Abstract:
The signal regeneration circuit recovers a digital signal from an input signal that is supplied via metallic isolation (galvanic separation). The circuit has two input terminals for the input signal and one output terminal for the recovered digital signal. A current direction sensor detects the current direction prevailing between the input terminals and outputs the signal in accordance with the last prevailing current direction. The circuit is advantageously used in connection with digital circuits that require potential isolation at their input terminals.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a circuit for recovering a digital signal from an input signal received by way of galvanic separation, comprising two input terminals for the input signal and one output terminal for the recovered digital signal. 
     In the transmission of electrical signals between two systems A and B, it is frequently required that systems A and B must not be galvanically (metallically) connected to one another, that is to say there must not be a direct metallic connection between systems A and B. To galvanically isolate systems A and B, which are intended to exchange signals with one another, the insertion of a capacitor in each of the signal transmission lines or the use of a transformer for a pair of signal lines is possible in line-connected signal transmission. Accordingly, the coupling elements of capacitor or transformer, respectively, allow the transmission of signals between systems A and B without connecting these to one another galvanically. 
     However, the use of the coupling elements causes a change in the signal variation so that in the transmission of a signal from system A to system B, a device must be provided in system B by means of which the signal variation originally output by system A can be recovered. This is required especially in the case of digital signals because the digital signal processing devices in the receiving system B only operate reliably with defined signal structures. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a signal regeneration circuit, which overcomes the abovementioned disadvantages of the heretofore-known devices and methods of this general type and which allows a digital signal supplied to a receiver system by way of metallic isolation to be recovered from the received signal. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a signal regeneration circuit for recovering a digital signal from an input signal supplied via metallic isolation, comprising: 
     two input terminals receiving an input signal; 
     one output terminal outputting a recovered digital signal; and 
     a current direction sensor connected to the two input terminals for detecting a current direction prevailing between the two input terminals and outputting a signal representing the prevailing current direction. 
     In accordance with an added feature of the invention, the current direction sensor includes a voltage clamping configuration for fixing a voltage between the input terminals and a current mirror comprising an output driver configuration outputting the signal representing the prevailing current direction. 
     In accordance with an additional feature of the invention, the voltage clamping configuration includes a first series circuit of at least two diodes each connected in a given direction and a second series circuit of at least two diodes each connected in a direction opposite the given direction, the first and second circuits being connected to one another at a center node. 
     In other words, a preferred signal regeneration circuit has a current direction sensor which detects the current direction prevailing between the input terminals and outputs a signal corresponding to the prevailing current direction. In this configuration, the current direction sensor detects the direction of the current which flows between the two input terminals. The current direction sensor preferably includes a voltage clamping configuration and a current mirror. The voltage clamping configuration fixes the voltage between the input terminals to a predetermined maximum value. The current mirror detects the current flowing between the input terminals and supplies the detected current value to an output driver configuration at which the output signal can be picked up. It is especially preferred in this configuration that the voltage clamping configuration has a first and a second series connection of in each case at least two diodes connected in the same direction. In this configuration, the diodes of the first series circuit are connected in the opposite direction to the diodes of the second series circuit, and the series circuits are connected to one another at a center node. 
     In accordance with another feature of the invention, the current mirror includes a first current mirror circuit and a second current mirror circuit, the first current mirror circuit reflecting a current flow through the voltage clamping configuration in a first current flow direction and the second current mirror circuit reflecting a current flow through the voltage clamping configuration in a second current flow direction. 
     In accordance with again an added feature of the invention, the current mirror further includes a third current mirror circuit and a fourth current mirror circuit, the third current mirror circuit being connected to an output current path of the first current mirror circuit and the fourth current mirror circuit being connected to an output current path of the second current mirror circuit. 
     In accordance with again another feature of the invention, a connecting node connects an output current path of the third current mirror circuit with an output current path of the fourth current mirror circuit, the output current paths of the third and fourth current mirror circuits form the output driver configuration, and the connecting node outputs the output signal representing the prevailing current direction. 
     These foregoing features define a further, especially preferred exemplary embodiment of the signal regeneration circuit. The current mirror of the current direction sensor thereby has a first and a second current mirror circuit. In this configuration, the first current mirror circuit reflects the current flow through the voltage clamping configuration in a first current flow direction and the second current mirror circuit reflects the current flow through the voltage clamping configuration in a second current flow direction. In addition, it is especially preferred that the current mirror includes the third and fourth current mirror circuits. Here, the third current mirror circuit is connected to the output current path of the first current mirror circuit and the fourth current mirror circuit is connected to the output current path of the second current mirror circuit. The embodiment in which the output current path of the third current mirror circuit and the output current path of the fourth current mirror circuit are connected to one another and form the output driver configuration is especially preferred. The output signal can then be picked up at the connecting node of the output current paths of the third and fourth current mirror circuit. 
