To input an analog voltage into PLC the continuous voltage value must be sample and then converted to a numerical value by an Analog to Digital Converter (ADC). Figure below shows continuous voltage change overtime. There are three samples on the figure.
The process of sampling data is not instantaneous, so each sample has a start and stop time. The time required to acquire the sample is called sampling time. A/D converter can only acquire a limited number of samples per second. The time between samples is called the sampling period T, and the inverse of sampling period is the sampling frequency or sampling rate. The sampling time is often much smaller than sampling period. The sampling frequency is specified when buying hardware, but for a PLC maximum sampling rate might be 20 Hz.
The data value sampled will be somewhere between the voltage at the start and end of the sample. The maximum (Vmax) and minimum (Vmin) voltages are a function of the control hardware. These are often specified when purchasing hardware, but usually are:
• 0V to 5V
• 0V to 10V
• -5V to 5V
• -10V to 10V
The number of bits of the A/D Converter is the number of bits in the result word. If the A/D Converter is 8 bits the result can read up to 256 different voltage levels. Most of A/D Converters have 12 bit, 16 bit converters are used for precision measurement.