LOW SKEW, 1-TO16, DIFFERENTIAL-TO-2.5V LVPECL FANOUT
BUFFER
7 REVISION G 06/26/15
8530 DATA SHEET
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 V
REF
= V
CC
/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 V
REF
in the center of the input voltage
swing. For example, if the input clock swing is 2.5V and V
CC
= 3.3V,
R1 and R2 value should be adjusted to set V
REF
at 1.25V. The values
below are for when both the single ended swing and V
CC
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
CC
+ 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
