Posted on Feb 5, 2014

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

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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...

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