Multiplexed signal reference level equalizer
BACKGROUND OF THE INVENTION 
     1. Field Of The Invention 
     The present invention is directed to signal multiplexers. More specifically, the present invention is directed to a signal multiplexer circuit having a multiplexed signal reference level equalizer. 
     2. Description Of The Prior Art 
     The multiplexing of an input signal to a contemporary digital computer process control system involves a selection of a process signal from a plurality of remotely generated signals for application to a digital computer interface such as an analog-to-digital converter. In order to ascertain the operability of the interface, a known calibration signal is provided for selection according to a predetermined sequence. The calibration signal, thus, provides a known reference against which the operation of the interface can be checked. On the other hand, the calibration signal is usually generated locally while the process signals are remote, or field, generated signals which gives rise to a difference in signal common, or reference, levels. In order to allow the interface to accept either type of input signals, the local and remote signals must be equalized, i.e., the offset of their reference levels is compensated for, to allow either type of input signal to be utilized by the process control system. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an improved signal multiplexer circuit having a signal equalizing capability for providing an output signal having a fixed relationship to selectable input signals generated either locally or remotely. 
     In accomplishing this and other objects, there has been provided, in accordance with the present invention, a signal multiplexing circuit having an input multiplexer arranged to select one of a plurality of input signals. The input signals are either remotely generated signals or locally generated signals. The remotely generated signals have a common mode signal which is the difference between the common signal level of the remote signal and a local common level superimposed thereon. A locally generated signal has the common mode signal superimposed thereon by the multiplexing circuit. The output signal from the multiplexer is applied to the non-inverting input of an operational amplifier while the inverting input of the operational amplifier is provided with the common mode signal. The operational amplifier is arranged to subtract the common mode signal from the selected input signal. The multiplexer circuit components are selected to provide an output signal from the multiplexer circuit which is 1.5 times the selected input signal for both the remote input signal and the local input signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention may be has when the following detailed description is read in connection with the accompanying drawing in which the single figure is the schematic illustration of a multiplexer circuit embodying the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to the single figure drawing in more detail, there is shown a multiplexer and equalizer circuit having a multiplexer circuit 2 arranged to select one of a plurality of input signals. For purposes of illustration, the local multiplexer 2 is shown as being provided with three input signals applied to input lines 4, 6, and 8 and two locally generated signals applied to input lines 10 and 12. The output signal from the multiplexer is applied through a first resistor 14 to the non-inverting input of a first operational amplifier 16. The output signal from the operational amplifier 16 is applied to an output terminal 18 as a multiplexer output signal V o  and through a feedback resistor 20 to the inverting input of the operational amplifier 16. A second input signal for the inverting input of the operational amplifier 16 is obtained from a circuit arranged to produce an output signal which is the common mode, or difference, signal between the local common signal level and the remote common signal level. Specifically, a second operational amplifier 22 is arranged to have its output connected through a resistor 24 to the inverting input of the first operational amplifier 16 and through a feedback resistor 26 to the inverting input of the second amplifier 22. The inverting input of the second amplifier 22 is also connected through a resistor 28 to the local common connection 30. The non-inverting input of the second amplifier 22 is connected through a resistor 32 to the remote common connection 34. Calibration, or local, signals to be used as two of the inputs for the multiplexer 2 are obtained from a signal dividing network connected between a reference signal source V ref  and a local common connection 30. Thus, a series string of six resistors 36, 38, 40, 42, 44 and 46 are connected between the reference signal V ref  and the local common 30. The third resistor 40 is a potentiometer having its slider connected to input line 12 of the multiplexer 2 while the fifth resistor 44 is also a potentiometer having its slider connected to input line 10 of the multiplexer 2. Thus, the third resistor 40 is arranged as a source of a high calibration signal while the fifth resistor is a source of a low calibration signal. A common mode signal from the second amplifier 22 is applied to applied to the calibration signal circuit from the junction of the first and second resistors 36 and 38 to the junction between the fifth and sixth resistors 44 and 46. Specifically, a pair of series resistors 48 and 50 are connected between the aforesaid junctions while the junction between these resistors is connected to the output line from the second operational amplifier 22 before the input resistor 24 of the first operational amplifier 16. 
     MODE OF OPERATION 
     In operation, the multiplexer circuit of the present invention shown in FIG. 1 is effective to provide an output voltage which is a function of a preselected one of a plurality of input signals to a multiplexer 2. While, for purposes of illustration, three remote input signals and two calibration input signals have been shown it should be noted that other numbers of remote signals and calibration signals could be handled by the circuit with a conventional modification of the multiplexer 2 and the calibration signal circuitry. Referring to the single figure drawing, the output signal V o  can be a function of one of the remote input signals V 1 , V 2  and V 3  or one of the calibration signals V cal-lo  and V cal-li . If a remote input signal is selected by the multiplexer 2, the multiplexer output signal will be a function of the selected remote signal and a common mode signal which is the difference between the signal level of the remote common and the local common. The reference level equalizing portion of the multiplexer circuit of the present invention is arranged to subtract the common mode signal from the selected remote signal to produce a multiplexer circuit output signal V o  that is independent of the common mode signal and has a fixed relationship to the selected remote input signal. Similarly, for a local input signal selected by the multiplexer 2, the common mode signal is added to the selected local signal and is subsequently subtracted as in the case of the selected remote input signal. Thus, in either case, the multiplexer circuit of the present invention is effective to produce an output signal V o  which is a fixed function of the selected input signal independently of the common mode signal, or common signal levels of the input signals. 
     The multiplexer selected remote signal with the common mode signal superimposed upon it is applied to the non-inverting (+) input of the first amplifier 16 through the input resistor 14. Concurrently, the common mode signal is separately developed by applying the local common and remote common levels to the respective inputs of the second amplifier 22. Thus, the remote common signal level is applied to the non-inverting input of the amplifier 22 over line 34 and through input resistor 32 while the local common signal level is applied to the inverting input of the amplifier 22 through the input resistor 28. The output signal from the second amplifier 22 is, accordingly, the common mode signal V cm , multiplied by the transfer function of the second amlifier 22. The output signal V o  is: ##EQU1## which can be simplified to: ##EQU2## If the circuit resistors are selected as follows: 
     
