AD9518-2ABCPZ datasheet(33/64 Pages) AD

33 / 64 page

Data Sheet

AD9518-2

Rev. C | Page 33 of 64

PROGRAMMABLE

N DELAY

REFIN (REF1)

REFIN (REF2)

CLK

REF1

REF2

STATUS

DIVIDER

VCO STATUS

PROGRAMMABLE

R DELAY

REFERENCE

SWITCHOVER

REF_SEL

CPRSET VCP

GND

RSET

DISTRIBUTION

REFERENCE

REFMON

STATUS

P, P + 1

PRESCALER

A/B

COUNTERS

N DIVIDER

BYPASS

LOW DROPOUT

REGULATOR (LDO)

VCO

PHASE

FREQUENCY

DETECTOR

LOCK

DETECT

CHARGE

PUMP

HOLD

DIVIDE BY

2, 3, 4, 5, OR 6

Figure 39. Reference and VCO Status Monitors

VCO Calibration

The AD9518 on-chip VCO must be calibrated to ensure proper

operation over process and temperature. VCO calibration centers

the dc voltage at the internal VCO input (at the LF pin) for the

selected configuration; this is normally required only during

initial configuration and any time the PLL settings change. VCO

calibration is controlled by a calibration controller driven by the

R divider output. The calibration requires that the input reference

clock be present at the REFIN pins, and that the PLL be set up

properly to lock the PLL loop. During the first initialization after

a power-up or a reset of the AD9518, a VCO calibration sequence

is initiated by setting Register 0x018[0] = 1b. This can be done

during initial setup, before executing an update registers operation

(Register 0x232[0] = 1b). Subsequent to initial setup, a VCO

calibration sequence is initiated by resetting Register 0x018[0] = 0b,

executing an update registers operation, setting Register 0x018[0] =

1b, and executing another update registers operation. A readback

bit (Bit 6 in Register 0x1F) indicates when a VCO calibration is

finished by returning a logic true (that is, 1b).

The sequence of operations for the VCO calibration is as follows:

Program the PLL registers to the proper values for the PLL

loop. Note that that automatic holdover mode must be

disabled, and the VCO divider must not be set to “Static.”

Ensure that the input reference signal is present.

For the initial setting of the registers after a power-up or reset,

initiate VCO calibration by setting Register 0x018[0] = 1b.

Subsequently, whenever a calibration is desired, set

A sync operation is initiated internally, causing the outputs

to go to a static state determined by normal sync function

operation.

The VCO calibrates to the desired setting for the requested

VCO frequency.

Internally, the SYNC signal is released, allowing outputs

to continue clocking.

The PLL loop is closed.

The PLL locks.

A sync is executed during the VCO calibration; therefore, the

outputs of the AD9518 are held static during the calibration,

which prevents unwanted frequencies from being produced.

However, at the end of a VCO calibration, the outputs may

resume clocking before the PLL loop is completely settled.

The VCO calibration clock divider is set as shown in Table 44

(Register 0x018[2:1]).

The calibration divider divides the PFD frequency (reference

frequency divided by R) down to the calibration clock. The

calibration occurs at the PFD frequency divided by the

calibration divider setting. Lower VCO calibration clock

frequencies result in longer times for a calibration to be

completed.

The VCO calibration clock frequency is given by

fCAL_CLOCK = fREFIN/(R × cal_div)

where:

fREFIN is the frequency of the REFIN signal.

R is the value of the R divider.

cal_div is the division set for the VCO calibration divider

(Register 0x018[2:1]).

The VCO calibration takes 4400 calibration clock cycles.

Therefore, the VCO calibration time in PLL reference clock

cycles is given by

Time to Calibrate VCO =

4400 × R × cal_div PLL Reference Clock Cycles

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