1/3 ts613 as driver for adsl line interfaces - a single supply implementation with passive or active impedance matching by c. prugne june 2000 adsl concept asymmetric digital subscriber line (adsl), is a new modem technology, which converts the exist- ing twisted-pair telephone lines into access paths for multimedia and high speed data communica- tions. adsl transmits more than 8 mbps to a subscriber, and can reach 1mbps from the subscriber to the central office. adsl can literally transform the ac- tual public information network by bringing mov- ies, television, video catalogs, remote cd-roms, lans, and the internet into homes. an adsl modem is connected to a twisted-pair telephone line, creating three information chan- nels: a high speed downstream channel (up to 1.1mhz) depending on the implementation of the adsl architecture, a medium speed upstream channel (up to 130khz) and a pots (plain old telephone service), split off from the modem by filters. the line interface - adsl remote terminal (rt): the figure1 shows a typical analog line interface used for adsl. the upstream and downstream signals are separated from the telephone line by using an hybrid circuit and a line transformer. on this note, the accent will be made on the emission path. the ts613 is used as a dual line driver for the up- stream signal. for the remote terminal it is required to create an adsl modem easy to plug in a pc. in such an ap- plication, the driver should be implemented with a +12 volts single power supply. this +12v supply is available on pci connector of purchase. the figure 2 shows a single +12v supply circuit that uses the ts613 as a remote terminal trans- mitter in differential mode. the driver is biased with a mid supply (nominaly +6v), in order to maintain the dc component of the signal at +6v. this allows the maximum dy- namic range between 0 and +12 v. several op- tions are possible to provide this bias supply (such as a virtual ground using an operational amplifier), such as a two-resistance divider which is the cheapest solution. a high resistance value is re- quired to limit the current consumption. on the other hand, the current must be high enough to bias the inverting input of the ts613. if we consid- er this bias current (5 m a) as the 1% of the current through the resistance divider (500 m a) to keep a stable mid supply, two 47k w resistances can be used. the input provides two high pass filters with a break frequency of about 1.6khz which is neces- sary to remove the dc component of the input sig- nal. to avoid dc current flowing in the primary of the transformer, an output capacitor is used. figure 1 : typical adsl line interface impedance matching twisted-pair telephone line hybrid circuit digital treatment lp filter ts613 line driver reception (analog) emission (analog) digital to analog analog to digital high output current reception circuits upstream downstream figure 2 : ts613 as a differential line driver with a +12v single supply r1 r3 r2 _ + _ + vi vi vo vo 3 2 6 5 1 7 4 gnd 8 +12v 25 w 100w 1:2 hybrid & transformer gnd +12v 47k 47k 10 100n 100n 100n 1k 1k 12.5 12.5 10n 1 AN1281 application note
AN1281 2/3 the 1 m f capacitance provides a path for low fre- quencies, the 10nf capacitance provides a path for high end of the spectrum. in differential mode the ts613 is able to deliver a typical amplitude signal of 18v peak to peak. the dynamic line impedance is 100 w . 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 w ) 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 w series resistance (12.5 w 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 ts613. in this case the load impedance is 25 w 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 ts613. 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 : ts613 as a differential line driver with an active impedance matching r1 r4 r2 _ + _ + vi vi vo vo 3 2 6 5 1 7 4 gnd 8 +12v 25 w 100w 1:2 hybrid & trans former gnd +12v 47k 47k 10 100n 100n 100n 1k 1k 12.5 12.5 10n 1 r3 r5 vo vo figure 4 : single ended equivalent circuit 1/2 r1 r2 r3 + _ vi vo rs1 -1 vo 1/2 rl 2 vi r 1 --------- vi vo C () r 2 -------------------------- and vi vo + () r 3 ----------------------- - , g vo noload () vi ------------------------------- 1 2 r 2 r 1 ---------- - r 2 r 3 ------- ++ 1 r 2 r 3 ------- C ---------------------------------- - == gl vo withload () vi ------------------------------------ 1 2 -- - 1 2 r 2 r 1 ---------- - r 2 r 3 ------- ++ 1 r 2 r 3 ------- C ---------------------------------- - 1 () , == vo vig () roiout () C = 2 () , vo vi 1 2 r 2 r 1 ---------- - r 2 r 3 ------- ++ ? ?? 1 r 2 r 3 ------ - C ---------------------------------------------- - rs 1 iout 1 r 2 r 3 ------- C --------------------- 3 () , C =
AN1281 3/3 information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result f rom its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specificati ons mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of stmicroelectronics. ? the st logo is a registered trademark of stmicroelectronics ? 2000 stmicroelectronics - printed in italy - all rights reserved stmicroelectronics group of companies australia - brazil - china - finland - france - germany - hong kong - india - italy - japan - malaysia - malta - morocco singapore - spain - sweden - switzerland - united kingdom ? http://www.st.com by identification of both equations (2) and (3), the synthesized impedance is, with rs1=rs2=rs: unlike the level vo required for a passive imped- ance, vo will be smaller than 2vo in our case. let us write vo=kvo with k the matching factor vary- ing between 1 and 2. assuming that the current through r3 is negligeable, it comes the following resistance divider: after choosing the k factor, rs will equal to 1/2rl(k-1). a good impedance matching assumes: from (4) and (5) it becomes: by fixing an arbitrary value for r2, (6) gives: finally, the values of r2 and r3 allow us to extract r1 from (1), and it comes: with gl the required gain. capabilities the table below shows the calculated compo- nents for different values of k. in this case r2=1000 w and the gain=16db. the last column displays the maximum amplitude level on the line regarding the ts613 maximum output capabilities (18vpp diff.) and a 1:2 line transformer ratio. measurement of the power consumption in the adsl application conditions: passive impedance matching transformer turns ratio: 2 maximun level required on the line: 12.4vpp maximum output level of the driver: 12.4vpp crest factor: 5.3 (vp/vrms) the ts613 power consumption during emission on 900 and 4550 meter twisted pair telephone lines: 360mw figure 5 : equivalent schematic. ro is the syn- thesized impedance ro rs 1 r 2 r 3 ------- C ---------------- - 4 () , = ro vi.gi iout 1/2 rl ro kvorl rl 2 rs 1 + --------------------------- = ro 1 2 -- - rl 5 () , = r 2 r 3 ------- 1 2 rs rl --------- - 6 () , C = r 3 r 2 1 2 rs rl --------- - C ------------------- = r 1 2 r 2 21 r 2 r 3 ------- C ? ?? gl 1 C r 2 r 3 ------ - C --------------------------------------------------------- 7 () , = gl (gain for the loaded system) gl is fixed for the application requirements gl=vo/vi=0.5(1+2r2/r1+r2/r3)/(1-r2/r3) r1 2r2/[2(1-r2/r3)gl-1-r2/r3] r2 (=r4) abritrary fixed r3 (=r5) r2/(1-rs/0.5rl) rs 0.5rl(k-1) active matching k r1 ( w ) r3 ( w ) rs ( w ) ts613 output level to get 12.4vpp on the line (vpp diff) maximum line level (vpp diff) 1.3 820 1500 3.9 8 27.5 1.4 490 1600 5.1 8.7 25.7 1.5 360 2200 6.2 9.3 25.3 1.6 270 2400 7.5 9.9 23.7 1.7 240 3300 9.1 10.5 22.3 passive matching 12.4 18
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