Bus Design Boot Camp Chapter 8 Differential Signaling
Bus Design Boot Camp Chapter 8 Differential Signaling Instructor: Howard Heck 1
Schedule Bus Design Boot Camp ü Day 1 Ø Ø Ø Introduction – Bus design Overview Transmission Line Fundamentals Digital Timing Analysis Crosstalk Non-ideal Interconnect Falconer 0. 25 Leddige 2. 0 Heck 2. 0 Falconer 2. 0 Hall 2. 0 ü Day 2 Ø Ø Connectors, Packages and Vias NIRPS, SSO, Power Delivery Differential Signaling Buffer Modeling Mix 2. 0 Hall 2. 0 Heck 2. 0 Nelson 2. 0 ü Day 3 Ø Ø Introduction to network analysis Equalization Overview Design Methodology (DOE & PD) High Speed Meas & Validation Hall 2. 0 Heck 2. 0 Shykind 2. 0 Mc. Call 2. 0 2
Bus Design Boot Camp Contents ü Introduction ü Common and Differential Modes ü Common Mode Noise Rejection ü Termination ü Transmitters & Receivers ü Differential Printed Circuit Boards Ø Structures Ø Losses Ø Common Mode Conversion in PCBs ü Differential S-Parameters ü Summary ü References 3
Bus Design Boot Camp Introduction ü Differential signaling uses two conductors per signal. Ø The transmitter translates the single input signal into a pair of outputs that are driven 180° out of phase. Ø The receiver, a differential amplifier, recovers the signal as the difference in the voltages on the two lines. ü The cost of differential signaling seems clear – 2 x the # of signal pins (die, package) and PCB traces. ü So, why do this? What’s the benefit? 4
Bus Design Boot Camp Differential Signaling Description ü Recall from the crosstalk chapter that for a system with 2 signal conductors (& 1 reference conductor) there are 2 modes (even and odd mode). ü Treating them as completely isolated, all signals on the lines propagation as a combination of the two modes. ü Since the two lines are driven 180° out of phase, the waves will propagate in the odd mode. E H Even Mode Odd Mode 5
Differential Signaling Description ü If the pair is not isolated, signals on the other conductors will affect the propagation. Bus Design Boot Camp Ø More modes will exist, and signals will be a combination of all of those modes. ü Noise from those conductors, and from any other sources, can be decomposed into two “modes” – common and differential. ü These aren’t actual modes, but they provide a convenient and useful way of looking at the components of the signal. E H Even Mode Odd Mode 6
Common and Differential Modes Bus Design Boot Camp ü Differential mode signals propagate 180° out of phase. ü Common mode signals propagate in phase. ü The receiver is essentially a differential sense amplifier. Ø The output depends on the difference between the inputs. Ø Since common mode signals are in phase, a purely common mode signal tends to put the receiver into an unstable state. ü The answer to “why differential” lies in understanding how injection of a common mode noise signal on top of a differential signal affects the operation of the receiver. 7
Common Mode Noise Rejection ü The signal to be transmitted is represented by voltage, V(t). The two out-of-phase waveforms are defined as: Bus Design Boot Camp [8. 1] [8. 2] where V 0 is a constant ü Modal decomposition of the signal pair (see Young, chapters 10 & 11) allows us to analyze coupled lines without explicitly using mutual circuit elements. [8. 3] 8
Common Mode Noise Rejection #2 ü Combing the first three equations: Bus Design Boot Camp [8. 4] [8. 5] ü Veven carries only a DC component, so it generates no noise due to reactive parasitics, or SSO. Benefit #1 ü Vodd carries a scaled version of the signal. ü If lines are tightly coupled (not always the case), then noise from external sources affects both lines as common mode noise. Benefit #2 ü Transients on the two conductors tend to be self canceling, greatly reducing power supply noise. Benefit #3 9
Common Mode Noise Rejection #3 Bus Design Boot Camp ü In summary, differential signaling offers excellent immunity to SSO & crosstalk. i. e. the receiver rejects the common mode noise. Ø Even mode picks up the common mode noise. Ø Odd mode remains noise free. ü To prove it, we can superimpose common mode noise, Vnoise(t), in the mode voltages at the receiver: [8. 4] [8. 5] 10
