Selasa, 02 Agustus 2011

Low-voltage differential signaling

http://www.national.com/assets/en/appnotes/National_LVDS_Owners_Manual_4th_Edition_2008.pdf"
Differential vs. single-ended signaling

LVDS is a differential signaling system, meaning that it transmits information as the difference between the voltages on a pair of wires; the two wire voltages are compared at the receiver. In a typical implementation, the transmitter injects a small current, typically 3.5 mA, into one wire or the other, depending on the logic level to be sent. The current passes through a resistor of about 100 to 120 ohms (matched to the characteristic impedance of the cable) at the receiving end, then returns in the opposite direction along the other wire. From Ohm's law, the voltage difference across the resistor is therefore about 350 mV. The receiver senses the polarity of this voltage to determine the logic level. This type of signalling is called a current loop.

The small amplitude of the signal and the tight electric- and magnetic-field coupling between the two wires reduces the amount of radiated electromagnetic noise (and power lost to conductor resistance).

The low common-mode voltage (the average of the voltages on the two wires) of about 1.25 V allows LVDS to be used with a wide range of integrated circuits with power supply voltages down to 2.5 V or lower. The low differential voltage, about 350 mV as stated above, causes LVDS to consume very little power compared to other systems. For example, the static power dissipation in the LVDS load resistor is 1.2 mW, compared to the 90 mW dissipated by the load resistor for an RS-422 signal. Without a load resistor the whole wire has to be loaded and unloaded for every bit of data. Using high frequencies and a load resistor so that a single bit only covers a part of the wire (while traveling near light speed) is more power efficient.

Logic Levels:[1]
Vee VOL VOH Vcc VCMO
GND 1.0 V 1.4 V 2.5–3.3 V 1.2 V

LVDS is not the only differential signaling system in use, but is currently the only scheme that combines low power dissipation with high speed.
Applications

LVDS became popular in the latter half of the 1990s. Before that, computers were too slow to make use of such fast data rates, and the cost of twice as many wires for the same amount of data outweighed the speed benefits. Yet multimedia and supercomputer users, both of which needed to move large amounts of data over links several meters long (from a disk drive to a workstation, for instance) maintained a widespread interest in LVDS.

The first widespread application for LVDS was to transport video data from graphics adapters to computer monitors, particularly flat panels in notebook computers, using the Flat Panel Display Link (FPD-Link) by National Semiconductor (NSC). The LCD display module vendors commonly use the term LVDS instead of FPD-Link when referring to the input signal to their modules. The higher performance follow-ons were LVDS Display Interface (LDI) and OpenLDI standards. These standards allow a maximum pixel clock of 112 MHz, which suffices for a display resolution of 1400 x 1050 (SXGA+) at 60 Hz refresh. A dual link can boost the maximum display resolution to 2048 x 1536 (QXGA) at 60 Hz. FPD-Link works with cable lengths up to about 5 m, and LDI extends this to about 10 m. Mobile devices have begun to use Display Serial Interface which utilizes LVDS signaling.

Two examples of LVDS use in computer buses come from HyperTransport and FireWire, both of which trace their development back to the post-Futurebus work which also led to SCI. LVDS is supported in SCSI standards (Ultra-2 SCSI and later) to allow higher data rates and longer cable lengths. Serial ATA, PCI Express, RapidIO, and SpaceWire utilize LVDS to allow high speed data transfer.

Intel and AMD expect that the LVDS LCD-panel interface would no longer be supported in their product lines by 2013, with Embedded DisplayPort and Internal DisplayPort as preferred solutions.[2]
Comparison with parallel transmission

LVDS is often used for serial data transmission, which involves sending data bit-by-bit down a single pair of wires. This is in contrast to parallel transmission, in which several wires are used with a common ground to carry several signals at once. The high speed of LVDS, and its use of in-channel synchronisation, allows more data to be sent using fewer wires than can be accomplished with a parallel bus. The device for converting between serial and parallel data is called a serializer/deserializer, abbreviated to SerDes.
Comparison with 8b/10b transmission

LVDS does not specify a bit encoding scheme (i.e. start bit and stop bit). LVDS is the physical mechanism to transport bits across wires. Any user-specified encoding scheme can be sent/received across an LVDS link, including 8b/10b data. An 8b/10b encoding scheme is typically utilized when DC balance is desired, a necessity for AC-coupled transmission paths (such as capacitive or transformer-coupled paths).
Parallel LVDS

When serial data transmission is not fast enough, data can be transmitted in parallel form using an LVDS pair for each bit or even byte (as in PCI Express or HyperTransport). This system is called parallel LVDS.
Multipoint LVDS

Bus LVDS, or BLVDS (by NSC) was the first variation of LVDS designed to drive multiple LVDS receivers, and uses 100-ohm terminations at each end of the differential transmission line to maintain the signal integrity. Standard LVDS transmitters are designed for point-to-point links, but multipoint bus systems can be made using modified LVDS transmitters with high-current outputs that can drive multiple termination resistors. Bus LVDS and LVDM (by TI) are de facto multipoint LVDS standards. Multipoint LVDS (MLVDS) is the TIA standard (TIA-899) that has evolved and is used in AdvancedTCA for some clock distribution.

