Application Report
SCAA062–March2003
DC-Coupling Between Differential LVPECL,LVDS,HSTL,and CM Kal Mustafa/Chris Sterzik High Performance Analog
ABSTRACT
This report describes various methods of interfacing different logic levels.The focus is dc-
coupling between the following differential signaling:LVPECL(low-voltage positive-
referenced emitter coupled logic),LVDS(low-voltage differential signals),HSTL(high-
speed transceiver logic),and CML(current-mode logic).The report discuss sixteen
various interface cas between the aforementioned differential signaling levels.
Contents
1AC-Coupling (2)
1.1LVPECL (3)
1.2LVDS (7)
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1.3CML (11)
1.4HSTL (13)
2References (15)
Figures
Figure1.Input and Output Parameters (2)
Figure2.LVPECL to LVPECL (3)赞扬老师的话
Figure3.LVPECL to LVPECL (4)
Figure4.LVPECL to LVDS (4)
Figure5.LVPECL to LVDS (5)
Figure6.LVPECL to CML (5)
怎么克服紧张恐惧心理Figure7.LVPECL to CML Converter (6)
Figure8.LVPECL to HSTL (6)
Figure9.LVDS to LVPECL (7)
Figure10.LVDS to LVPECL (8)
Figure11.LVDS to LVDS Without On-Chip Termination (8)
Figure12.LVDS to LVDS With On-Chip Termination (9)
Figure13.LVDS to CML Converter (9)
Figure14.LVDS to HSTL (10)
Figure15.CML to LVPECL Translator (11)
Figure16.CML to LVDS (11)
Figure17.CML to CML (12)
Figure18.CML to HSTL (13)
新闻部Figure19.HSTL to LVPECL Converter (13)
Figure20.HSTL to LVDS (14)
Figure21.HSTL to CML Translator (14)
Figure22.HSTL to HSTL (15)
Tables
Table1.Typical LVPECL,LVDS,HSTL,and CML Outputs (2)
Table2.Typical LVPECL,LVDS,CML,and HSTL Input Levels (2)
Table3.Interface Table (3)
1
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2DC-Coupling Between Differential LVPECL,LVDS,HSTL,and CM
1AC-Coupling
DC-coupling is ud in a system when there is a need for a wide bandwidth,or when dc-unbalanced code is ud.Both interfaces must have the same ground potential on the same board or system.DC-coupling directly connects the components together without any coupling capacitors.Among the advantages of dc-coupling are:simple board design,no dc-wander issues,and i
t is uful in all coded-data streams including SONET and NRZ data applications.One disadvantage of dc-coupling is that it requires careful power supply design.Figure1,Table 1,and Table 2provide the I/O levels for the differential signals.
Figure 1.
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各种福字图片Input and Output Parameters
Table 1.
Typical LVPECL,LVDS,HSTL,and CML Outputs
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Output LVPECL LVDS HSTL CML V OH (Min) 2.275V 1.249VDDQ 1
-0.4
V CC
2
V OL (Max)
1.68V
1.252
0.4
V CC -0.4V
Table 2.
Typical LVPECL,LVDS,CML,and HSTL Input Levels
Input LVPECL LVDS HSTL CML V IH (Min)
2.135V 1.249VRef +0.2V CC VRef or VCM 2 1.20.75V CC -0.2V V IL (Max) 1.825V 1.252VRef -0.2V CC -0.4V V ID (Min)
310mV
200mV
400mV
400mV
1VDDQ =1.5V ±10%2
V CC =3.3V ±10%
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Table3.Interface Table
1.1LVPECL
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Figure2.LVPECL to LVPECL
PECL and LVPECL are normally terminated though50Ωto(V CC-2V).Most systems normally do not have dual power supplies of both3.3V and1.3V;therefore Figures2and Figure3show alternative methods to terminate LVPECL output signals.The pullup and pulldown combination terminates the50-Ωtransmission line and establish the LVPECL common-mode voltage of2V at the receiver.
DC-Coupling Between Differential LVPECL,LVDS,HSTL,and CM3
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4DC-Coupling Between Differential LVPECL,LVDS,HSTL,and CM
Figure 3.LVPECL to LVPECL
The Y-termination in Figure 3is another alternative to LVPECL termination where a VTT supply is not readily available.This scheme saves one resistor over the scheme in Figure
2.
Figure 4.LVPECL to LVDS
The 33-Ωresistor is usually required when the LVPECL output is too high for the LVDS receiver input stage.For the LVDS receivers listed above,it is not required and Figure 5is recommended.
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Figure5.LVPECL to LVDS
Most LVDS receivers are capable of accepting LVPECL signals and it is not necessary to attenuate the LVPECL signal prior to the LVDS receiver.This is due to the wide common-mode range of the LVDS receivers listed above.
Figure6.LVPECL to CML
The most widely ud method of translating from LVPECL to CML is through ac-coupling.AC-coupling is recommended for dc-balanced signals.AC-coupling generates ba-line wander in high-s
peed rial data transmission such as SONET and NRZ encoded data(non dc-balanced). Therefore,dc-coupling is recommended for such data streams.For detailed derivation,e the SCAA056.
DC-Coupling Between Differential LVPECL,LVDS,HSTL,and CM5
