AD9546SLASHPCBZ datasheet(146/205 Pages) AD

146 / 205 page

AD9546

Data Sheet

Rev. 0 | Page 146 of 205

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 101 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

AD9546

3.9nF

Figure 101. APLL Loop Filter

To program R1, C2 and R3 use Register 0x1082 (for APLL0)

and Register 0x1482 (for APLL1). Bits[7:5] (unsigned) control

R1, Bits[4:2] (unsigned) control C2, and Bits[1:0] (unsigned)

control R3. Table 90 shows how the decimal value of the bit

fields that select R1, C2 and R3 relate to the physical loop filter

component value.

Table 90. APLL Loop Filter Component Values

Bit Field Value

(Decimal)

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 optimal performance.

Although altering the response of the APLL loop filter is not

necessary, 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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