AD9549 datasheet(43/78 Pages) AD

43 / 78 page

Preliminary Technical Data

AD9549

Rev. PrA | Page 43 of 78

programmed via the I/O Register Map via a 5-bit word that

spans a range of 0 to 31, but the internal logic automatically

adds a bias of 2 to the value entered, extending the range to 33.

Care should be taken when choosing these values so as to not

exceed the maximum input frequency of the Sysclk PLL phase

detector or bipolar edge detector. These values can be found in

the AC Electrical Characteristics section of this datasheet.

External Loop Filter (SysClk PLL)

The loop bandwidth of the SysClk Multiplier PLL can be

adjusted by means of three external components as shown in

the diagram below. The nominal gain of the VCO is

800MHz/V. The recommended component values are shown in

Table 4 below. They establish a loop bandwidth of

approximately 1.6MHz with the charge pump current set to

250µa. The default case is N=40, and assumes a 25MHz SysClk

input frequency and generates an internal DAC sampling

frequency (fS) of 1GHz.

Figure 30: External Loop Filter for SysClk PLL

Sysclk

Multiplier

Series

Shunt

< 8

390Ω

1 nF

82 pF

470Ω

820pF

56 pF

1kΩ

390pF

27 pF

40 (default)

2.2kΩ

180 pF

10 pF

2.7kΩ

120 pF

5 pF

Table 4: Recommended Loop Filter Values for a nominal 1.5MHz

Sysclk PLL loop Bandwidth

Detail of SysClk Differential Inputs

A diagram of the SysClk input pins is provided here. Included

are details of the internal components used to bias the input

circuitry. These components have a direct effect on the static

levels at the SysClk input pins. This information is intended to

aid in determining how best to interface to the device for a

given application.

Note that the SysClk PLL Bypassed and SysClk PLL Enabled

input paths are internally biased to a DC level of ~1V. Care

should be taken to ensure that any external connections do not

disturb the DC bias, as this may significantly degrade

performance. Generally, it is recommended that the SysClk

inputs be AC coupled to the signal source (except when using a

crystal resonator).

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