AN409: Output Termination Options for the Si500S and Si500D Silicon Oscillators

AN409
O UTPUT TERMINATION O PTIONS FOR TH E
Si500S AND Si500D S I L I C O N O SC ILLA TORS
1. Introduction
The Si500D Silicon Oscillator can be ordered with one of five different output buffer types: CMOS, SSTL, LVPECL,
LVDS, or HCSL. Each output type has its own particular benefits and limitations. This document describes each
buffer type, proper biasing and termination schemes, and related technical trade-offs.
2. CMOS
Complementary metal-oxide semiconductor (CMOS) totem pole output buffers are used to drive capacitive loads to
CMOS logic levels. The Si500 CMOS driver output impedance is a nominal 36 . An external series resistor can
be added to provide optimum impedance matching with higher impedance traces as shown in Figure 1.
Unlike most CMOS output drivers, the Si500 provides two outputs which can be ordered as complementary or inphase. When in-phase, the two outputs may be shorted together to produce a single nominal 18  driver to drive
large capacitive loads. The CMOS option can be ordered for all three of the supported supply levels (1.8, 2.5, and
3.3 V).
Figure 1. Example of External Source Termination Resistors
to Improve Trace Impedance Matching
Rev. 0.2 5/10
Copyright © 2010 by Silicon Laboratories
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3. SSTL
SSTL (Stub Series Terminated Logic) is an electrical interface commonly used with DDR (Double Data Rate)
DRAM memory ICs and memory modules. The Si500 support 3.3, 2.5, and 1.8 V SSTL outputs which can be
single-ended, differential, or in-phase. The termination scheme for SSTL is shown in Figure 2.
VTT
VTT
50 
50 
VDD
VDD
2.43 k
2 k
0.50*VDD
VTT
2 k
0.45*VDD
VTT
2 k
0.1 uF
Practical VTT for SSTL-2, SSTL-18
Practical VTT for SSTL-3
Figure 2. SSTL Termination Schemes
2
0.1 uF
Rev. 0.2
AN409
4. LVPECL
Low-voltage positive emitter-coupled logic (LVPECL) differential outputs are typically chosen for their superior jitter
performance. The Si500 devices offer two LVPECL options: a standard LVPECL option and a low-power LVPECL
output option for ac coupling only. LVPECL is offered with both 3.3 V and 2.5 V supplies.
The standard LVPECL output option requires external biasing and proper termination of 50 to VDD-2 V for each
side of the differential output. Many well-known LVPECL biasing and termination schemes are supported by the
Si500 devices. The most common are shown in Figures 3, 4, and 5. The primary disadvantages of this output
format are increased power consumption (due to dc biasing) and incompatibility with 1.8 V supplies. The primary
advantage of the LVPECL signal format is jitter performance. LVPECL provides the best jitter performance
because of its large swing and fast edge rates.
50 
50 
50 
50 
VDD-2V
Figure 3. Traditional Biasing and Termination for Standard LVPECL Output Option
R1
R1
3.3V LVPECL
R1 = 130 , R2 = 82 
2.5V LVPECL
R1 = 250 , R2 = 62.5 
R2
R2
Figure 4. Alternative Biasing and Termination for Standard LVPECL Output Option
Rev. 0.2
3
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0.1 µF
130 
100 
130 
Figure 5. Alternative Biasing and Termination for Standard LVPECL Output Option
(100  Line Termination May Be Internal to the Receiving IC)
The low-power LVPECL option eliminates the need for external dc biasing, which reduces overall power
consumption without sacrificing jitter performance.
100 
0.1 µF
Figure 6. Termination for Low-Power LVPECL Option
(100  Line Termination May be Internal to the Receiving IC)
4
Rev. 0.2
AN409
5. LVDS
Low-voltage differential signaling (LVDS) differential outputs are typically chosen for their ease of use. LVDS is a
common input and output type used with FPGAs. LVDS outputs require no external biasing or termination when
connected to LVDS inputs and are very power-efficient. Also, the LVDS specification allows for significant dc
biasing drift from transmitter to receiver, further simplifying system-level design. LVDS outputs are connected as
shown in Figure 7.
100 
Figure 7. Typical Transmission Line Connection for LVDS
(100  Line Termination May Be Internal to the Receiving IC)
6. HCSL
High-Speed Current Steering Logic (HCSL) outputs are commonly used for PCI Express applications. The Si500D
integrates the 50  termination resistors to ground, simplifying the connection to an HCSL receiver.
50 
50 
50 
50 
Internal To HCSL Output Driver
Figure 8. HCSL Connection Using Internal Termination Resistors
Rev. 0.2
5
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