Common Mode Noise Rejection #4 Bus Design Boot Camp ü Now translate the voltages back to V+ and V-: [8. 6] [8. 7] 11
Common Mode Noise Rejection #3 Bus Design Boot Camp ü The differential receiver detects the signal as the difference between V+ and V- : [8. 8] ü Vóila! The common mode noise has been removed. ü In practice, receivers aren’t perfect and some common mode noise gets through. Ø Differential receivers typically spec a common mode rejection ratio (CMRR). 12
Final Thoughts on CM Rejection Bus Design Boot Camp ü Breaks in symmetry cause mode conversion between even and odd modes, which will inject common mode noise onto the signal. Examples: Ø Ø serpentines reference plane changes crosstalk electrical length mismatch ü Note that while differential signaling requires 2 x the pins, the immunity to SSO noise allows us to reduce the number of power and ground pins in packages, sockets, and connectors. Ø It may also allow us to remove some decoupling. ü Finally, realize that the common mode rejection ratio (CMRR) of the receiver will affect performance, too. 13
Bus Design Boot Camp Benefit of Differential Signaling ü A differential pair shares a common return path, but the common mode noise introduced by the return path is rejected by the differential receiver. ü This makes differential signaling much quieter than single ended signaling. Ø Remember Shannon’s theorem: SNR limits performance. ü So, differential signaling can operate at much Higher data rates. Ø Must be at >2 x to make it worthwhile. ü High speed links operating in excess of ~1 Gb/s use differential signaling (e. g. Infiniband, PCI-Express). ü In fact, differential signals are already used for high speed clocks in desktop PCs. 14
Bus Design Boot Camp Termination of Differential Signals ü The goal is to terminate the signals in a resistor network that terminates each mode. ü We can use a pi network to terminate both the even and odd modes. ü We can again use modal decomposition. [8. 9] ü The optimum termination will give us reven = rodd = 0. [8. 10] [8. 11] 15
Bus Design Boot Camp Differential Transmitters ü Differential signaling typically uses current-mode transmitters. One example is shown here (a source coupled pair). ü Features: Ø provides an extremely sharp transient response because the current switches from 0 to k·Iref over a half volt input swing. Ø Draws constant current from the supply, which reduces the AC component of power supply noise. Ø The source voltage, VS, is stable, reducing the turn-on transient that results with a switched current-source configuration. ü Other options include using a cascode current mirror to reduce the output capacitance. 16
Bus Design Boot Camp Differential Receivers ü Source coupled FET receivers are often used with differential signaling. ü An example is shown here (self-biasing Chappell Amplifier). ü Dally provides a good reference on differential transmitter and receiver circuits. 17
Differential PCBs ü It is possible to implement tightly coupled differential interconnects, e. g. using twisted pair wires. Bus Design Boot Camp Ø Coupling is 99. 9% ü It is not practical to do so in a PCB: Ø Typical coupling for differential traces is 20 -50%. Ø This is OK, as long as the traces are symmetrical. Ø Routing with minimum spacing is OK, but must be maintained or we’ll get an impedance discontinuity. Ø Trace lengths must be matched, or common mode current will be generated. The amount of current imbalance can be expressed as: [8. 12] 18
Differential PCBs #2 ü Differential impedance is defined as: Bus Design Boot Camp [8. 13] ü Differential impedance control in HVM PCBs is typically 15 -20%. Ø Versus single ended impedance (10 -15%). Ø Strongly influenced by the etch profile (W 1, W 2). ü Skin effect will show up differently than with single ended lines. 19
