Rev C 1/5/15

6

2:1, DIFFERENTIAL-TO-3.3V DUAL LVPECL/ECL CLOCK

MULTIPLEXER

85356 DATA SHEET

Application Information

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

Recommendations for Unused Input and Output Pins

Inputs:

CLK/nCLK Inputs

For applications not requiring the use of the differential input, both

CLK and nCLK can be left floating. Though not required, but for

additional protection, a 1k

 resistor can be tied from CLK to ground.

Control Pins

All control pins have internal pull-ups or pull-downs; additional

resistance is not required but can be added for additional protection.

A 1k

 resistor can be used.

Inputs:

LVPECL Outputs

All unused LVPECL outputs can be left floating. We recommend that

there is no trace attached. Both sides of the differential output pair

should either be left floating or terminated.