LMH7324_14 Datasheet, PDF (23/30 Pages) Texas Instruments – Quad 700 ps High Speed Comparator with RSPECL Outputs

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LMH7324
SNOSAZ2F – SEPTEMBER 2007 – REVISED JULY 2010
top copper
signal line
PCB FR4
bottom copper
stripline
signal line
Top Copper
PCB FR4
bottom copper
Microstrip
signal lines
Top Copper
PCB FR4
bottom copper
differential microstrip
Figure 33. PCB Lines
Differential Microstrip Line
The transmission line which is ideally suited for complementary signals is the differential microstrip line. This is a
double microstrip line with a narrow space in between. This means both lines have strong coupling and this
determines the characteristic impedance. The fact that they are routed above a copper plane does not affect
differential impedance, only CM-capacitance is added. Each of the structures above has its own geometric
parameters, so for each structure there is a different formula to calculate the right impedance. For calculations on
these transmission lines visit the TI website or order RAPIDESIGNER. At the end of the transmission line there
must be a termination having the same impedance as that of the transmission line itself. It does not 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
microstrip line, line width is fixed for a given impedance and for a specific board material. Other line widths will
result in different impedances.
Advantages of Differential Microstrip Lines
Impedances of transmission lines are always dictated by their geometric parameters. This is also true for
differential microstrip lines. Using this type of transmission line, the distance of the track determines the resulting
impedance. So, if the PCB manufacturer can produce reliable boards with low track spacing the track width for a
given impedance is also small. The wider the spacing, the wider tracks are needed for a specific 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 to create optimal connections to the receiving part or the
terminating resistor, in accordance to 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 the
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 the open lines must be to prevent signal degradation.
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