Analog input module

Abstract

An analog input module includes an activation unit, an analog/digital converter, an input circuit with a first input and a second input and a voltage divider disposed between the first input and the second input. The activation unit is connected to the first input and the second input and is connected upstream of the analog/digital converter. The voltage divider has center tap which is connected to a first comparator input of a comparator of a closed-loop control unit. The closed-loop control unit has a switching device disposed between the second input and a reference point. The comparator includes a second comparator input which is connected to a reference source. The comparator has a control output which is connected to a control input of the switching device. The closed-loop control unit is configured such that the switching device enables or disables a current path to the reference point.

What is claimed is: 
     
       1. An analog input module comprising;
 an activation unit; 
 an analog-to-digital (AD) converter; 
 an input circuit with a first input and a second input; and 
 a voltage divider disposed between the first input and the second input, 
 wherein the activation unit is connected to the first input and the second input and is connected upstream of the analog/digital converter, 
 wherein the voltage divider has center tap which is connected to a first comparator input of a comparator of a closed-loop control unit, 
 wherein the closed-loop control unit has a switching device disposed between the second input and a reference point, 
 wherein the comparator includes a second comparator input which is connected to a reference source, 
 wherein the comparator has a control output which is connected to a control input of the switching device, 
 wherein the closed-loop control unit is configured such that the switching device,
 (1) on connection of a two-wire transducer to the first input and while the second input remains unconnected, enables a current path to the reference point; and 
 (2) on connection of a four-wire transducer to the first input and the second input, disables the current path to the reference point. 
 
 
     
     
       2. The analog input module of  claim 1 , wherein the voltage divider has a first resistor and a second resistor in a series circuit, and wherein the center tap is disposed between the first and the second resistor. 
     
     
       3. The analog input module of  claim 1 , wherein the comparator is an operational amplifier and the switching device is semiconductor switch.