AD6649EBZ datasheet(19/40 Pages) AD

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AD6649

Rev. 0 | Page 19 of 40

VOLTAGE REFERENCE

A stable and accurate voltage reference is built into the AD6649.

The full-scale input range can be adjusted by varying the reference

voltage via SPI. The input span of the ADC tracks reference voltage

changes linearly.

CLOCK INPUT CONSIDERATIONS

For optimum performance, the AD6649 sample clock inputs,

CLK+ and CLK−, should be clocked with a differential signal.

The signal is typically ac-coupled into the CLK+ and CLK− pins

via a transformer or via capacitors. These pins are biased

internally (see Figure 31) and require no external bias. If the

inputs are floated, the CLK− pin is pulled low to prevent spurious

clocking.

AVDD

CLK+

4pF

CLK–

0.9V

Figure 31. Simplified Equivalent Clock Input Circuit

Clock Input Options

The AD6649 has a very flexible clock input structure. Clock

input can be a CMOS, LVDS, LVPECL, or sine wave signal.

Regardless of the type of signal being used, clock source jitter

is of the most concern, as described in the Jitter Considerations

section.

Figure 32 and Figure 33 show two preferable methods for clocking

the AD6649 (at clock rates of up to 625 MHz). A low jitter clock

source is converted from a single-ended signal to a differential

signal using an RF balun or RF transformer.

The RF balun configuration is recommended for clock frequencies

between 125 MHz and 625 MHz, and the RF transformer is recom-

mended for clock frequencies from 10 MHz to 200 MHz. The

back-to-back Schottky diodes across the transformer secondary

limit clock excursions into the AD6649 to approximately 0.8 V p-p

differential. This limit helps prevent the large voltage swings of

the clock from feeding through to other portions of the AD6649

while preserving the fast rise and fall times of the signal, which

are critical to low jitter performance.

390pF

SCHOTTKY

DIODES:

HSMS2822

CLOCK

INPUT

50Ω

100Ω

CLK–

CLK+

ADC

Mini-Circuits®

ADT1-1WT, 1:1Z

XFMR

Figure 32. Transformer-Coupled Differential Clock (Up to 200 MHz)

390pF

CLOCK

INPUT

25Ω

CLK–

CLK+

SCHOTTKY

DIODES:

HSMS2822

ADC

Figure 33. Balun-Coupled Differential Clock (Up to 625 MHz)

If a low jitter clock source is not available, another option is to

ac couple a differential PECL signal to the sample clock input pins

as shown in Figure 34. The AD9510, AD9511, AD9512, AD9513,

AD9514, AD9515, AD9516, AD9517, AD9518, AD9520, AD9522,

AD9523, AD9524, and ADCLK905/ADCLK907/ADCLK925 clock

drivers offer excellent jitter performance.

100Ω

0.1µF

240Ω

PECL DRIVER

50kΩ

CLK–

CLK+

CLOCK

INPUT

CLOCK

INPUT

AD95xx

ADC

Figure 34. Differential PECL Sample Clock (Up to 625 MHz)

A third option is to ac-couple a differential LVDS signal to the

sample clock input pins, as shown in Figure 35. The AD9510,

AD9511, AD9512, AD9513, AD9514, AD9515, AD9516, AD9517,

AD9518, AD9520, AD9522, AD9523, and AD9524 clock drivers

offer excellent jitter performance.

100Ω

0.1µF

50kΩ

CLK–

CLK+

CLOCK

INPUT

CLOCK

INPUT

AD95xx

LVDS DRIVER

ADC

Figure 35. Differential LVDS Sample Clock (Up to 625 MHz)

Input Clock Divider

The AD6649 contains an input clock divider with the ability to

divide the input clock by integer values between 1 and 8. The

duty cycle stabilizer (DCS) is enabled by default on power-up.

The AD6649 clock divider can be synchronized using the external

SYNC input. Bit 1 and Bit 2 of Register 0x3A allow the clock

divider to be resynchronized on every SYNC signal or only on

the first SYNC signal after the register is written. A valid SYNC

causes the clock divider to reset to its initial state. This synchro-

nization feature allows multiple parts to have their clock dividers

aligned to guarantee simultaneous input sampling.

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