A METHOD AND AN ARRANGEMENT FOR ADAPTING, FROM A DC POINT OF VIEW, A FIRST CIRCUIT TO AT LEAST ONE SECOND CIRCUIT

Abstract: In a method and an arrangement for adapting, from a DC point of view, the signal input (4) of a first circuit (3), lying at a first DC voltage, to the signal output (2) of at least one second circuit (1), lying at a second DC voltage, the signal input terminal (4) is connected to the respective signal output terminal (2) of the respective second circuit (1) via a respective first resistance (R1), whereby first DC currents (IR1) will be allowed to flow through the respective first resistance (R1). The signal input (4) is adapted to be virtually short-circuited to a source for the first DC voltage (U), which is of a predetermined value, separate from zero. The first circuit (3) is adapted to internally generate a DC cancellation current (I) and bring it to flow in the signal input (4). The signal input (4) is to be connected to ground via a selectable, second resistance (R2), whereby a second DC current (IR2) will flow through the second resistance (R2). This second resistance (R2) is to be selected in such a manner that the sum of the DC cancellation current (I), the first DC currents (IR1), and the second DC current (IR2), equals zero.

Description: A METHOD AND AN ARRANGEMENT FOR ADAPTING, FROM A DC POINT OF VTEW, A FIRST CIRCUIT TO AT LEAST ONE SECOND CIRCUIT TECHNICAL FIELD The invention relates, generally, to an arrangement for adapting, from a DC point of view, a first circuit to at least one second circuit and, more specifically, to an arrangement for adapting, from a DC point of view, the signal input terminal of a first circuit, lying at a first DC reference level, to the signal output terminal of at least one second circuit, lying at a second DC reference level. BACKGROUND OF THE INVENTION On e.g. line interface boards provided with line interface circuits and code converters, the signal input terminals of the line interface circuits as well as the signal output terminals of the code converters were earlier related to ground which enabled a direct connection between the signal input terminal of a line interface circuit and the signal output terminal of a code converter. However, modem code converters have a reference voltage for their signal output terminal somewhere between ground and +5V, most often around 2.25V, while the signal input terminal of the line interface circuits lies at ground potential. If a line interface circuit whose signal input terminal lies at ground potential, is interconnected with a code converter whose signal output terminal lies at 2.25 V, an offset current is obtained in the line current outputted by the line interface circuit. Upon a polarity reversal of the line current, this offset current will maintain its polarity. Thus, different line currents are obtained for normal and reversed polarity, respectively, of the line current. Also, the offset current can cause other problems. One way of solving this problem is to interconnect a coupling capacitor between the signal input terminal of the line interface circuit and the signal output terminal of the code converter. However, this coupling capacitor has to be large in order for the system not to become unstable. SUMMARY OF THE INVENTION The object of the invention is to generally adapt, from a DC point of view, the signal input terminal of a first circuit, e.g. a line interface circuit, lying at a first DC reference level to the signal output terminal of at least one second circuit, e.g. a code converter, lying at a second DC reference level without the use of any coupling capacitor. This is attained according to the invention in that a DC current is deliberately allowed to flow in the signal input terminal of the first circuit, the effect of this current being eliminated in the first circuit by generating a corresponding current of opposite polarity. More in detail, this object is attained in accordance with the invention mainly in that the signal input terminal is adapted to be virtually short-circuited to the first DC reference voltage level which is of a predetermined value separate from zero, that the signal input terminal is connected, via a respective first resistance, to the respective signal output terminal, whereby first DC currents will be allowed to flow through the respective first resistance, that the signal input terminal is connected to ground via a selectable, second resistance, whereby a second DC current will flow through the second resistance which is to be selected in such a manner that the DC current sum formed by the first DC currents and the second DC current in the signal input teirninal of the first circuit, amounts to a predetermined value, and that the line interface circuit is adapted to internally generate a DC cancellation current corre- sponding to the DC current sum but being of opposite polarity in order to eliminate the effect of the DC current sum. Hereby, the need of a coupling capacitor between the signal input terminal of the first circuit and the signal output terminal of the respective second circuit, will be eliminated. Thus, the first circuit will be easily adaptable to the second circuits by a suitable selection of the resistance connected to ground. BRIEF DESCRIPTION OF THE DRAWING The invention will be described more in detail below with reference to the appended drawing, on which the single figure shows an embodiment of an arrangement according to the invention. PREFERRED