AD9531 datasheet(87/88 Pages) AD

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Data Sheet

AD9531

DRIVING REF2 OR REF3 WITH 3.3 V CMOS LOGIC

For applications with a 3.3 V CMOS driver as the reference

source, a single-ended to differential conversion is necessary, as

shown in Figure 47.

AD9531

REF2_P

REF2_N

3.3V

CMOS

1.2R

REF3_P

REF3_N

3.3V

CMOS

1.2R

1.8V

Figure 47. Interfacing the REF2 or REF3 Inputs to 3.3 V CMOS

Choose the value of R and C in Figure 47 based on the input

frequency, fREF. Typically, choose R to limit the current load on

the CMOS driver as given by R = 1.5/ILOAD. Then, choose C

based on the input frequency such that C > 10/(R × fREF). The

remaining shunt resistor is 1.2× the value of R and provides the

necessary voltage scaling from 3.3 V to 1.8 V.

For example, ILOAD = 1 mA and fREF = 25 MHz results in

R = 1.5 kΩ (RSHUNT = 1.8 kΩ) and C > 270 pF.

USING REF2 OR REF3 WITH A CRYSTAL

RESONATOR

For applications using a crystal resonator as the reference input,

the XTAL amplifier bit in the appropriate programming register

must be set to Logic 1. Register 0x0200, Bit D4 applies to PLL2

and Register 0x0300, Bit D4 applies to PLL3.

Figure 48 shows a typical crystal connection for both the PLL2

and PLL3 reference inputs. The internal input circuitry converts

to an amplifier configuration to promote oscillation of the crystal

resonator. The value of the shunt capacitors (C) connected to the

crystal depends on the crystal manufacturer load capacitance

(CLOAD) specification for parallel resonance, as follows:

C = 2 × CLOAD

For example, a CLOAD specification of 10 pF means that the value

of each of the two capacitors connected to the crystal is 20 pF.

AD9531

REF2_P

REF2_N

REF3_P

REF3_N

Figure 48. Interfacing the REF2 or REF3 Inputs to a Crystal Resonator

Rev. 0 | Page 87 of 88

AD9531 Datasheet(PDF) 87 Page - Analog Devices

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