LMH7322_14 Datasheet, PDF (26/33 Pages) Texas Instruments – Dual 700 ps High Speed Comparator with RSPECL Outputs

LMH7322
SNOSAU8H – MARCH 2007 – REVISED MAY 2011
www.ti.com
These cables have a characteristic impedance determined by their geometric parameters. Widely used
impedances for the coaxial cable are 50Ω and 75Ω. Twisted pair cables have impedances of about 120Ω to
150Ω.
Other types of transmission lines are the strip line and the micro strip line. These last types are used on PCB
boards. They have the characteristic impedance dictated by the physical dimensions of a track placed over a
metal ground plane (see Figure 38).
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 38. PBC 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 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 given impedance and a board material. Other line widths will result in
different impedances.
Advantages of Differential MicrostripLines
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 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 the open lines must be to prevent signal degradation.
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