     In accordance with yet an added feature of the invention, the voltage clamping configuration includes a center node and a reference potential is added at the center node of the voltage clamping configuration. In this configuration, the reference potential is fixed with respect to the ground potential of the current direction sensor and thus of the signal-receiving system. The circuit structures of the current mirror can then be designed with respect to the ground potential of the receiving system. 
     In accordance with yet another feature of the invention, the reference potential has a value that lies within a range of threshold potentials of CMOS transistors in an integrated circuit. 
     In accordance with yet an additional feature of the invention, the current direction sensor is constructed with a plurality of transistors including bipolar transistors and CMOS transistors. In accordance with yet a further feature of the invention, the input signal is supplied to the inputs via capacitive coupling where capacitances are connected upstream of the two inputs in the signal flow direction. 
     In accordance with a concomitant feature of the invention, the current direction sensor comprises a buffer for temporarily storing a last prevailing current direction. In a preferred embodiment the buffer is constructed as a flip flop. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a signal regeneration circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit schematic of a preferred exemplary embodiment of the signal regeneration circuit according to the invention; and 
     FIG. 2 is a circuit schematic of a novel circuit for generating a reference potential. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a circuit that is essentially subdivided into four circuit blocks. A circuit block CC relates to the capacitive coupling between two signal lines. Circuit blocks VC, CM and OL together form a current direction sensor according to a preferred exemplary embodiment of the invention. The current direction sensor according to FIG. 1 includes a voltage clamping configuration VC, a current balancing configuration or current mirror CM and a buffer configuration OL. The current direction sensor is supplied via the capacitive coupling CC with a typically differential input signal that is evaluated by the current direction sensor and output as recovered input signal at its output terminal OUTPUT. The capacitive coupling configuration, in each of the signal lines, includes one coupling capacitor CHP 1  and CHP 2  and a series-connected resistor R 3  and R 4 , respectively. The free terminals of the resistors R 3  and R 4 , respectively, are connected to the input signal terminals INPUT 1  and, respectively, INPUT 2  of the voltage clamping cofiguration VC of the current direction sensor. 
     The voltage clamping configuration includes two series of diodes D 1 , D 2 , D 3 , D 4  and, respectively, D 5 , D 6 , D 7 , D 8  which are connected in antiparallel and which are connected at their free ends to the input signal terminals INPUT 1  and, respectively, INPUT 2 . A series circuit of resistors R 1  and R 2  is connected in parallel with the diode series circuits. The free ends of the resistor series circuit R 1 +R 2  are also connected to the signal input terminals INPUT 1  and, respectively, INPUT 2 . The two diode series circuits and the resistor series circuit are connected to one another at a center node B. The center node B is connected to a reference potential Vref which is fixed with respect to the ground potential of the receiving system. The signal input terminals INPUT 1  and, respectively, INPUT 2  are thus above and, respectively, below the reference voltage Vref by two diode conducting voltages V D  with respect to the ground potential of the receiving system to which the current direction sensor belongs. In an integrated circuit, the diodes D 1  to D 8  are preferably constructed as bipolar transistors, the collector and base of which are short-circuited and thus emulate the function of a diode in a simple manner in the integrated circuit. 
     In the current mirror CM of the current direction sensor, the input current paths of a first and of a second current mirror circuit are connected between the center node B and the node of the input signal terminal INPUT 2 . The input current path of the first current mirror circuit includes a resistor RX and a bipolar transistor QX 1  of the npn type which is short-circuited to form a diode. The input current path of the first current mirror circuit is thus connected in the same conducting direction as the diode series circuit D 1  to D 4 . The input current path of the second current mirror circuit includes a resistor RY and a bipolar transistor QY 1  of the pnp type which is short-circuited to form a diode. Thus, the input current path of the second current mirror circuit has the same conducting direction as the diode series circuit D 5  to D 8 . The maximum current IX through the input current path of the first current mirror circuit and the maximum current IY through the input current path of the second current mirror circuit are identical in amounts. The current direction, however, is opposite in accordance with the diode switching direction. The amounts are: 
     
       
         [1 ]=|IX|=|IY |=(2 V   D   −V   BE )| RX    
       
     
     where V D  is the conducting voltage of a diode of the diode series circuits D 1  to D 8  and where V BE  is the base-emitter voltage of transistor QX 1  and, respectively, QY 1 . Since V D ≈V BE ≈0.7 V, the following result is obtained: 
     
       
         [2 ]=|IX|=|IY|= 0.7V/ RX    
       
     
     whereby the resistors RX and RY have the same dimensions for reasons of symmetry. Accordingly, the same amounts of current are thus obtained for IX and IY. 
