ (see Aliasing). The quantity ½ cycles/sample × f s samples/sec = f s/2 cycles/sec ( hertz) is known as the Nyquist frequency of the sampler. Therefore, s( t) is usually the output of a low-pass filter, functionally known as an anti-aliasing filter.
Without an anti-aliasing filter, frequencies higher than the Nyquist frequency will influence the samples in a way that is misinterpreted by the interpolation process. In practice, the continuous signal is sampled using an analog-to-digital converter (ADC), a device with various physical limitations. Various types of distortion can occur, including: This results in deviations from the theoretically perfect reconstruction, collectively referred to as distortion. Some amount of aliasing is inevitable because only theoretical, infinitely long, functions can have no frequency content above the Nyquist frequency. Aperture error results from the fact that the sample is obtained as a time average within a sampling region, rather than just being equal to the signal value at the sampling instant.Īliasing can be made arbitrarily small by using a sufficiently large order of the anti-aliasing filter.In a capacitor-based sample and hold circuit, aperture errors are introduced by multiple mechanisms. Jitter or deviation from the precise sample timing intervals.įor example, the capacitor cannot instantly track the input signal and the capacitor can not instantly be isolated from the input signal.Noise, including thermal sensor noise, analog circuit noise, etc.Slew rate limit error, caused by the inability of the ADC input value to change sufficiently rapidly.
0 kommentar(er)