EMBODIMENT The invention will be described in connection with a line interface circuit and a single code converter connected thereto but it is to be understood that the invention is generally applicable when it comes to adapting the signal input terminal of a first circuit, lying at a first DC reference voltage level, to the signal output terminal of at least one second circuit, lying at another DC reference voltage level. On the drawing, 1 denotes a code converter which is adapted to transfer analog signal voltages on its signal voltage output terminal 2 to the signal current input terminal 4 of a line interface circuit 3. The signal output terminal 2 of the code converter 1 is supposed, in the embodiment shown, to lie at a DC reference voltage level of 2.25V, while the signal input terminal 4 of the line interface circuit 3 is supposed to lie at a DC reference voltage level which is equal to ground. As mentioned in the introductory portion, this difference in DC reference voltage level would give rise to an offset current in the line interface circuit, which offset current, in its turn, would cause line currents of different values to be outputted from the line interface circuit upon a polarity reversal. To adapt the signal output level of the code converter 1 to the signal input level of the line interface circuit 3, according to the invention, the signal input terminal 4 of the line interface circuit 3 is adapted to be virtually short-circuited to a DC refer- ence voltage level U which, in accordance with the invention, is of a predetermined value separate from zero. Thus, the DC reference voltage level of the signal input terminal 4 will be at a DC voltage level U which in the embodiment shown, is supposed to be e.g. 1.25V. In accordance with one embodiment of the invention, the signal input terminal 4 of the line interface circuit 3 is connected to the DC voltage level U via a schematically illustrated current amplifier 5 in the line interface circuit 3. The input current of the current amplifier 5 controls its output current in a manner known per se. In accordance with the invention, the signal output terminal 2 of the code converter 1 is connected to the signal input terminal 4 of the line interface circuit 3 via a resistance Rl whose value can differ. Since the DC voltage level on the signal output terminal 2 in the embodiment illustrated, is supposed to be higher than the DC voltage level U on the signal input terminal 4 of the line interface circuit 3, a DC current LR1 is, thus, allowed to flow through the resistance Rl. Also, in accordance with the invention, the signal input terminal 4 of the line interface circuit 3 is connected to ground via a selectable resistance R2. A DC current LR2 will, thus, flow to ground through the resistance R2. According to the invention, the resistance R2 is to be selected in such a manner that the sum of the currents IR1 and IR2 in the signal input terminal 4 of the line interface circuit 3 amounts to a predetermined value which is independent of the value of the resistance Rl. Thus, this predetermined DC current sum value is line interface circuit specific. In the embodiment shown on the drawing, to eliminate the effect of this DC current sum of a predetermined value in the line interface circuit 3, the line interface circuit 3 is adapted to internally generate a DC cancellation current I by means of a current generator 6 in the line interface circuit 3. This DC cancellation current I corresponds to the DC current sum but is of opposite polarity. In the embodiment shown on the drawing, the current generator 6 is connected to the signal input terminal 4 of the line interface circuit 3 and, consequently, the current I will be of the same value as the current sum in the signal input teirninal 4. In case the current generator 6, in a manner not shown, is instead connected to the output of the current amplifier 5, the current generated by the current generator would have to be equal to the current sum in the signal input terminal of the line interface circuit 3 multiplied by the gain of the current amplifier 5. However, it is to be understood that the current generator 6, of course, can be inserted in any place in the signal path from the signal input terminal 4. Since the signal input terminal 4 is virtually short-circuited to the DC reference voltage level U, the value of the resistance R2 will only affect the DC current in the signal input terminal 4. Thus, the AC signal transmission between the signal output terminal 2 and the signal input terminal 4 will not be affected by the value of the resistance R2. Thus, the resistance Rl may first be dimensioned for the desired AC signal transmission, whereupon the resistance R2 can be adapted for DC current balance so that the DC current in the signal input terminal 4 will be the desired current. Thus, by means of the arrangement according to the invention, no coupling capacitor will be needed between the respective signal output terminal 2 and the signal input terminal 4. At the same time, the signal input terminal 4 of the line interface circuit 3 can easily be adapted to the DC reference voltage levels of different code converters by a simple selection of the value of the resistance R2. It should be pointed out that it is to be understood that the signal output terminals of more than one code converter can be connected via a respective resistance to one and the same signal input terminal of the line interface circuit 3.

