IDT8SLVD1208I Data Sheet 1:8, LVDS OUTPUT FANOUT BUFFER
IDT8SLVD1208NBGI REVISION A MAY 1, 2013 12 ©2013 Integrated Device Technology, Inc.
Applications Information
Recommendations for Unused Input and Output Pins
Inputs:
PCLK/nPCLK Inputs
For applications not requiring the use of a differential input, both the
PCLK and nPCLK pins can be left floating. Though not required, but
for additional protection, a 1k resistor can be tied from PCLK to
ground.
Outputs:
LVDS Outputs
All unused LVDS output pairs can be either left floating or terminated
with 100 across. If they are left floating, there should be no trace
attached.
Wiring the Differential Input to Accept Single-Ended Levels
Figure 1 shows how a differential input can be wired to accept single
ended levels. The reference voltage V1
= V
DD
/2 is generated by the
bias resistors R1 and R2. The bypass capacitor (C1) is used to help
filter noise on the DC bias. This bias circuit should be located as close
to the input pin as possible. The ratio of R1 and R2 might need to be
adjusted to position the V1
in the center of the input voltage swing.
For example, if the input clock swing is 2.5V and V
DD
= 2.5V, R1 and
R2 value should be adjusted to set V1
at 1.25V. The values below are
for when both the single ended swing and V
DD
are at the same
voltage. This configuration requires that the sum of the output
impedance of the driver (Ro) and the series resistance (Rs) equals
the transmission line impedance. In addition, matched termination at
the input will attenuate the signal in half. This can be done in one of
two ways. First, R3 and R4 in parallel should equal the transmission
line impedance. For most 50 applications, R3 and R4 can be 100.
The values of the resistors can be increased to reduce the loading for
slower and weaker LVCMOS driver. When using single-ended
signaling, the noise rejection benefits of differential signaling are
reduced. Even though the differential input can handle full rail
LVCMOS signaling, it is recommended that the amplitude be
reduced. The datasheet specifies a lower differential amplitude,
however this only applies to differential signals. For single-ended
applications, the swing can be larger, however V
IL
cannot be less
than -0.3V and V
IH
cannot be more than V
DD
+ 0.3V. Though some
of the recommended components might not be used, the pads
should be placed in the layout. They can be utilized for debugging
purposes. The datasheet specifications are characterized and
guaranteed by using a differential signal.
Figure 1. Recommended Schematic for Wiring a Differential Input to Accept Single-ended Levels
