Abstract:
The invention relates to an evaluation device ( 4 ) connected to the output of an A/D converter ( 3 ) for comparing the direct component of a digitally converted input signal having a threshold value and at least one power source ( 13, 14, 17, 18 ) that may be connected to the differential input ( 1 ) by the evaluation device ( 4 ) in such a way that the differential input ( 1 ) can be loaded or unloaded with a current to increase or reduce the direct component in the direction of the threshold value.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of differential input circuits, and in particular to a differential input circuit for a digital signal processing system that includes an analog difference input and an analog/digital converter, as well as a device for adjusting the DC component in a digitally converted input signal. 
     Digital signal processing systems are widespread. As known, these systems are technically and economically superior to analog circuits in numerous ways, and are replacing analog circuits in many applications. 
     In many cases signals are transmitted as analog signals due to the limited bandwidth available in the transmission medium. Consequently, an analog/digital converter (A/D converter) must be connected before the digital signal processing system. To improve noise immunity, input circuits with a difference input are frequently used for this purpose, with the input signal frequently being fed into the A/D converter through an anti-aliasing filter. 
     However, transmitting signals with the usual modulation methods has the consequence that the received signal does not have a DC component (signal component at frequency zero). Since such a DC component is important for several signals, for example video signals with a porch, this must be specially reconstructed by using information transmitted together with the signal. 
     Differential input circuits for this purpose are known, in which a DC component of the signal can be recovered and adjusted by resistors and capacitors. However, a disadvantage of these circuits is that, if the resistors are mismatched or if there is an offset in a subsequent anti-aliasing filter and/or the A/D converter, the DC component of the digitally converted input signal will be defective (“clamping error”). In case of video signals, this can result in color garbling or, in a black and white picture, it can lead to color tinge. 
     European Patent Application EP-A-0539259 describes a differential input circuit with an analog-digital converter, and a microprocessor that evaluates the analog input signals converted by the analog-digital converter and performs corrections to attain a design value. 
     European Patent Application EP 0714169 A1 describes an input circuit with an analog-digital converter and a comparison circuit, which compares the input signal that has been digitally converted by the analog-digital converter to a design value, to adjust the DC component of the analog input signal. 
     Japanese Patent Application JP 05055918A describes a differential input circuit with an analog difference input for an operational amplifier. An analog-digital converter is connected after the operational amplifier, and it converts the analog output signal of the operational amplifier into a digital value. The digitally converted value is compared with a design value by an evaluation device. To adjust the DC component of the analog input signal, the analog difference input is charged or discharged in the direction toward the design value. 
     Therefore, there is a need for a differential input circuit that accurately recovers a DC component of a received signal. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to create a differential input circuit of the type mentioned in the introduction, so that the errors in the DC component of an analog transmitted and digitally converted input signal can essentially be avoided. 
     Briefly, according to an aspect of the present invention, a differential input circuit for a digital signal processing system receives a differential input signal and provides an analog input signal indicative thereof. The circuit digitizes the analog input signal and provides a digital input signal indicative thereof, and compares the DC component of the digital input signal with a DC-component design value. The circuit regulates the magnitude of the analog input signal such that the DC-component of the digital input signal is driven towards the DC-component design value. 
     An advantage of the inventive input circuit is that, by evaluating the digitally converted input signal, errors caused by an anti-aliasing filter and the A/D converter and possibly other components can be compensated. 
     Further details, features, and advantages of the invention are apparent from the following description of a preferred embodiment in terms of the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates a basic circuit diagram of a differential input circuit for a digital signal processing system; and 
     FIG. 2 illustrates a circuit diagram of a preferred embodiment differential input circuit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a differential input circuit that includes a difference input  1 , which is connected to an anti-aliasing filter  2 . The outputs of this filter  2  are connected to an A/D converter  3 , which provides a digitally converted difference input signal (adout) to a digital signal processing system (not shown) and an evaluation device  4 . 
     The difference input  1  includes a first, non-inverting input connection  11  (inp) and a second, inverting input connection  12  (inn). Each of the input connections  11 ,  12  is equipped in usual fashion with a capacitor C 1 , C 2  to block DC voltages from a preceding stage. 
     A first current source  13  is connected between a positive reference potential and the first input connection  11 , and a second current source  14  is connected between the first input connection  11  and ground. A first switch  15  is connected between the first current source  13  and the first input connection  11 , and a second switch  16  is connected between the first input connection  11  and the second current source  14 . 
     A third current source  17  is situated between the positive reference potential and the second input connection  12 , and a fourth current source  18  is connected between the second input connection  12  and ground. Here, too, both current sources  17 ,  18  are connected to the second input connection  12 , through a third and fourth switch  19 ,  20 , respectively. 
