AD9528BCPZ datasheet(27/68 Pages) AD

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AD9528

Data Sheet

Rev. C | Page 26 of 67

PLL1 has the flexibility to operate with a narrow loop bandwidth.

This relatively narrow loop bandwidth gives the AD9528 the

ability to suppress jitter that appears on the input references

(REFA and REFB). The low phase noise output of PLL1 acts as

the reference to PLL2 and can be routed to the clock

distribution section.

PLL1 Reference Clock Inputs

The AD9528 features two separate reference clock inputs, REFA

and REFB. These inputs can be configured to accept differential

or single-ended signals. REFA and REFB are self biased in

differential mode and high impedance in single ended CMOS

mode. If REFA or REFB is driven single-ended, decouple the

unused side (REFA, REFB) via a suitable capacitor to a quiet

ground. These inputs may be dc-coupled, but set the dc

operation point as specified in the Specifications section.

The differential reference input receiver is powered down when

the differential reference input is not selected, or when the PLL1

is powered down. The single-ended buffers power down when

the PLL1 is powered down, when their respective individual

power-down registers are set, or when the differential receiver is

selected.

PLL1 Loop Filter

The PLL1 loop filter is mostly external from LF1 (Pin 7) to

ground. The value of the external components depend on the

external VCXO and the configuration parameters, such as input

clock rate and desired PLL1 loop bandwidth.

Figure 28. PLL1 Loop Filter

An external RC low-pass filter is recommended at the VCXO_VT

output for the best noise performance at 1 kHz offset. The pole

of this filter must be sufficiently high enough in frequency to

avoid stability problems with the PLL loop bandwidth.

Figure 29. Input PLL (PLL1) Block Diagram

CEXT_POLE

REXT_ZERO

LF1EXT_CAP

RPOLE2

165kΩ

CPOLE2

72pF

CHARGE

PUMP

EXTERNAL

VCXO

LF1

BUFFER

1kΩ

0.47µF

VCXO_VT

OPTIONAL

FILTER

AD9528

SWITCH-

OVER

CONTROL

CHARGE

PUMP

LOCK

DETECT

REFA

LF1

VCXO_VT

VCXO_IN

REFA

REF_SEL

VCXO

REFB

AD9528

10-BIT

DIVIDER

10-BIT

DIVIDER

PLL 1

10-BIT

DIVIDER

LOOP

FILTER

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