14©2016 Integrated Device Technology, Inc Revision B November 16, 2016
831724 Datasheet
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
1
= 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 V
1
in the center of the input voltage
swing. For example, if the input clock swing is 2.5V and V
DD
= 3.3V,
R1 and R2 value should be adjusted to set V
1
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
Receiv er
+
-R4
100
R3
100
RS Zo = 50 Ohm
Ro
Driver
VCC
VCC
R2
1K
R1
1K
C1
0.1uF
Ro + Rs = Zo
V1
VC C VC C