     The basic function of the circuit is to filter and digitize a differential input signal applied to the difference input  1 , and provide the digitzed signal to the evaluation device  4 . The evaluation device determines whether the DC component of the input signal is greater than or less than a specified threshold, which is determined digitally, for example by evaluating information transmitted together with the input signal. If the DC component is too small, the evaluation device  4  closes the first switch  15  through a suitable switching signal, to charge the first input connection  11  with the first current source  13 . Alternatively, the evaluation device  4  can command the fourth switch  20  to close in order to increase the DC component by discharging the second input connection  12 . The second and third switches  16 ,  19  here remain in their opened position. 
     If the DC component is too high, the first input connection  11  is discharged through the second current source  14  by closing the second switch  16 , or the second input terminal  12  is charged through the third current source  17  by closing the third switch  19 . The first and the fourth switches  15 ,  20  here remain in their open state. All the switches are opened or closed by the evaluation device  4  via appropriate switching signals. 
     The DC level can be tracked slowly. Regulation here can be made immune to noise by a digital filter. The control loop does indeed pull the difference of the analog signals toward the desired value. However, with this arrangement, there is no control circuit for the in-phase signal of the analog inputs. 
     To regulate the phase commonality, use is therefore made of the fact that the difference signal can be regulated at the first (non-inverting) or at the second (inverting) input connection  11 ,  12 , respectively. Therefore, if the difference is to be increased, this can be accomplished by closing the first switch  15  at the first input connection  11  or by closing the fourth switch  20  at the second input connection  12 . The difference can be reduced if the second switch  16  at the first input connection  11  or the third switch  19  at the second input connection  12  is closed. The other switches are open in this control mode. 
     FIG. 2 shows a preferred embodiment of the inventive differential input circuit with a common-phase control. The current sources  13 ,  14 ,  17 ,  18  are always designed as a controllable semiconductor element (e.g., especially as field effect transistors), and are actuated via their base or gate connection to adjust the amplitude of the current. Furthermore, the elements shown in FIG. 1 are also found in this circuit and are designated respectively by the same reference symbols. In addition, an actuation circuit  100  for the current sources is present, which contains a difference stage  110 . 
     The in-phase voltage of the input signals and a desired design in-phase voltage Vcm are both conducted to this difference stage  110 . 
     The voltage applied to the first input connection  11  is taken off by a field effect transistor T 1 , connected as a source follower, with a drain current source I 1 , as well as a first resistor R 1 .The voltage applied to the second input connection  12  is conducted through a second field effect transistor T 2 , connected as a source follower, with a drain current source  12  as well as a second resistor R 2 . The other ends of the first and second resistors R 1 , R 2  are connected to one side of the difference stage  110 . The two source followers are used for decoupling and prevent the analog input voltages from being affected by the resistors R 1 , R 2 . 
     The other side of the difference stage  110  carries the desired in-phase voltage Vcm, which is conducted through a third field effect transistor T 3 , connected as a source follower, with drain current source  13 . 
     The two sides of the difference stage  110  are formed by the series-connected transistors T 4 , T 5  and T 6 , T 7 , respectively. The in-phase voltages are applied respectively to the gate connections of the fourth and sixth transistors T 4 , T 6 . The fifth transistor T 5  is connected, via transistors, T 51 , T 52 , to the control terminals of the first and third current sources  13 ,  15 , and the sixth transistor T 6  is connected to the control connections of the second and fourth current sources  14 ,  16 . 
     Three parallel-connected current sources Ig 1 , Ig 2 , Ig 3  are connected in series with the current source switches S 1 , S 2 , S 3 , respectively. Depending on the level of the current to be fed into the difference input, these switches are closed or opened by the evaluation device  4 . For this purpose, the current sources are connected, via two further transistors Ti 1 , Ti 2 , to the connection point of the fourth and sixth transistors T 4 , T 6 . 
     Since all three source followers (transistors T 1 , T 2 , T 3 ) are identical, the difference stage  110  evaluates only the difference between the in-phase voltage at the input connections and the desired in-phase voltage Vcm. The difference stage  100  distributes the current fed into its base in such a fashion that, depending on the deviation of the in-phase voltage of the input signal from the desired in-phase voltage, every readjustment of the difference value readjusts the in-phase voltage in the proper direction, by closing the first and fourth, or the second and third switches,  15 ,  20 ;  16 ,  19  respectively. 
     To ensure the in-phase voltage of the digitally converted input signal is equalized to the desired In-phase voltage Vcm as quickly as possible, the evaluation device  4  not only determines the direction in which the In-phase voltage is to be changed, but also the magnitude of the difference between the two voltages. Depending on the magnitude of this difference, the clamping current, in the case of large deviations, is made as large as possible through a control output Si of the evaluation device  4 , which appropriately closes the current source switches S 1 , S 2 , S 3 . Especially upon turn-on, this causes a rapid decay of initial transients to the desired value. 
     The circuit can be used both for continuous clamping (the regulation process is always active), and also for cycled clamping, for example in the case of video signals, in which the DC component is present as an information signal only at certain times. For this purpose, a clamping switching signal Sk can be conducted to the evaluation device  4 , so this device is switched active only at the clamping instants.