Balanced lines

1
Balanced lines

• tex_8000b, AN1406, lvds_ch3

2

• Characteristic impedance impedance between two
  conductors when there is no coupling to ground

3

• Differential impedance impedance between two
  conductors
• - when there is no coupling to ground, it is
  equal to the characteristic imp.
• Common -mode impedance impedance between the two
  conductors (connected to each other) and ground

4

• Odd-mode impedance impedance of one conductor to
  ground when the pair is driven differentially
• Even-mode impedance impedance of one conductor
  to ground when the pair is driven with equal
  polarity signals

5

• ZoddZdiff/2
• Zeven2Zcommon_mode
• if no coupling with ground ZoddZchar
• Measures from Zodd and Zeven (measured) we can
  calculate
• Zdiff2ZoddZodd1Zodd2
• Zcommon_modeZeven/2Zeven1 // Zeven2

6
Edge-coupled microstrip edge-coupled stripline
7
broad-side striplines

• Field solver needed to compute Z

8
Termination (e.g. for LVDS)

• Center tap capacitance termination may also be
  used in conjunction with two 50 ohm resistors to
  filter common-mode noise at the expense of extra
  components

9
The S Rule

• Using the edge-to-edge S distance between the
  traces of a pair, other separations can be
  defined
• The distance between two pairs should be gt2S.
• The distance between a pair and a TTL/CMOS
  signal should be gt3S. Even better,
• - locate the TTL/CMOS signals on a different
  plane isolated by a ground plane.
• If a guard ground trace or ground fill is used,
  it should be gt2S away.

10
AC-coupling

• Why would you want to AC-couple a signal?
• To change the DC bias voltage when interfacing
  logic families with different input thresholds
  (such as optical systems where LVDS and PECL are
  used), or
• To protect drivers from possibly getting shorted
  when used in a removable interface (such as on
  cards used in network switches and routers), or
• To prevent DC currents flowing between systems
  with different ground potentials
• AC-coupling works best with encoded data that
  provides an equal number of ones and zeros (a
  DC-balanced signal).