AD9545 datasheet(102/157 Pages) AD

102 / 157 page

AD9545

Data Sheet

Rev. A | Page 102 of 157

In general, the default values of the APLLx dc offset current

value and APLLx manual charge pump current bit fields yield

the best overall performance. For this reason, the recommendation

is to use only the manual charge pump mode (that is, enable

APLLx manual charge pump current = 1).

APLL LOOP FILTER

The phase errors detected by the PFD drive the charge pump.

The charge pump transfers charge to the loop filter in proportion

to the amount of phase error. The loop filter stores the charge,

which creates a dc voltage that steers the VCO frequency in a

manner that nulls out the phase error.

The loop filter is a resistor and capacitor filter with a low-pass

response characteristic that serves three purposes:

•

Storage of charge originating from the charge pump

•

Attenuation of the high frequency components of the

phase error signal

•

Stabilization of the feedback loop

Figure 82 shows the loop filter schematic. Three of the

components (R1, R3, and C2) are programmable, allowing the

user to tailor the response of the loop filter. The two external

capacitors (3.9 nF and 220 nF) are connected to the LFx and

LDOx pins. These two components are necessary for proper

operation of the loop filter and the internal LDO, respectively.

FROM

CHARGE

PUMP

VCO

LFx

LDOx

220nF

30pF

AD9545

3.9nF

Figure 82. APLL Loop Filter

To program R1, use the 3-bit unsigned APLLx loop filter zero

resistor (R1) bit field (where x is 0 or 1) in Bits[D7:D5] of

2-bit unsigned APLLx loop filter second pole resistor (R3) bit

field (where x is 0 or 1) in Bits[D1:D0] of Register 0x1082 and

loop filter pole capacitor (C2) bit field (where x is 0 or 1) in

Bits[D4:D2] of Register 0x1082 and Register 0x1482. Table 49

shows the relationship between the bit field values and the

values of the loop filter components.

Table 49. APLL Loop Filter Component Values

Bit Field Value

R1 (Ω)

C2 (pF)

R3 (Ω)

8 (default)

200 (default)

250

500

333

750

500

1000

n/a

1250

n/a

1500

104

n/a

1750 (default)

120

n/a

In general, the default filter settings yield the best performance.

While it is generally not necessary to alter the response of the

APLL loop filter, the user can contact Analog Devices for

assistance with determining alternative component values for

the APLL loop filter to tailor the response to specific requirements.

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