     In the output current path of the first current mirror circuit, the collector-emitter path of an npn transistor QX 2  is located and in the output current path of the second current mirror circuit, the collector-emitter path of a pnp transistor QY 2  is located. The base terminals and the emitter terminals of transistors QX 1  and QX 2  and of transistors QY 1  and QY 2  are connected to one another in order to form a current mirror circuit. The output current path of the first current mirror circuit is connected to the input current path of a third current mirror circuit and the output current path of the second current mirror circuit is connected to the input current path of a fourth current mirror circuit. In the input current path of the third current mirror circuit, the load path of a p-channel MOS transistor MX 1  is located. A free end of the load path of the transistor MX 1  is connected to the supply voltage terminal of the receiving system. In addition, the gate terminal of the transistor MX 1  is connected to the connecting node of the transistors QX 2  and MX 1 . Similarly, the output current path of the second current mirror circuit is connected to the input current path of a fourth current mirror circuit. In the input current path of the fourth current mirror circuit, the load path of an n-channel MOS transistor MY 1  is located, the free end of the load path being connected to the ground potential of the receiving system. The gate terminal of transistor MY 1  is connected to the connecting node of transistors QY 2  and MY 1 . 
     The output current path of the third current mirror circuit is formed by a p-channel MOS transistor MX 2  and the output current path of the fourth current mirror circuit is formed by the load path of an n-channel MOS transistor MY 2 . A free end of the load path of the transistor MX 2  is connected to the supply voltage terminal of the receiving system and a free end of the load path of the transistor MY 2  is connected to the ground terminal of the receiving system. The gate terminals of the transistors MX 1  and MX 2  and of transistors MY 1  and MY 2  are connected to one another in order to form a current mirror circuit. The load paths of the transistors MX 2  and MY 2  of the third and fourth current mirror circuit are connected to one another at a node S. At the node S, a logic signal can be picked up which indicates the current direction that is instantaneously prevailing between the input terminals INPUT 1  and INPUT 2 . 
     The current mirror CM is followed by the output buffer configuration OL in the signal flow direction. The output buffer configuration OL stabilizes the output signal at the node S of the current mirror CM. The output buffer circuit OL includes a flip flop FF consisting of CMOS inverters, the input terminal of which is connected to the output node S of the current mirror CM. One of the inverters of the flip flop FF also includes load transistors ML 1  and ML 2  which are also configured in complementary MOS technology. At the output terminal of the flip flop, three series-connected inverters A 1 , A 2  and A 3  are connected. The inverter A 3  is configured and dimensioned as a CMOS output driver. The output signal OUTPUT of the signal regeneration circuit of the invention is available at the output terminal of the inverter A 3 . 
     The voltage clamping configuration with the antiparallel-connected diode series circuits D 1  to D 4  and, respectively, D 5  to D 8  clamps the voltage between the signal input terminals INPUT 1  and INPUT 2  to a value which is four times a diode conducting voltage V D  by amount. With respect to the center node B, the voltage at the signal input terminal INPUT  2  moves between Vref+2V D  and Vref−2V D  with respect to the ground potential of the receiving system. It should be mentioned at this point that in the case of a rectangular differential input signal present at capacitors CHP 1  and CHP 2 , the capacitors CHP 1  and CHP 2  discharge exponentially via resistors R 1  and, respectively, R 2  after each change in voltage level in the input signal. In this case, the current direction of the discharge current determines the logic state of the input signal. A first current direction is detected in the input current path of the first current mirror circuit and the second current direction is detected in the input current path of the second current mirror circuit. Although the input current paths of the first and second current mirror circuit according to FIG. 1 are connected between the center node B and the node of the input terminal INPUT 2 , the circuit operation can also be implemented with only a slightly different configuration if the input current path of the first or second current mirror or of the first and second current mirror is connected between the center node B and the node of the input terminal INPUT 1 . 
     The current IX in the input current path of the first current mirror circuit is reflected in its output current path and, at the same time, supplied to the input current path of the third current mirror circuit. Similarly, the current IY in the input current path of the second current mirror circuit is reflected in its output current path and supplied to the input current path of the fourth current mirror circuit. Since at a particular time, either only the first current mirror circuit or only the second current mirror circuit is in each case active, the output current paths of the third and fourth current mirror circuit can be connected to one another in order to pick up the output signal at the connecting node S. 
