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Zero-order hold



The Zero-order hold (ZOH) is a mathematical model of the practical reconstruction of sampled signals done by conventional digital-to-analog converters (DAC). A mathematical model such as the ZOH (or possibly the first-order hold) is necessary because, in the sampling and reconstruction theorem, a sequence of dirac impulses, xs(t), representing the discrete samples, x(nT), is low-pass filtered to recover the original signal that was sampled, x(t). However, outputting a sequence of dirac impulses is decidedly impractical. Most conventional DACs output a voltage proportional to the discrete sample value and hold that voltage to a constant value for the duration of the sampling interval and then change that voltage rapidly to the value corresponding to the next discrete sample value.



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