Download Terminations

Terminations
Chris Allen ([email protected])
Course website URL
people.eecs.ku.edu/~callen/713/EECS713.htm
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Termination resistors
Purpose
Reduce transmission-line reflections
Complete circuit for ECL and GaAs outputs
Issues
Reflection characteristics
Current / power loading
Component placement / routing issues
Resistor type, packaging
Options
End termination (L  0)
Source termination (S  0)
Other termination schemes
Differential line termination
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End termination
VHI  VTT 

RT

 must not exceed maximum output current of driver
VLO  VTT 


RT
I HI 
I LO
 0.8   2
 24 mA , I LO  4 mA
50
ECL output driver can sink/source 50 mA
GaAs (GigaBit Logic) can sink/source 60 mA
I HI 
both could drive two 50- loads
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End termination
Fanout – How many inputs can one driver support?
not limited by the input current ( 500 A) per input
 current available is 50 mA – 24 mA = 26 mA
driver output term
available
based on this analysis ECL or GaAs could drive over 50 inputs
26 mA / 0.5 mA = 52 inputs
instead fanout is limited by the capacitance (3 pF to 5 pF per input)
maximum fanout is about 10 inputs
capacitance slow the signal rise time (RC circuit)
rise time of RC circuit is, Tr RC   1.1 Z o C
Tr total  Tr2RC   Tr2
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End termination
Daisy-chain layout – one termination (only) at the far end
Issues – stub connections to inputs must be short (> l / 6)
so that each gate input appears as an open circuit
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End termination
Daisy-chain layout
termination resistor should be at the far end of the transmission line
to minimize reflections
or
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End termination
Alternative layout option with end termination
The driver ‘sees’ a single
50- transmission line that
is properly terminated
The challenge is fabricating
100- lines
For CMOS and TTL technology
A 50- termination may exceed the driver’s current capacity
To accommodate CMOS and TTL, can increase Zo and R so that the
current required is within the driver’s ability
Also, can reduce power dissipation in
termination resistor with AC coupling
During transient (Tr), capacitor looks like
short circuit Xc = Tr/C
if Xc << RT, then reflection is small
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End termination
Similar approach could also be used with ECL / GaAs
Reduce power dissipation in termination resistor with AC coupling
R1 = 50 
R1 + R2 >> 50 
For differential lines in CMOS and TTL technology
The termination pair could be capacitively coupled, taking advantage
of the inherent DC-balance
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Source termination
Source (or series) termination (S = 0)
Another approach to reduce reflections is to focus on the source end
Assumption – far end impedance, ZL >> Zo (~ open circuit)  L ~ 1
For RO + RS = Zo, S = 0
RO is driver resistance
Issues –
Only valid for driving far-end
load (intermediate positions
do not see full transition
in single step)
 no daisy chaining
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Source termination
What is the driver’s output impedance? (CMOS, TTL, ECL, GaAs)
Digital circuit output stages have output resistances that depend on
logic state
Family
Ro(HI)
Ro(LO)
Rin
Cin
Fanout
CMOS
28 
10 
5 M
10 pF
~5
TTL
50 
8
10 k
5 pF
20 to 30
ECL
6
8
50 k
GaAs
8  to 12 
200 
10 k
3 pF to
5 pF
1.5 pF to
5 pF
~ 10
~ 10
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Source termination
Why aren’t Ro(LO) higher for ECL and GaAs?
Output transistor is not biased to cutoff mode
Ro ~ 1/slope
~ 60 
~ 600 
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Source termination
Equivalent circuit for analyzing output
For RO(HI)  RO(LO)
cannot find RS that
Satisfy
RO + RS = ZO for
both HI and LO cases
For Zo = 50 , CMOS
ECL
GaAs
Rs ~ 30 
Rs ~ 40 
Rs ~ 40  (but a big S when LO)
However to pull down the output for ECL and GaAs,
a pull-down resistor is needed as well
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Source termination
How to determine value for Rpd?
First, find the current, Ipd, required to
pull the voltage level down on the
transmission line when the output goes low
I pd 
V
, where  V  VOH  VOL 
2 Zo
2 Zo due to open transmission line (L = +1)
V ~ 1 V  1 V/(2  50 ) = 10 mA  Ipd  10 mA
Second, during HI to LO transition, I pd
think of the trace as a charged capacitor
Enforce these two conditions
for VOH = -0.8 V, VEE = -5 V
Rs = 40 , Zo = 50 
VOH  VEE