Claims: CLAIMS 1. A method of adapting, from a DC point of view, the signal input (4) of a first circuit (3), lying at a first DC reference voltage level, to the signal output (2) of at least one second circuit (1), lying at a second DC reference voltage level, comprising connecting the signal input (4) to the respective signal output (2) via a respective first resistance (Rl), characterized by - selecting the first DC reference voltage level (U) to be separate from zero, - virtually short-circuiting the signal input (4) to the first DC reference voltage level (U), - generating a DC cancellation current (I) in the first circuit (3) and bringing it to flow in the signal input (4), and - connecting the signal input (4) to ground via a selectable second resistance (R2), which is selected in such a manner that the sum of the DC cancellation current (I), currents (IRl) flowing through the respective first resistances (Rl), and current (IR2) flowing through the second resistance (R2), equals zero. 2. An arrangement for adapting, from a DC point of view, the signal input (4) of a first circuit (3), lying at a first DC voltage, to the signal output (2) of at least one second circuit (1), lying at a second DC voltage, the signal input (4) being connected to the respective signal output (2) via a respective first resistance (Rl), whereby first DC currents (IRl) will be allowed to flow through the respective first resistance (Rl), characterized in - that the signal input (4) is adapted to be virtually short-circuited to a source for the first DC voltage (U), which is of a predetermined value, separate from zero, - that the first circuit (3) is adapted to internally generate a DC cancellation current (I) and bring it to flow in the signal input (4), and - that the signal input (4) is intended to be connected to ground via a selectable, second resistance (R2), whereby a second DC current (IR2) will flow through the second resistance (R2), which is to be selected in such a manner that the sum of the DC cancellation current (I), the first DC currents (IRl) and the second DC current (IR2), equals zero. 3. An arrangement for adapting, from a DC point of view, the signal input (4) of a line interface circuit (3), lying at a first DC voltage, to the signal output (2) of at least one code converter (1) lying at a second DC voltage, the signal input (4) being intended to be connected to the respective signal output via a respective first resistance (Rl), whereby first DC currents (IRl) will be allowed to flow through the respective first resistance (Rl), characterized in - that the signal input (4) is adapted to be virtually short-circuited to a source for the first DC voltage (U), which is of a predetermined value, separate from zero, - that the line interface circuit (3) is adapted to internally generate a DC cancellation current (I) and bring it to flow in the signal input (4), and - that the signal input (4) is intended to be connected to ground via a selectable, second resistance (R2), whereby a second DC current (IR2) will flow through the second resistance (R2), which is to be selected in such a manner that the sum of the DC cancellation current (I), the first DC currents (IRl), and the second DC current (IR2), equals zero. 4. The arrangement as claimed in claim 3, characterized in that the signal input (4) is connected to the source for the first DC voltage (U) via the input terminals of a current amplifier (5). 5. The arrangement as claimed in claim 3 or 4, characterized in that a current generator (6), provided in the line interface circuit (3), is adapted to generate the DC cancellation current (I).

Inventors: ERIKSSON HANS, LARSSON ELISABETH, ERIKSSON, HANS, LARSSON, ELISABETH
Applicants: ERICSSON TELEFON AB L M, ERIKSSON HANS, LARSSON ELISABETH, ERIKSSON, HANS, LARSSON, ELISABETH, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1997
Main classification: H04B3
IPC codes: H03F3, H03K5, H04B3