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Description: 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 produced by certain types of sensors require conditioning to convert them into a higher-level standard signal suitable for transmission over wires to the receiving controller. Analog inputs are converted to digital signals by an analog-to-digital (A/D) converter typically located at the controller. The analog-to-digital conversion is confined to a limited range of DC voltage, necessitating the modification of non-compatible signal types to a DC voltage range that falls within the A/D converter's limits. The 4-20 mA signal has become the industry standard current signal for use with both analog and digital controllers. A variation of the 4-20 mA signal is the 0-20 mA signal. Resistance measurement is most commonly associated with direct inputs from temperature sensing devices, such as thermistors and RTDs. RTD nominal resistances are typically 100 Ω, 500 Ω, 1000 Ω, or 2000 Ω. Common thermistor nominal resistances are 2252 Ω, 3k Ω, 10k Ω, 20k Ω, or 100 kΩ. A schematic diagram illustrates a typical externally powered 4-20 mA analog input utilizing a loop-powered 4-20 mA temperature transmitter. In this circuit configuration, the typical power supply voltage is approximately 24 VDC. The circuitry within 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 employed at the controller terminals to complete the circuit. The resistance of the A/D converter in the circuit is significantly higher than that of the resistor, ensuring that nearly all of the current flows through the resistor. The value of the resistor is selected to align with the input voltage range of the controller.

The described analog input system is fundamental in various industrial applications, where accurate temperature measurement and control are critical. The 4-20 mA signal standard is favored due to its robustness against electrical noise and its ability to transmit over long distances without significant signal degradation. The use of a loop-powered transmitter allows for a simplified wiring scheme, as the same two wires that carry the signal also provide the necessary power to the transmitter.

In the schematic, the temperature sensor (RTD or thermistor) is connected to the input of the transmitter. The transmitter converts the resistance change, which corresponds to temperature variations, into a current signal that adheres to the 4-20 mA standard. The controller, designed to interpret this current signal, employs an A/D converter to digitize the analog input for processing and monitoring. The selection of the fixed resistor at the controller terminals is crucial, as it determines the voltage drop across the resistor, allowing the controller to accurately measure the current flowing through the loop.

The design considerations include ensuring that the A/D converter's input impedance is sufficiently high to avoid loading effects that could distort the measurements. Additionally, the choice of the fixed resistor value must be compatible with the expected range of current flow to maintain linearity and accuracy in signal representation. Overall, this configuration exemplifies the integration of analog sensors with digital control systems, facilitating effective monitoring and control in automated processes.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 signa

l 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.

The 4-20 mA signal has become the industrys 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, 500W, 1000 W or 2000 W. Common thermistor nominal resistances are 2252 W, 3k W, 10k W, 20 kWor 100 kW. 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.