Sample & Hold
The Signal given to the digital controller is a sampled data signal and in turn the controller gives the controller output in digital form. But the system to be controlled needs an analog control signal as input. Therefore the digital output of controllers must be converters into analog form.
This can be achieved by means of various types of hold circuits. The simplest hold circuits are the zero order hold (ZOH). In ZOH, the reconstructed analog signal acquires the same values as the last received sample for the entire sampling period.
The high frequency noises present in the reconstructed signal are automatically filtered out by the control system component which behaves like low pass filters. In a first order hold the last two signals for the current sampling period. Similarly higher order hold circuit can be devised. First or higher order hold circuits offer no particular advantage over the zero order hold.