TECHNICAL NOTE TERMINATION FOR POINT- TO-POINT SYSTEMS TN TERMINATON FOR POINT-TO-POINT SYSTEMS. Zo = L C. ω - angular frequency = 2πf

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1 TECHNICAL NOTE TERMINATION FOR POINT- TO-POINT SYSTEMS INTRODUCTION Because dgtal sgnal rates n computng systems are ncreasng at an astonshng rate, sgnal ntegrty ssues have become far more mportant to desgners. At hgher frequences, PCB traces can no longer be treated as just wres used to connect dgtal components together they become RF (Rado Frequency) transmsson lnes. Therefore, a basc understandng of transmsson lne theory s mportant to nsure sgnal ntegrty n today s hgh-speed dgtal systems. There are two major areas that affect sgnal ntegrty. The frst are transmsson lne effects such as rngng and reflectons. The second area s n the nteracton between crcuts, such as cross talk and nose on the power plane. Wth modelng, these transmsson lnes and nteracton of adjacent sgnals can be passed wth mnmal attenuaton and dstorton. Mcron provdes both IBIS and Spce models to help n ths area (see ww.mcron.com/models). R L R L G C G C Fgure Transmsson Lne TRANSMISSION LINE THEORY Understandng electromagnetc waves and the theores (well s Equatons) that defne ther behavor s not smple, but understandng the effects n your applcaton can be done wthout too much trouble. One of the basc concepts of a transmsson lne, s characterstc mpedance (see Fgure ). R + jωl Zo = = G + jωc R + jωl jωc ω - angular frequency = πf j - - Equaton Z O Assumng G= The characterstc mpedance of a lne s defned as Z o (Equaton ), where, R s the resstance per unt length, L s the nductance per unt length, G s the conductance per unt length, and C s the capactance per unt length. The conductance per unt length s due to loss through delectrc materal separatng the two traces. You can safely assume G= at speeds below GHz. In some crcumstances you can also assume R to be zero but as the frequences are ncreasng you have to account for the skn effect. If you assume both G and R are zero, then the transmsson lne would be consdered loss less (Equaton ). Zo = L C Equaton Z o Loss Less Tranmsson Lne The skn effect s a phenomena where the conductvty decreases as the frequency ncreases. The sgnal s carred n a very thn layer of the conductor s surface. The thckness of ths layer s dependent on the frequency of the sgnal (Equaton ) causng the ress- : Sgnal Integrty and Termnaton, Mcron Technology, Inc.

2 δ = Rs = n nches δ = n cm f f Equaton Skn Depth for Copper πf µ σ f - frequency µ - permeablty σ - conductvty Ra.c. = Rs w f - frequency µ - permeablty σ - conductvty - length w - wdth Equaton Skn Depth Resstance the characterstc mpedance) reflectons of the sgnal wll occur. Reflectons on the lne dstort the desred sgnal and f the dstorton s large enough the sgnal cannot be detected relably by the recever. TERMINATION Termnaton s used to match the drver mpedance to the transmsson lne mpedance when msmatch case exsts. Where Output drver mpedance + termnaton mpedance = Z of the lne. If the characterstc output drve mpedance s not known, t can easly be approxmated wth the dv/di curves. These curves can be found n the IBIS model for the devce drver. To extract the drver mpedance from the curves, dvde the delta V by delta I n the operatng range of the output drver as shown n Fgure. From Fgure take the delta on the two lnear lnes drawn from to about.v. PULL DOWN tance (R) of the transmsson lne to ncrease wth frequency. At the speed some of the pont-to-pont DDR systems are runnng, the skn depth s reachng to a few tenths of a ml (. nches) for the conductng layer n turn ncreasng the AC resstance. The skn effect s the man reason one cannot gnore the resstance n calculatng the characterstc mpedance of a transmsson lne. Characterstc mpedance has unts of ohms. When a transmsson lne s termnated at both ends wth ts characterstc mpedance, the lne s sad to be matched (see Fgure ). Beng matched means no reflectons occur on the lne. A matched lne s the optmum lne for passng hgh frequency sgnals. If a transmsson lne s not matched (termnatng mpedances don t match Current ma Volts V Fgure Standard Pull Down (Operatng Range -.V) MIN MAX Source() Fgure Matched Transmsson Lne (Source Impedance = ) As the drve strength of a drver s ncreased ts mpedance goes down. However, there are lmts to the range of mpedances that can be easly obtaned for a drver. The characterstc mpedance of mcrostrp s also lmted to a range because of physcal dmensons. The characterstc mpedance of mcrostrp s a functon of the delectrc constant of the substrate, the thckness of the substrate, and the wdth of the trace and frequency of the sgnal. As ts wdth ncreases the characterstc mpedance drops. For. ml FR substrate and a ml lne the characterstc mpedance s ~6 ohms. If, because of layout concerns, the wdest the mcrostrp lne can be s mls, the drver mpedance would have to be 6 ohms. If the mcrostrp lnes can be : Sgnal Integrty and Termnaton, Mcron Technology, Inc.