         R48 =R50 = R 
    
     
         r36 = r46 = r/2 
    
     
         r24 = 2r20 
    
     
         r26 = 2r28 
    
     then the above expression for V o  when a remote signal is selected can be simplified as follows: ##EQU3## (2. ) V o  = 1.5 V remote   
     Similarly, if a local calibration signal is selected, the common mode signal from the amplifier 22 is superimposed on it by the resistor 48 and 50 to enable the equalizing circuit of the present invention to handle the remote and local signals in a similar manner. The expression for determining V cal-lo  is: ##EQU4## where R T  = R38 +R40 + R42 + R44 
     α = position of slider of first potentiometer 40 
     V ref  = stable signal level source 
     The expression for the output signal V o  when V cal-lo  is selected as an input signal is: ##EQU5## 
     This can be simiplified using the aforesaid resistance value relationships to: ##EQU6## (4. ) V o  = 1.5 V cal-lo  - 1.5 V cm  Substituting the expression for V cal-lo  , Eq. 3 into Eq. 4 gives: ##EQU7## Hence, since the common mode signal, V cm , quantities cancel, V o  is effectively 1.5 times the V ref . Thus, the output signal V o  is 1.5 times either a selected remote signal or effectively 1.5 times V ref  when V cal-lo  is selected. The 1.5 factor is based on the particular circuit components while the equalization circuit of the present invention, in general, handles these input signals in the same manner even though these two multiplexed input signals have different signal common levels. The same relationship can be shown to exist for a selection of V cal-hi  with the position of the slider of potentiometer 40 being used, e.g.,β, instead of the α term used for the V cal-lo  expression. 
     Accordingly, it may be seen that there has been provided in accordance with the present invention, a reference voltage equalizing circuit for compensating multiplexed input signals having different signal common levels.