AD9957SLASHPCBZ datasheet(35/61 Pages) AD

35 / 61 page

Data Sheet

AD9957

Rev. F | Page 35 of 61

TERMINATION

REF_CLK

DIFFERENTIAL SOURCE,

DIFFERENTIAL INPUT.

SINGLE-ENDED SOURCE,

DIFFERENTIAL INPUT.

SINGLE-ENDED SOURCE,

SINGLE-ENDED INPUT.

0.1µF

PECL,

LVPECL,

LVDS

DRIVER

REF_CLK

50Ω

0.1µF

BALUN

(1:1)

REF_CLK

0.1µF

50Ω

Figure 49. Direct Connection Diagram

PHASE-LOCKED LOOP (PLL) MULTIPLIER

An internal phase-locked loop (PLL) provides users of the

AD9957 the option to use a reference clock frequency that is

significantly lower than the system clock frequency. The PLL

supports a wide range of programmable frequency multiplica-

tion factors (12× to 127×) as well as a programmable charge

pump current and external loop filter components (connected

via the PLL_LOOP_FILTER pin). These features add an extra

layer of flexibility to the PLL, allowing optimization of phase

noise performance and flexibility in frequency plan develop-

ment. The PLL is also equipped with a PLL_LOCK pin.

The PLL output frequency range (fSYSCLK) is constrained to the

range of 420 MHz ≤ fSYSCLK ≤ 1 GHz by the internal VCO. In

addition, the user must program the VCO to one of six operating

ranges such that fSYSCLK falls within the specified range. Figure 50

and Figure 51 summarize these VCO ranges.

Figure 50 shows the boundaries of the VCO frequency ranges

over the full range of temperature and supply voltage variation

for all devices from the available population. The implication is

that multiple devices chosen at random from the population and

operated under widely varying conditions may require different

values to be programmed into CFR3[26:24] to operate at the

same frequency. For example, Part A chosen randomly from the

population, operating in an ambient temperature of −10°C with

a system clock frequency of 900 MHz may require CFR3[26:24] to

be set to 100b. Whereas Part B chosen randomly from the

population, operating in an ambient temperature of 90°C with a

system clock frequency of 900 MHz may require CFR3[26:24]

to be set to 101b. If a frequency plan is chosen such that the

system clock frequency operates within one set of boundaries

(as shown in Figure 51), the required value in CFR3[26:24] is

consistent from part to part.

Figure 51 shows the boundaries of the VCO frequency ranges

over the full range of temperature and supply voltage variation

for an individual device selected from the population. Figure 51

shows that the VCO frequency ranges for a single device always

overlap when operated over the full range of conditions.

In conclusion, if a user wants to retain a single default value for

CFR3[26:24], a frequency that falls into one of the ranges found

in Figure 50 should be selected. Additionally, for any given

individual device, the VCO frequency ranges overlap, meaning

that any given device exhibits no gaps in its frequency coverage

across VCO ranges over the full range of conditions.

VCO0

VCO1

VCO2

VCO3

VCO4

VCO5

395

495

595

695

795

895

995

FLOW = 400

FHIGH = 460

FLOW = 455

FHIGH = 530

FLOW = 530

FHIGH = 615

FLOW = 760

FHIGH = 875

FLOW = 920

FHIGH = 1030

FLOW = 650

FHIGH = 790

(MHz)

Figure 50. VCO Ranges Including Atypical Wafer Process Skew

335

435

535

635

735

835

935

1035

1135

VCO0

VCO1

VCO2

VCO3

VCO4

VCO5

FLOW = 342

FHIGH = 522

FLOW = 402

FHIGH = 602

FLOW = 469

FHIGH = 709

FLOW = 574

FHIGH = 904

(MHz)

FLOW = 646

FHIGH = 966

FLOW = 810

FHIGH = 1180

Figure 51. Typical VCO Ranges

Table 7. VCO Range Bit Settings

VCO SEL Bits (CFR3[26:24])

VCO Range

000

VCO0

001

VCO1

010

VCO2

011

VCO3

100

VCO4

101

VCO5

110

PLL Bypassed

111

PLL Bypassed

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