Bus Design Boot Camp Current Distribution & Differential Losses ü For coupled differential lines, the virtual ground will pull the current to the edges. ü Current flows in a smaller area, which increases resistance. ü For very narrow spacing, the current area will asymptote to t·dskin. ü For very wide spacing, the current area will asymptote to W·dskin. 20
Differential Transitional ü Ports are matched to Zdiff. ü Current distributions effect the loss. ü Evidence of a minimum loss “sweet spot”. Single Line 0. 40 0. 35 Loss (1 -|S 21|) Bus Design Boot Camp Current Distribution and Differential Losses 0. 30 10 GHz 0. 25 5 GHz 0. 20 0. 15 0. 10 0. 05 0. 00 1 10 100 Spacing [mils] 1000 21
Bus Design Boot Camp Common Mode Conversion in PCBs ü Phenomenon: Differential Phenomenon: pairs see variation in effective dielectric constant due to local non-uniformity. ü Root Cause: Different Root Cause dielectric constants (er): glass ~ 6, epoxy ~ 3 Ø A line routed over a glass bundle travels more slowly due to the higher er (& vice versa). Ø Converts differential signals to common mode thru electrical length mismatch caused by the er difference. FR 4 Glass Cloth w/ Differential Signals er = 3. 5 er = 3. 3 D+ D- D+ 10 mils Glass D- Epoxy Glass 16. 7 mils 22
Common Mode Conversion in PCBs #2 Mechanism Transmitter Receiver Bus Design Boot Camp D+ D- Vdiff = D+ - D- Vcomm = + D +D 2 V V 0 0 -V -V Differential phase skew degrades voltage & timing margins. 23
ü Impact: Max data rate Impact: degradation. 1 st order model: 0. 25 0. 15 a = 1 d. B/in 0. 10 er, eff (D+) = 3. 3 0. 05 0. 00 where 5 Gb/s 10 Gb/s 0. 20 Timing Noise [ps] Bus Design Boot Camp Ø Noise , SNR . % Voltage Noise Mode Conversion in PCBs #3 er, eff (D-) = 3. 5 0 5 10 Length [in] 15 5 Gb/s 200 160 120 80 ~15 ps/in Phase skew 40 0 10 Gb/s 0 5 10 Length [in] 15 In the plots, er 1, eff = 3. 3 and er 2, eff = 3. 5. 24
Bus Design Boot Camp Example 1: Balanced Ckt/Length Skew Ckt Circuit: ü 3 differential pairs. ü 12” traces with 5 mil space between all traces. ü Terminated in Zodd at both ends. ü 16. 67 m. A current source transmitter. ü Only the middle pairs driven. ü Results are plotted for D 2 & D 2. ü In the skewed case, D 2 is 0. 1” longer than D 2. Balanced Skewed 25
Bus Design Boot Camp Example 1: Balanced Ckt/Length Skew Ckt Here are the common mode and differential mode waveforms at the receiver. Differential Mode Common Mode 26
Example 2: Crosstalk Same 3 differential pairs Trace Case 1 Case 2 Bus Design Boot Camp D 1 Quiet D 1 bar Quiet D 2 L H D 2 bar H L D 3 H L L H D 3 bar L H H L Common Mode Case 1 Differential Mode 27
Differential S-Parameters Bus Design Boot Camp üDifferential S-Parameters are derived from a 4 -port measurement. üTraditional 4 -port measurements are taken by driving each port, and recording the response at all other ports while terminated in 50. üAlthough, it is perfectly adequate to describe a differential pair with 4 -port single ended s-parameters, it is more useful to convert to a multi-mode port. 28
Multi-Mode S-Parameters Bus Design Boot Camp üSpecify the differential S-parameters in terms of differential and common mode responses. ØDifferential stimulus, differential response ØCommon mode stimulus, common mode response ØDifferential stimulus, common mode response (aka ACCM Noise) ØCommon mode stimulus, differential response üThis can be done either by driving the network with differential and common mode stimulus, or by converting the traditional 4 -port s-matrix. ØConverting the s-matrix al. Ls the use of the 4 -port VNA. é bdm 1 ù é Sdd 11 Sdd 12 ê ú ê b ê dm 2 ú = ê Sdd 21 Sdd 22 ê bcm 1 ú ê Scd 31 Scd 32 ê ú ê ëbcm 2 û ë Scd 41 Scd 42 Sdc 13 Sdc 23 Scc 33 Scc 43 Sdc 14 ù é adm 1 ù úê ú Sdc 24 ú êadm 2 ú Scc 34 ú ê acm 1 ú úê ú Scc 44 û ë acm 2 û Matrix assumes differential and common mode stimulus. 29
Conversion to Multi-Mode S-Parameters ü Converting the S-parameters into the multi-mode is a matter of performing some algebra. ü Example: Differential return loss, Sdd 11: Bus Design Boot Camp The stimulus is equal, but opposite: & [8. 15] For a symmetrical network: & Also use: [8. 16] 30