R pd  R s  Z o
VOH  VEE
0 .5

R pd  R s  Z o Z o
4 .2
 10 mA  R pd  90  420 
R pd  90
Rpd  330 
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Source termination
Disadvantages of source termination
The source termination scheme produces the desired output voltage
only at the far end of the transmission line
Therefore daisy-chaining is not permitted
since gates attached midline will see irregular waveforms
Therefore all receiving gates must be clustered at the end of the line
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Source termination
Can drive more than one series terminated line, however
must not exceed the max output current limit
Driving two or more lines required a lower pull-down resistor value
Drawing more quiescent current through the termination (IOH) reduces
VOH (due to the gate’s internal resistance) resulting in a decreased
noise margin
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Source termination
Alternatively could use a circuit with multiple outputs, e.g.,
a fanout buffer, to drive more than one series terminated
line
Each output treated individually
Load distribution
Line impedance
Pull-down resistor value
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Other termination challenges
When there are several signal drivers on a given transmission line and
they are not located together,
where should the termination be located ?
Example, TTL or CMOS tri-state outputs on bus, or ECL (or GaAs)
gates connected in Wired-OR configuration on a transmission line
Possible solutions include:
1.
2.
3.
4.
Adding a source termination to each driver
Adding an end termination to each receiver
Adding a series resistance between every junction of branches
Adding a shunt termination in the middle of the network
Issues
Option 1 well defined, requires little power, provides damping, reduces settling time
Option 2 requires a lot of drive power but works in star configuration
Options 1 and 2 provides a perfect solution except it wastes power and attenuates
the signal
Option 3 attenuates the signal at each junction 
Option 4 is the middle termination
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Other termination challenges
Driving multiple lines with single source, single termination
Can drive a mix of terminated
and unterminated lines
Distorted waveforms
result as the unterminated
stub length increases
The ripple following the LO HI
transition is a result of inadequate
quiescent IOH level to cause a full
signal level on all lines
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Other termination challenges
Data busing involves connecting two or more outputs and
one or more inputs to the same signal line
Any driver can be enabled to apply data to the line
Termination resistors matching the line impedance connected to both
ends to prevent reflections
In analyzing such a configuration, the capacitance of a disabled
(unactive) driver should be taken as 2 pF for ECL
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Other termination challenges
Examples of how to terminate twisted pair cables
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Other termination challenges
Examples of how to terminate coaxial cables
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Termination resistor specifications
Accuracy
R = Rnom  R (%), R is the tolerance
R  Zo
Impacts the reflection coefficient,  
R  Zo
Variations in R and Zo affect 
Ideally R = 0
R 1 
Typical choices are 10%, 5%, 1%

Zo 1  
Recall   noise margin 1.35  R  0.74
Zo
for ECL, NM ~ 15%
for R  10%  Zo +22% or -19%
for R  5%  Zo +28% or -22%
for R  1%  Zo +34% or -25%
for R  0%  Zo +35% or -26%
Zo impacts board
manufacturing tolerances
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Termination resistor specifications
Power rating
Found using worst case condition (steady state)
for ECL or GaAs,
2

VHI  VTT 
P
RT
VHI ~ -0.8 V, VTT = -2 V, RT = 50   P = 29 mW
 1/8-W resistors appropriate
Resistor composition & package options
Ideally the resistor should be purely resistive
ZR = R + jXR , XR  0
However real resistors have reactance
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Termination resistor specifications
Resistor composition & package options
High-frequency circuit model for resistor
Wire-wound resistors have a large
inductive compontent
Carbon-film or printed resistors have lower inductance
Leaded resistors have lead inductance
Surface-mount (leadless) resistors have much less inductance
For high-frequency applications (low Tr) systems
inductance is a significant issue
Printed resistors
 Use leadless, chip resistors
Added benefit, doesn’t require vias for mounting
vias cost money and restrict routing areas
Laser trimmed to obtain
precise resistance values
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Termination resistor specifications
Resistor composition & package options
Surface-mounted chip resistors also have less inductive crosstalk
than leaded parts when placed close together
Multi-resistor packs (packages
containing multiple resistors, often
in a single in-line package – SIP or
dual in-line package – DIP)
have unacceptable crosstalk due to
proximity and due to a common current path
DO NOT USE THESE IN HIGH SPEED DESIGNS
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