Reference voltage circuit

#The invention relates to a reference voltage circuit for generating at least one constant reference voltage independently of fluctuations of a reference potential.

#Reference voltage circuits for generating reference voltages are required in many circuits. There are applications in which exact reference voltages Vref with respect to a common mode voltage (VCM), which varies within certain voltage limits, are required. FIG. 1 shows such an application. On an SLIC circuit (SLIC: Subscriber Line Interface Circuit), a terminal device is connected via a twisted two-wire telephone line. The SLIC circuit generates from the line currents measured via current sensors a transversal current IT for the voice signal and a longitudinal current IL for the necessary control information. The currents IIL, IIT generated by the SLIC circuit are converted by means of resistors RIL, RIT into corresponding voltages for further processing by a CODEC circuit. In this case, the two resistors RIL, RIT are respectively wired between an input-terminal pad IT, IL of the CODEC circuit and a common-mode reference voltage terminal VCM.

#The CODEC circuit includes a reference voltage source for generating a reference voltage Vfloat. This reference voltage Vfloat is applied to the common-mode voltage terminal VCM via a buffer B. The reference voltage generated by the reference voltage source may vary within certain voltage limits, typically by +/−100 mV.

#In the drawing:

#FIG. 1 shows a CODEC circuit with an integrated reference voltage circuit according to the prior art;

#FIG. 2 shows a CODEC circuit with a reference voltage circuit according to the invention integrated in it.

#FIG. 2 shows a CODEC circuit 1 with a reference circuit 2 integrated in it for generating a plurality of constant reference voltages, which are given out via reference voltage lines 3 to comparator circuits 4 within the CODEC circuit 1.

#The CODEC circuit 1 is connected to an SLIC circuit 5 and receives via the input terminals 6, 7 an input voltage UTT, which is picked off at the external resistor 8, and an input voltage UIL, which is picked off at the external resistor 9. At the external resistor 8 there is a transversal current IIT, which in the case of the example shown reproduces a voice signal. At the resistor 9 there is a voltage UIL which is proportional to the lateral current IIL and essentially contains control information for the signal processing. The SLIC circuit 5 is connected via terminals 10, 11 and via a twisted 2-wire telephone line 12a, 12b to a terminal device 13. The SLIC circuit 5 essentially includes a current sensor circuit 14, which measures via internal lines 15, 16 the line currents flowing through the lines 12a, 12b and delivers them as a transversal current IIT or lateral current IIL via the terminals 17, 18 to the CODEC circuit 1. The SLIC circuit 5 also has input terminals 19, 20, which are connected to a differentially constructed operational amplifier 21. The signals emitted by the operational amplifier 21 are also amplified in the amplifiers 22, 23 and delivered to the terminal device 13.

#The CODEC circuit 1 includes in the voice signal path an analog pre-filter 24, a downstream analog-digital converter 25, a digital filter 26 and a programmable filter 27 within a DSP signal processor 28. The programmable filter 27 is connected on the output side to a PCM interface 29.