ICS8745BI-21 Datasheet, PDF (10/21 Pages) Integrated Device Technology – Output frequency range

ICS8745BI-21 Data Sheet
Applications Information
Power Supply Filtering Technique
As in any high speed analog circuitry, the power supply pins are
vulnerable to random noise. To achieve optimum jitter performance,
power supply isolation is required. The ICS8745BI-21 provides
separate power supplies to isolate any high switching noise from the
outputs to the internal PLL. V_DD, V_DDA and V_DDO should be
individually connected to the power supply plane through vias, and
0.01µF bypass capacitors should be used for each pin. Figure 1
illustrates this for a generic V_DD pin and also shows that V_DDA
requires that an additional 10Ω resistor along with a 10µF bypass
capacitor be connected to the V_DDA pin.
1:1 DIFFERENTIAL-TO-LVDS ZERO DELAY CLOCK GENERATOR
3.3V
VDD
.01µF _10Ω
V_DDA
.01µF
10µF
Figure 1. Power Supply Filtering
Wiring the Differential Input to Accept Single-Ended Levels
Figure 2 shows how a differential input can be wired to accept single
ended levels. The reference voltage V_REF = 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_REF 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_REF 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 50Ω applications, R3 and R4 can be 100Ω.
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 2. Recommended Schematic for Wiring a Differential Input to Accept Single-ended Levels
ICS8745BMI-21 REVISION D JULY 28, 2010
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©2010 Integrated Device Technology, Inc.