Example High Speed I/O Ac Coupled Mode - Texas Instruments TLK3134 Data Manual

TLK3134
SLLS838F – MAY 2007 – REVISED DECEMBER 2009
TDP
TDN
TRANSMITTER
Current Mode Logic (CML) drivers often require external components. The disadvantage of the external
component is a limited edge rate due to package and line parasitic. The CML driver on TLK3134 has
on-chip 50Ω termination resistors terminated to VDDT, providing optimum performance for increased
speed requirements. The transmitter output driver is highly configurable allowing output amplitude and
de-emphasis to be tuned to a channel's individual requirements. Software programmability allows for very
flexible output amplitude control. AC Coupled and Direct Coupled modes are supported. When AC
coupling is selected, the receiver input is internally biased 0.8 × VDDT which is the optimum voltage for
input sensitivity. As the input and output references are derived from VDDT, the tolerance of this supply
will dominate the accuracy of the internal reference.
When transmitting data across long lengths of PCB trace or cable, the high frequency content of the signal
is attenuated due to the skin effect of the media. This causes a "smearing" of the data eye when viewed
on an oscilloscope. The net result is reduced timing margins for the receiver and clock recovery circuits. In
order to provide equalization for the high frequency loss, 1-tap finite impulse response (FIR) transmit
de-emphasis is implemented. A highly configurable output driver maximizes flexibility in the end system by
allowing de-emphasis and output amplitude to be tuned to a channel's individual requirements. A total of
15 de-emphasis settings and 8 output amplitude settings can be independently selected.
V
CMT
Figure 2-31. Output differential voltage with 1-tap FIR de-emphasis
46 Detailed Description
50 W transmission line
50 transmission line
W
MEDIA
Figure 2-30. Example High Speed I/O AC Coupled Mode
V (p)
OD
V (d)
OD
bit bit
time time
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Product Folder Link(s):
RDP
VDDT 0.8*VDDT
DIR
50 COUP
50
GND
RDN
RECEIVER
V (d)
OD
V (p)
OD
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TLK3134
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AC
COUP
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V (pd)
V (pp)
OD OD