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©2015 Integrated Device Technology, Inc.

Revision C, December 1, 2015

85352 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 VREF = VCC/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 VREF in the center of the input voltage

swing. For example, if the input clock swing is 2.5V and VCC = 3.3V,

R1 and R2 value should be adjusted to set VREF at 1.25V. The values

below are for when both the single ended swing and VCC 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 VIL cannot be less

than -0.3V and VIH cannot be more than Vcc + 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