Datasheet

FR4
top copper
bottom copper
signal line
PCB
stripline
FR4
Top Copper
signal line
bottom copper
PCB
Microstrip
FR4
Top Copper
signal lines
bottom copper
PCB
differential microstrip
LMH7220
www.ti.com SNOSAL3E –SEPTEMBER 2006–REVISED MAY 2013
Figure 35. PCB Transmission Lines
Differential Microstrip Line
The transmission line which is ideally suited for LVDS signals is the differential micro strip line. This is a double
micro strip line with a narrow space in between. This means both lines have a strong coupling and this
determines mainly the characteristic impedance. The fact that they are routed above a copper plane doesn't
affect differential impedance, only CM-capacitance is added. Each of the structures above has its own geometric
parameters so for each structure there is another formula to calculate the right impedance. For calculations of
these transmission lines visit the Texas Instruments website or feel free to order the RAPIDESIGNER. For some
formula’s given in the ‘LVDS owners manual’ see chapter 3 (see INTRODUCTION for the URL). At the end of the
transmission line there must be a termination having the same impedance as of the transmission line itself. It
doesn’t matter what impedance the line has, if the load has the same value no reflections will occur. When
designing a PCB board with transmission lines on it, space becomes an important item especially on high density
boards. With a single micro strip line, line width is fixed for given impedance and a board material. Other line
widths will result in different impedances.
Advantage of Differential Microstrip
Impedances of transmission lines are always dictated by their geometric parameters. This is also true for
differential micro strip lines. Using this type of transmission lines, track distance determines mainly the resulting
impedance. So, if the PCB manufacturer can produce reliable boards with narrow track spacing the track width
for a given impedance is also small. The wider the spacing, the wider tracks are needed for a certain impedance.
For example two tracks of 0.2 mm width and 0.1 mm spacing have the same impedance as two tracks of 0.8 mm
width and 0.4 mm spacing. With high-end PCB processes, it is possible to design very narrow differential
microstrip transmission lines. It is desirable to use these phenomena to create optimal connections to the
receiving part or the terminating resistor, in accordance with their physical dimensions. Seen from the
comparator, the termination resistor must be connected at the far end of the line. Open connections after the
termination resistor (e.g. to an input of a receiver) must be as short as possible. The allowed length of such
connections varies with the received transients. The faster the transients the shorter open lines must be to
prevent signal degradation.
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