ADMV4530 datasheet(33/64 Pages) AD

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Data Sheet

ADMV4530

THEORY OF OPERATION

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Rev. C | 33 of 64

CHARGE PUMP CURRENT SETUP

For a specifically designed loop filter, set the ICP by adjusting the

CP_CURRENT value in Bits[7:4], Register 0x01E. Calculate ICP by

using the following the equation:

ICP = (CP_CURRENT + 1) × 350 µA

(11)

where CP_CURRENT is an integer value (0 to 15).

The default value for a 100 MHz fPFD for CP_CURRENT = 11,

which yields a current of 4.2 mA. The applicable range is 0.35 mA

to 5.6 mA, with 0.35 mA steps.

To change the fPFD, if no change has been made to the existing

loop filter components, it is recommended to scale ICP by using the

following equation:

ICP(NEW)= ICP(DEFAULT)×fPFD(DEFAULT)

fPFD(NEW)

(12)

where:

ICP (NEW) is the new desired ICP.

ICP (DEFAULT) is the default ICP.

fPFD (DEFAULT) is the default fPFD.

fPFD (NEW) is the new desired fPFD.

When ICP (NEW) is obtained, the CP_CURRENT value in Bits[7:4],

function:

CP_CURRENT=ROUND ICPNEW350μA −1 (13)

where ROUND is the mathematical round function.

BLEED CURRENT (CP_BLEED) SETUP

The charge pump includes a binary scaled bleed current (IBLEED)

that is set by using the CP_BLEED value in Register 0x026.

The bleed current introduces a slight phase offset in the phase

frequency detector to improve integer boundary spurs and phase

noise when operating in fractional-N mode. To enable the bleed

current for fractional-N mode, set CP_BLEED_EN = 1 (Register

0x027, Bit 3). For integer mode, CP_BLEED_EN must be set to 0.

Generally, the optimum bleed current value is 115 (431.25 µA),

and this value provides optimal performance for most applications.

However, there can be additional performance improvements by

empirically determining the appropriate bleed current value from the

actual measurements for the intended application. The applicable

range is 0 µA to 956.25 µA with 3.75 µA steps.

IBLEED = CP_BLEED × 3.75 µA

(14)

where CP_BLEED is an integer value (0 to 255).

Figure 93 shows an example of 100 kHz offset phase noise vs.

CP_BLEED.

Figure 93. 100 kHz Offset Phase Noise vs. CP_BLEED, LO Frequency =

29.995 GHz, CP_CURRENT = 4.2 mA

Figure 94 is an example of 100 kHz offset phase noise vs. LO fre-

quency with a CP_BLEED value of 115 (431.25 µA). The CP_CUR-

RENT is 4.2 mA and the LO frequency step size is 10 MHz. At

each integer boundary, the CP_BLEED is disabled, resulting in

approximately −1 dBc/Hz improvement.

Figure 94. 100 kHz Offset Phase Noise vs. LO Frequency, CP_BLEED = 115,

CP_CURRENT = 4.2 mA

ADMV4530 Datasheet(PDF) 33 Page - Analog Devices

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