Digital Systems Terminations II CMPE 6501 (4/24/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6AC Biasing for End TerminatorsCapacitors are used in end-terminations to reduce the quiescent power dissi-pation.If the drive circuit spends half its time in each state (also called DC-balanced),the average value accumulated on C1 will be halfway between HI and LOvoltages.Z0transmission lineWorks only if signalis DC balancedR1 = Z0C1+5VV1+-Z0transmission lineWastedR2 = 2Z0+5V+5VR3 = 2Z0FlowCurrentR1C >> Signal Clock Time(DC-balanced)Split terminationCapacitive terminationDigital Systems Terminations II CMPE 6502 (4/24/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6AC Biasing for End TerminatorsThe voltage across R1 will be ∆V/2. The power dissipated in R1 will beIn the split terminator either R2or R3 always has the full ∆V across it, buteach resistor is twice as big as R1 so, the average power dissipation isThe additional wasted power dissipation flows from VCC directly to groundthrough R2 and R3.From driving circuit’s perspective, the two terminations are indistinguish-able w.r.t. power dissipation.Only the dissipation in the terminating resistors differs.whereZ0 = Value of terminating resistor, Ω∆V= HI-LO Logic voltages, VPR1∆V 2⁄( )2Z0----------------------∆V24Z0-----------= =Twice the power of DC-balancedPR2R3+∆V( )22Z0---------------=Digital Systems Terminations II CMPE 6503 (4/24/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6End Terminations for Differential LinesGiven two signals which are complementary (a differential pair), we can con-nect their end-terminating resistors together onto a single capacitor.This provides a power saving end terminator with a guaranteed correct volt-age always present on C1.Resistor Selection: Accuracy in Terminating ResistorsPurpose of terminating resistor is to reduce/eliminate unwanted reflec-tions on a Tx-line.Z0C1+5VZ0+5VR1R2R1= R2= Z0Digital Systems Terminations II CMPE 6504 (4/24/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Resistor SelectionThis is possible only when its value matches the characteristic impedance ofthe Tx-line (Z0).To compute the worst-case terminating mismatch, the uncertainty in Z0 isadded to the uncertainty in the terminating resistor.The uncertainty in Z 0 is usually larger than that of the terminating resistor.E.g., with a 10% uncertainty in Z0, designers would use a resistor with a1% tolerance.For high fidelity signal requirements, use both source and end terminations.Cuts the received signal in half, but reflections are greatly reduced.Also, this relaxes the tolerance required for termination matching ateither end.This technique is used extensively in microwave circuits but for digital elec-tronics, receivers must be able to discriminate between reduced levels.Digital Systems Terminations II CMPE 6505 (4/24/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Resistor SelectionAlways calculate the worst-case power dissipation in each terminator regard-less of the operating speed, i.e., do not assume 50% duty cycle.Worst case for split terminationNo Current Flow in R2R2LO = 0VVCC = +5VR1PR1 = I1VCC = 0.25W (R1 = 100Ω)I1 = VCC/R1No Current Flow in R1R2HI = VCCVCC = +5VR1PR2 = I2VCC = 0.25W (R2 = 100Ω)I2 = VCC/R2Pworst5V( )2100Ω---------------=The worst case power dissipationDigital Systems Terminations II CMPE 6506 (4/24/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Power Dissipation in Terminating ResistorsStandard 1/8-W resistors will over heat in this case at room temperature.Even 1/4-W resistors may overheat at elevated ambient temperatures.Power handing capacity of many resistors declines at elevated temperatures.Resistor bodies have thermal resistance rating (Degree Celsius rise per Watt)just like IC packages.However, resistors (especially ceramic) can tolerate much higher operat-ing temperatures than ICs.Unlike IC packages, resistors can be mounted in two different configurations:Horizontal Mount Vertical MountDigital Systems Terminations II CMPE 6507 (4/24/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Power Dissipation in Terminating ResistorsThe vertical mount has a lower thermal resistance in still air than the horizon-tal mount.The VERY short leg between the resistor and circuit board reduces ther-mal resistance.The horizontal mount has a lower inductance because the leads stay low.As a consequence of overheating, the resistance value may drift, causingreflections.In extreme cases a resistor may crack open, unterminating the circuit.Along with resistance value, a tolerance, and power rating, the next mostimportant factor is the parasitic series inductance.Every resistor has a parasitic series inductance depending on its internal con-struction, external lead type, and mounting configuration.Digital Systems Terminations II CMPE 6508 (4/24/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Series Inductance of Terminating ResistorsThe effect of series inductance is a function of operating frequency.The magnitude of inductive reactance seen by a rising edge as a function ofrise time is given by: Parasitic inductance causes termination mismatch just like an error in theterminating resistance.Every 1% of reactance causes 1/2% of reflection.E.g., when |X(Tr)| equals 10% of terminating resistance, the reflection is 5%.Typical series inductance of resistors:X Tr( )πLTr-------=Resistor Type Series Inductance (nH)1/4-W axial1/8-W axial1/8-W 1206, surface-mount2.51.00.92.2Ω carbon-film0Ω 0.12"x0.06"Digital Systems Terminations II CMPE 6509 (4/24/08)UMBCU M B CUNIVERSITY OF MARYLAND BALTIMORE COUNTY1 9 6 6Series Inductance of Terminating ResistorsLab Experiment using:• 1/8-W axial resistors to terminate a signal with rise and fall time of 1ns.• Split termination of 100Ω to VCC (5V) and 100Ω to ground.• Transmission line impedance of 50Ω.Magnitude of inductive reactance:Calculate the inductive reactance magnitude to resistance ratio |X(Tr)|/R:The reflection due to this inductance is 1.5% because of the parallel, split ter-mination (single terminating resistor of 50 Ohms twice as much -> 3.14%because of 50 in demoninator).In general, putting resistors in parallel is a good way to make low inductancestructures.X Tr( )π 1nH( )1ns------------------- 3.14Ω= =X Tr( )100Ω------------------ 3.14%=Digital Systems Terminations