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TS612ID Scheda tecnica(PDF) 9 Page - STMicroelectronics |
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TS612ID Scheda tecnica(HTML) 9 Page - STMicroelectronics |
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9 / 11 page  TS612 9/11 1 µF capacitance provides a path for low frequen- cies, the 10nF capacitance provides a path for high end of the spectrum. In differential mode the TS612 is able to deliver a typical amplitude signal of 18V peak to peak. The dynamic line impedance is 100 Ω. The typical value of the amplitude signal required on the line is up to 12.4V peak to peak. By using a 1:2 trans- former ratio the reflected impedance back to the primary will be a quarter (25 Ω) and therefore the amplitude of the signal required with this imped- ance will be the half (6.2 V peak to peak). Assum- ing the 25 Ω series resistance (12.5Ω for both out- puts) necessary for impedance matching, the out- put signal amplitude required is 12.4 V peak to peak. This value is acceptable for the TS612. In this case the load impedance is 25 Ω for each driv- er. For the ADSL upstream path, a lowpass filter is absolutely necessary to cutoff the higher frequen- cies from the DAC analog output. In this simple non-inverting amplification configuration, it will be easy to implement a Sallen-Key lowpass filter by using the TS612. For ADSL over POTS, a maxi- mum frequency of 135kHz is reached. For ADSL over ISDN, the maximum frequency will be 276kHz. INCREASING THE LINE LEVEL BY USING AN ACTIVE IMPEDANCE MATCHING With passive matching, the output signal ampli- tude of the driver must be twice the amplitude on the load. To go beyond this limitation an active maching impedance can be used. With this tech- nique it is possible to keep good impedance matching with an amplitude on the load higher than the half of the ouput driver amplitude. This concept is shown in figure3 for a differential line. Component calculation: Let us consider the equivalent circuit for a single ended configuration, figure4. Let us consider the unloaded system. Assuming the currents through R1, R2 and R3 as respectively: As Vo° equals Vo without load, the gain in this case becomes : The gain, for the loaded system will be (1): As shown in figure5, this system is an ideal gener- ator with a synthesized impedance as the internal impedance of the system. From this, the output voltage becomes: with Ro the synthesized impedance and Iout the output current. On the other hand Vo can be ex- pressed as: Figure 3 : TS612 as a differential line driver with an active impedance matching R1 R4 R2 Vi Vi Vo Vo 25 Ω 100Ω 1:2 Hybrid & Transformer GND +12V 47k 47k 10µ 100n 100n 100n 1k 1k 12.5 12.5 10n 1µ R3 R5 Vo° Vo° 9 8 19 12 4,5,6,7,14,15,16,17,18 GND 20 +12V 11 +12V + _ + _ 3 2 Figure 4 : Single ended equivalent circuit 1/2R1 R2 R3 + _ Vi Vo Rs1 -1 Vo° 1/2RL 2Vi R1 --------- Vi Vo ° – () R2 --------------------------and Vi Vo + () R3 ------------------------ , G Vo noload () Vi ------------------------------- 1 2R2 R1 ----------- R2 R3 ------- ++ 1 R2 R3 ------- – ----------------------------------- == GL Vo withload () Vi ------------------------------------ 1 2 --- 1 2R2 R1 ----------- R 2 R 3 ------- ++ 1 R2 R3 ------- – ----------------------------------- 1 () , == Vo ViG () RoIout () – = 2 () , Vo Vi 1 2R2 R1 ----------- R2 R3 ------- ++ 1 R2 R3 ------- – ----------------------------------------------- Rs1Iout 1 R2 R3 ------- – --------------------- 3 () , – = |
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