     If, for example, a negative input signal is present, that is to say the voltage difference between the input terminal INPUT 1  and input terminal INPUT 2  is negative and a current flows from the input terminal INPUT 2  to the input terminal INPUT 1 , a voltage which is twice a diode conducting voltage V D  above the reference voltage Vref with respect to the ground potential of the receiving system is present at the input terminal INPUT 2 . Thus, a current IY calculated as above flows through the input current path of the second current mirror circuit. This current IY is reflected to the output current path of the second current mirror circuit, multiplied by the fourth current mirror circuit and applied via the node S to the flip flop FF in order to switch the state of the latter. Thus, the flip flop switches between the input terminals INPUT 1  and INPUT 2  in accordance with the currently prevailing current direction. The switching currents of the flip flop FF can be set by means of the load transistors ML 1  and ML 2 . 
     Referring now to FIG. 2, there is shown a circuit for generating the reference potential Vref. For this purpose, a current I B  with a very low value is supplied to a node A. This current flows via npn transistors QB 1  and QB 2 , which are connected together to form diodes, and the load path of an n-channel MOS transistor away to the ground terminal of the receiving system. This results in a voltage drop of twice the base-emitter voltage V BE  of transistors QB 1  and QB 2  and the threshold voltage VT of MOS transistor MB 1 . At the n-channel MOS transistor MB 2 , this current is reflected into an output current path via transistors MB 1  and MB 2  which are connected together to form a current mirror circuit. The output current path consists of the load path of transistor MB 2 , the collector-emitter path of an npn transistor QB 7  which is also connected together with transistor QB 2  to form a current mirror circuit, and the load path of a p-channel MOS transistor MB 5 . Via transistor MB 5 , the current in the output current path of the current mirror is reflected to the load path of a p-channel MOS transistor MB 6 . The latter activates a pnp transistor QB 4  and drives a further current mirror consisting of n-channel or MOS transistors MB 3  and MB 4 . In the output current path of this current mirror circuit, the load path of transistor MB 4  and the collector-emitter path of an npn transistor QB 3  is located, which, as a result, is also activated. The base of an output transistor QB 5  of the npn type is connected to a connecting node Y of transistor MB 6  and of transistor QB 4 . The base of an output transistor QB 6  of the pnp type is connected to a connecting node X between the emitter of transistor QB 3  and the load path of transistor MB 4 . The emitters of output transistors QB 5  and QB 6  are connected to one another and are at the required voltage Vref. 
     In this configuration, the voltage Vref is at the same magnitude as the potential at the node A. This is due to the fact that the bases of the transistors QB 3  and QB 4  are also connected to the node A and the transistors QB 3  and QB 4  are kept activated by the current mirrors of transistors MB 3  and MB 4  and, respectively, MB 5  and MB 6 . Thus, the node Y is above the potential of the node A by the voltage drop across the base-emitter diode of the transistor QB 4 . The voltage drop across the base-emitter diode of the transistor QB 5  then brings one back to the potential of the node A. The same result is obtained from the node A via the base-emitter path of the transistor QB 3  to the node X and from there via the base-emitter path of the transistor QB 6  to the output terminal Vref. Through the output stage of the transistors QB 5  and QB 6 , the reference voltage Vref can be picked up with low impedance and push-pull capability at the output terminal. 
     The circuit according to FIG. 2 belongs to the receiving system and, accordingly, is referred to its ground potential. This also applies to the supply voltage terminals, illustrated in FIG. 2, at the free ends of the load paths of transistors MB 5 , MB 6 , QB 3  and QB 5 . Between node A and the ground potential, a capacitor C 1  is also connected which filters out high-frequency noise from a surrounding supply voltage source or clock system sources. The voltage at node A, and thus the output voltage Vref, is twice a base-emitter voltage above the threshold voltage of a MOS transistor with respect to the ground potential of the receiving system. 
     The circuit according to the invention makes it possible to regenerate a differential input signal which is supplied via coupling capacitors CHP 1  and CHP 2  to the receiving system via input terminals INPUT 1  and INPUT 2 , as such referred to the ground potential of the receiving system and to output this signal at an output terminal OUTPUT. Accordingly, no direct current flows between the transmitting system and the receiving system and only an alternating-current component is transmitted. The circuit according to the invention can be constructed in a simple manner completely integrated in a semiconductor structure.