MLVDS has two types of receivers. Type-1 are nearly compatible with LVDS and use a 0 Volt threshold. Type-2 use a 100 mV threshold to handle in a consistent way various errors such as open and short circuits. For MLVDS:
Output Input
Common mode Amplitude
Min 0.3 V 0.480 V −1.4 V
Max 2.1 V 0.650 V 3.8 V
SCI-LVDS

The present form of LVDS was preceded by an earlier attempt, SCI-LVDS, which was a subset of the Scalable Coherent Interconnect (SCI) specified in the IEEE 1596.3 1995 standard. It was designed for interconnecting multiprocessing systems.
Standards

The ANSI/TIA/EIA-644-A (published in 2001) standard defines LVDS. This recommends a maximum data rate of 655 Mbit/s over twisted-pair copper wire, but predicts a possible speed of over 1.9 Gbit/s for an ideal transmission medium.[3]

External links

LVDS Owner's Manual from National Semiconductor
M-LVDS Application Reports


Adie Dkhaz
LVDS adalah sistem pensinyalan diferensial, yang mengirimkan informasi sebagai perbedaan antara tegangan pada sepasang konduktor; Dalam implementasi nyata, pemancar menyuntikkan arus yang kecil, biasanya 3,5 mA, , tergantung pada tingkat logika yang akan dikirim. Melewati arus melalui sebuah resistor sekitar 100 sampai 120 ohm (disesuaikan dengan impedansi karakteristik konduktor/jalur) pada ujung penerima, kemudian kembali ke arah yang berlawanan sepanjang konduktor lainnya. Dari hukum Ohm, perbedaan tegangan di resistor karena itu sekitar 350 mV. Signal penerima polaritas tegangan ini untuk menentukan tingkat logika. Jenis sinyal disebut arus loop.

Amplitudo sinyal yang kecil dan loop listrik dan medan magnetik yang ketat antara dua konduktor mengurangi jumlah radiasi elektromagnetik (dan kehilangan daya untuk resistansi konduktor).

Tegangan yang umum-mode rendah (rata-rata tegangan pada dua konduktor) dari sekitar 1,25 V LVDS memungkinkan untuk digunakan berbagai sirkuit terpadu dengan catu daya tegangan turun ke 2,5 V atau lebih rendah. Tegangan diferensial rendah, sekitar 350 mV seperti disebutkan di atas, menyebabkan LVDS mengkonsumsi daya yang sangat sedikit dibandingkan dengan sistem lain. Sebagai contoh, disipasi daya statis di resistor beban LVDS adalah 1,2 mW, dibandingkan dengan 90 mW disebarkan oleh resistor beban untuk sinyal RS-422.

Tanpa beban resistor konduktor secara keseluruhan harus dimuat dan dibongkar untuk setiap bit data. Menggunakan frekuensi tinggi dalam sebuah resistor beban, sehingga satu bit hanya mencakup sebagian dari konduktor (saat bepergian mendekati kecepatan cahaya) yang lebih mengefisienkan daya.

Masalah yang sering terjadi pada LDVS

Beberapa masalah umum yang dihadapi dalam desain PCB adalah
Elektromagnetik Interference (EMI)
EMI adalah sebuah proses di mana energi elektromagnetik yang dihasilkan oleh satu sirkuit

mengganggu yang lain. perambatan
mode untuk EMI sebagian besar melalui radiasi, namun dapat terjadi juga melalui
konduksi. LVDS sirkuitmenghasilkan EMI rendah karena ayunan tegangan rendah digunakan untuk mengirimkan sinyal dan modus diferensial operasi mereka.
EMI dapat menjadi baik dengan transmisi tunggal-ended atau diferensial
IOS (TTL / CMOS) yang digunakan. Sebagian besar satu-end sinyal memiliki waktu transisi cepat dan tegangan tinggi , membuat lebih memungkinkan untuk memancarkan radiasi dan mengganggu sirkuit komponen sekitarnya.
EMI dapat terjadi pada intra-atau antar-sistem. Intra-sistem dapat menyebabkan gangguan peralatan yg membuatnya tidak berfungsi.. Antar-sistem interferensi dapat menciptakan masalah dengan instalasi / atau elektromagnetik
kompatibilitas (EMC) . Pengendalian interkoneksi antara driverr dan penerima
dapat menekan EMI pada sumbernya.
Mengendalikan EMI pada sumbernya melalui penggunaan hasil LVDS sinyal dalam
signifikan perbaikan dalam kinerja sistem secara keseluruhan

crosstalk
Salah satu contoh gangguan intrasystem adalah crosstalk yang terjadi antara konduktor yang signifikansaling induktansi dan / atau kapasitansi. Sirkuit LVDS yang terkena crosstalk pada input dan output sebagai
serta internal pada IC. Fast switching TTL / CMOS sinyal yang digunakan untuk input output driver atau penerima yang
sumber terburuk crosstalk dan harus diisolasi dari sirkuit sensitif. Isolasi dicapai
dengan mengurangi loop antara sirkuit dan jalur sinyal menjaga sesingkat mungkin. loop antara sirkuit,didefinisikan dalam hal induktansi bersama dan kapasitansi, berbanding terbalik dengan jarak antaradua sinyal jalur. Jadi, meminimalkan crosstalk biasanya melibatkan meningkatkan pemisahan
antara duakonduktor untuk mengurangi loop.
Bouncing ground crosstalk yang terjadi ketika ada pergeseran pada ground tegangan referensi internal karena Output switching. Masalah ini dapat mengakibatkan bagian yang salah beralih pada ground-direferensikan (single-end)
gerbang. Pada dasarnya, transien pada ground mengganggu tunggal end masukan yang menggunakan potensi ground sebagai referensi. Selama transisi tinggi ke output rendah, jumlah beban arus keluar dan semua harus beralih
melalui gerbang internal .perangkat mengalir melalui kabel ground. Laju perubahan arus ini (di / dt) mengembangkan tegangan induktansi yang memimpin ground dan menyebabkan overshoot positif atau bouncing ground di dalam ground . bouncing ground positif biasanya diikuti oleh terlalu rendah nya
bentuk gelombang tegangan pada terminal output. Output rendah ke tinggi akan menjadi transisi yang dapat menghasilkan bouncing ground

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