AD9549 datasheet(41/78 Pages) AD

41 / 78 page

Preliminary Technical Data

AD9549

Rev. PrA | Page 41 of 78

FDBK INPUTS

The FDBK pins serve as the input to the feedback path of the

digital PLL. Typically, these pins are used to receive the signal

generated by the DDS after it has been band-limited by the

external reconstruction filter.

A diagram of the FDBK input pins is provided here, which

includes some of the internal components used to bias the input

circuitry. Note that the FDBK input pins 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.

To S-Divider

and Clock

Output Section

VSS

~2pF

~1V

VSS

~1pF

FDBK_IN

FDBK_INB

Figure 26: Differential FDBK Inputs

REFERENCE INPUTS

Reference Clock Receiver

The Reference Clock receiver is the point at which the user

supplies the input clock signal that the Synchronizer will

synthesize into an output clock. The clock receiver circuit is

able to handle a relatively broad range of input levels as well as

frequencies from 8 kHz up to 750 MHz.

The following is a diagram of the REFA/B input pins, which

includes some of the internal components used to bias the input

circuitry. Note that the REF input pins are internally biased by

a DC source, Vb. Care should be taken to ensure that any

external connections do not disturb the DC bias, as this may

significantly degrade performance.

To Reference

Monitor and

Switching Logic

VSS

~1pF

REFA_IN

REFA_INB

(or REFB_IN)

(or REFB_INB)

GND

1pF

VDD

Figure 27: Reference Inputs

NOTE: Support for redundant reference clocks is achieved by

using the two Reference Clock receivers (REFA & REFB).

In order to accommodate a variety of input signal conditions

the value of Vb is programmable via a pair of bits in the I/O

pattern in Register 040F.

Reference Bias Level

R040F[1:0]

00 (default)

AVDD3-800mV

AVDD3-400mV

AVDD3-1600mV

AVDD3-1200mV

Table 3: Setting of Input Bias Voltage (Vb)

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