LMH7220_14 Datasheet, PDF (25/34 Pages) Texas Instruments – LMH7220 High Speed Comparator with LVDS Output

LMH7220
www.ti.com
SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
top copper
signal line
PCB
FR4
xxxx_{bottom copper}
stripline
xxxx _{signal line}
Top Copper
xx_{PCB FR4}
xxxx^{bottom copper}
Microstrip
signal lines
Top Copper
PCB FR4
xxx
bottom copper
xxxx ^{differential microstrip}
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.
Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LMH7220
Submit Documentation Feedback
25