An analog input is a measurable electrical signal with a defined range that is generated by a sensor and received by a controller. The analog input changes continuously in a definable manner in relation to the measured property.
The analog signals generated by some types of sensors must be conditioned by converting to a higher-level standard signal that can be transmitted over wires to the receiving controller. Analog inputs are converted to digital signals by the analog-to-digital (A/D) converter typically located at the controller. Analog-to-digital conversion is limited to a small range of DC voltage, so that internal or external input circuitry must change the character of non-compatible signal types to a DC voltage range within the limits of the A/D converter.
There are basically three types of analog input signals; voltage, current and resistance.
Common voltage signals used in the controls industry are 1-5 Volts Direct Current (VDC), 2-10 VDC, 3-15 VDC, 0-5 VDC, 0-10 VDC and 0-15 VDC.
The 4-20 mA signal has become the industry’s standard current signal for use with analog and digital controllers. A variation of the 4-20 mA signal is 0-20 mA.
Resistance measurement is most commonly associated with direct inputs from temperature sensing devices, such as thermistors and RTD's. RTD nominal resistances are typically 100 W, 500 W, 1000 W or 2000 W. Common thermistor nominal resistances are 2252 W, 3k W, 10k W, 20 kW or 100 kW .
The following circuit diagrams are examples of commonly used analog input configurations.
Figure 4 shows a voltage input circuit where the sensor output voltage does not match the controller.
Figure 5 shows the wiring schematic associated with a typical externally powered 4-20 mA analog input using a loop power 4-20 mA temperature transmitter. For this circuit type, typical power supply voltage is nominally 24 VDC. The circuitry in the transmitter regulates current flow in the loop between 4 and 20 mA in proportion to the temperature sensed by the sensor. A parallel fixed resistor is used at the controller terminals to complete the circuit. The resistance of the A/D converter in the circuit is very high in comparison to R, essentially all of the current flows through the resistor. The value of the resistor is chosen to match the input voltage range of the controller.
Figure 6 depicts the circuit for converting a resistance to voltage, in this case, a 10 kW Thermistor-type sensor.