3 ncreased to mls (characterstc mpedance of ~ ohms) the drver mpedance could be lowered to ohms. In some manner, drver mpedance should be matched to the characterstc mpedance of the transmsson lne. If the drver mpedance cannot be ncreased to the optmum mpedance, a seres resstor can be placed next to the drver to rase the mpedance to the desred level (see Fgure ). For example, f the drve mpedance s ~8 ohms the seres resstor would Source() Drver R 8 ohm Seres R Fgure Seres Resstor T Lne 6 ohm need to be s for a 6 ohm transmsson lne. If the lne needs to be drven from both ends (DQ lne) two seres resstors would be needed (see Fgure ). Source() Drver R 8 ohm Seres R T Lne 6 ohm Fgure Two Seres Resstor Seres R Recever 8 ohm Recever() As prevously ndcated, a fully matched transmsson lne s the optmum nterconnecton between devces. However, a fully matched lne also dvdes the sgnal n half (half the sgnal n the drver mpedance and half n the load). So the queston arses: Is there another way to connect the devces wthout reducng the sgnal by half? The answer s yes. If only one end of a transmsson lne s matched, sgnals wll reflect off the unmatched end and then termnate nto the matched end. The confguraton n whch the drvng mpedance s matched and the recevng end s not, s known as back termnatng (See Fgure 6). Sgnals that come from the source travel down the transmsson lne, reflect off the untermnated end then travel back through the transmsson lne and termnate n the source resstor. Although there s a reflecton, ths reflecton does not dstort the sgnal at the recevng end. Source() Fgure 6 Back Termnatng For pont-to-pont DQ connectons there are two man ssues, the system mpedance and the load capactance. Wth a partcular capactance and drve mpedance there s an ultmate lmt on the maxmum data rate that can be attaned n a dgtal system. As the capactance or drve mpedance goes up, maxmum data rate goes down. Lkewse, as the capactance or drve mpedance goes down, the maxmum data rate goes up. Ths s why t s so mportant to reduce the nput capactance on dgtal recevers. The ssue of reducng the drve mpedance s not qute so smple. Reducng the drve mpedance wthout reducng the mpedance of the connectng transmsson lne wll create sgnal ntegrty problems. The optmal soluton for pont-to-pont DQ connectons would be to match the drver mpedance to the characterstc mpedance of the transmsson lne and for ths mpedance to be as low as possble. Ths would allow a drect connecton wth the hghest possble speed and wth no addtonal components. : Sgnal Integrty and Termnaton, Mcron Technology, Inc.

4 One problem a drver has s that ts mpedance s not constant. As the levels change from low to hgh or hgh to low the mpedance vares. The drver also has some parastc capactance and nductance that makes the drve mpedance not purely resstve. A seres resstor can reduce the effect of drve mpedance varaton on sgnal ntegrty degradaton. If the above gudlnes are taken nto account pontto-pont connectons are smple to make, but they must be analyzed wth the known parastcs. Sometmes t may be benefcal to msmatch the lne wth a smaller drve mpedance. A smaller drve mpedance can sometmes allow a hgher data rate wth lttle sacrfce n sgnal ntegrty. When multple drops are requred (Clocks & Addresses) the confguraton s no longer smple and a more complcated analyss s requred. Varous parameters needed to be changed and balanced to get optmal performance. POINT-TO-POINT EXAMPLE DQ TOPOLOGY Ths topology was selected by consderng a combnaton of eye pattern, component count, and power consumpton. The topology selected for ths DQ, pontto-pont connecton uses a seres resstor located mdway between the drver and recever, and allows for both READs and WRITEs between the components. From the above dscusson ths s not an optmal confguraton. A compromse was made to allow the use of only one resstor. Note: All DQ pont to pont connecton analyss s done assumng a MB/sec data rate on. ml FR wth SSTL drver recever. () L= L= W= W= ohm 7 nh. ohm 6 ohm 7 nh pf pf () Fgure 7 Example Smulaton Net Assumng Inch Traces : Sgnal Integrty and Termnaton, Mcron Technology, Inc.

5 Eye <> j Eye < TR > Mn 6. nsec Rse Tme. 9. Eye <> j Eye < TR > Mn 6. nsec Rse Tme. 9 Fgure 8 Results for Example Topology (lnear smulaton, drver mpedance ohms). Note that the resstance used s 6 ohms. A 6 ohm resstor was used as a compromse between usng a sngle resstor and rse tme and overshoot. For ths topology a lower resstor could be selected f more over shoot can be tolerated. See the secton on resstor szes. Eye <> j Eye < TR > Mn Eye <> j Eye < TR > Mn ohm Fgure 9A Fgure 9B. 9 Fgure 9A-9D Varatons of Resstor Values. Eye <> j Eye < TR > Mn Eye <> j Eye < TR > Mn 7 ohm ohm. 6 Fgure 9C Fgure 9D. 9. The effect of resstor value on the example net (lnear smulaton, drver mpedance ohms) are shown n the next few smulaton graphs to demonstrate the effects of msmatchng the characterstc mpedance. Note that msmatchng the drve mpedance to the transmsson lne affects sgnal ntegrty. Also note that the case has better transtons than the selected topology but wth a lttle more overshoot. The rse tme can also play a large role n sgnal ntegrty. The next four graphs demonstrate the affects of reducng the rse tme to mprove sgnal ntegrty. It s always a good dea to use the slowest rse tmes that wll meet the requred tmng budget. : Sgnal Integrty and Termnaton, Mcron Technology, Inc.

6 Eye <> j Eye < TR >.nsec Rse Tme CONCLUSION The queston s a common one what does Mcron recommend for termnaton? Ultmately, the best soluton s to smulate your desgn to ensure clean sgnals and good data eyes. Mn Eye <> j Eye < TR >.nsec Rse Tme Mn Eye <> j Eye < TR > Mn Eye <> j Eye < TR > Mn 6. 9.nsec Rse Tme nsec Rse Tme S. Federal Way, P.O. Box 6, Bose, ID 877-6, Tel: E-mal: prodmktg@mcron.com, Internet: Customer Comment Lne: Mcron s a regstered trademark of Mcron Technology, Inc. : Sgnal Integrty and Termnaton 6, Mcron Technology, Inc.