Advantages/Disadvantages of Multi-Mode Matrix Bus Design Boot Camp Advantages: ü Describes 4 -port network in terms of 4 two port matrices. Ø Ø Differential Common mode Differential to common mode Common mode to differential ü Easier to relate to system specifications. Ø ACCM noise, differential impedance Disadvantages: ü Must convert from measured 4 -port scattering matrix. 31
Bus Design Boot Camp Summary ü Differential signaling offers much higher performance by minimizing common mode noise. ü Differential transmitters and receivers typically operate in current mode. ü Differential PCB traces must be symmetric to minimize the generation of common mode current. Ø Ditto for packages, connectors, and sockets. 32
Bus Design Boot Camp References ü S. Hall, G. Hall, and J. Mc. Call, High Speed Digital System Design, John Wiley & Sons, Inc. (Wiley Interscience), 2000, 1 st edition. ü W. Dally and J. Poulton, Digital Systems Engineering, Cambridge University Press, 1998. ü B. Young, Digital Signal Integrity, Prentice-Hall PTR, 2001, 1 st edition. ü Tektronix, Inc. , “Differential Oscilloscope Measurements, ” Application Note 51 W-10540 -1, July 1996. ü E. Bogatin, M. Resso, “Differential Impedance Measurement With Time Domain Reflectometry, ” Agilent Technologies Application Note 1382 -5, May 9, 2002 33
Appendix: Low & High Frequency Losses Bus Design Boot Camp ü Some additional details 34
Differential Microstrip Losses The plot shows mstrip losses as a function of frequency and loss tangent assuming smooth conductor (5/5/5). 0 tand=0. 01 y = -5 E-10 x - 1. 2079 R 2 = 0. 9953 Loss, d. B Bus Design Boot Camp -5 -10 tand=0. 03 -15 y = -1 E-09 x - 1. 1925 R 2 = 0. 9992 -20 -25 0 5 10 15 Frequency, GHz 20 25 This indicates that dielectric losses dominate beyond 2. 5 GHz to 4 GHz. (i. e. scale linearly with frequency) 35
Low Frequency Loss in Differential mstrips W/S/W=5/15/5 Bus Design Boot Camp W/S/W=5/5/5 Curves Intersect W 5 mils er 4. 2 h 4. 5 mils tand 0. 03 Zodd 50± 5 ü Low frequency losses are greater for narrowly spaced differential microstrips. ü Model predicts that loss curves for wide and narrow spaces intersect at: Ø 700 MHz when tand=0. 03, Ø 3 GHz when tand=0. 01 36
High Frequency Loss in Differential mstrips 0 w=5, s=5 w=5, s=10 w=5, s=15 w=5, s=20 W 5 mils er 4. 2 h 4. 5 mils -15 tand 0. 03 -20 Zodd 50± 5 loss [d. B] Bus Design Boot Camp -5 -10 -25 -30 0 5 10 Frequency 15 20 25 ü Model predicts losses that high frequencies increase with for wide spacing. Ø Worse high values of tand. Why? 37
Differential Microstrip Loss Mechanism Bus Design Boot Camp ü Conductor losses increase due to skin effect & proximity effect. ü In absence of dielectric losses, narrow spacing will produce higher losses due to proximity effect – area of current flow determines losses (approx. f 0. 5 behavior). Current Distribution Narrow Spacing Wide Spacing E-Fields ü Dielectric losses increase due to damped response of electric dipoles as a function of the frequency of applied oscillating electric field. Ø Dielectric loss increases linearly w/ freq. (assuming homogeneous media). ü Why does narrow spacing have the highest losses at low frequencies but the lowest loss at high frequencies? Ø At low frequencies, tand losses are small and losses are dominated by skin and proximity effects. • Narrow spacing = smaller area for current = high loss Ø At high frequencies, tand losses dominate. • Smaller spacing leads to more E-fields fringing through the air and less through the lossy dielectric. 38
High Frequency Loss in Differential Striplines 0 5 -5 strip, . 03 5 -10 strip, . 03 5 -15 strip, . 03 5 -20 strip, . 03 W 5 mils er 4. 2 -20 B 18 mils -25 tand 0. 03 Zodd 50± 5 -5 -10 loss, d. B Bus Design Boot Camp -15 -30 -35 -40 -45 0 5 10 frequency 15 20 ü Narrow spacing remains the highest loss configuration in a stripline at all frequencies. ü Since the dielectric media is homogeneous, all the fields are contained within the lossy material. ü With no fields fringing into a loss-free dielectric (air), the only conductor losses are affected by spacing. 39
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