AD9608 datasheet(28/41 Pages) AD

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Data Sheet

AD9608

Rev. C | Page 27 of 40

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 54. The AD9510/AD9511/AD9512/

AD9513/AD9514/AD9515/AD9516-0/AD9516-1/AD9516-2/

AD9516-3/AD9516-4/AD9516-5/AD9517-0/AD9517-1/

AD9517-2/AD9517-3/AD9517-4 clock drivers offer excellent

jitter performance.

100

Ω

0.1µF

240

Ω

240

Ω

50k

Ω

50k

Ω

CLK–

CLK+

CLOCK

INPUT

CLOCK

INPUT

ADC

AD951x

PECL DRIVER

Figure 54. Differential PECL Sample Clock (Up to 1 GHz)

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

sample clock input pins, as shown in Figure 55. The AD9510/

AD9511/AD9512/AD9513/AD9514/AD9515/AD9516-0/

AD9516-1/AD9516-2/AD9516-3/AD9516-4/AD9516-5/

AD9517-0/AD9517-1/AD9517-2/AD9517-3/AD9517-4 clock

drivers offer excellent jitter performance.

100

Ω

0.1µF

50k

Ω

50k

Ω

CLK–

CLK+

ADC

CLOCK

INPUT

CLOCK

INPUT

AD951x

LVDS DRIVER

Figure 55. Differential LVDS Sample Clock (Up to 1 GHz)

In some applications, it may be acceptable to drive the sample

clock inputs with a single-ended 1.8 V CMOS signal. In such

applications, drive the CLK+ pin directly from a CMOS gate, and

bypass the CLK− pin to ground with a 0.1 µF capacitor (see

Figure 56).

OPTIONAL

100

Ω

0.1µF

Ω1

150

Ω RESISTOR IS OPTIONAL.

CLK–

CLK+

ADC

VCC

Ω

CLOCK

INPUT

AD951x

CMOS DRIVER

Figure 56. Single-Ended 1.8 V CMOS Input Clock (Up to 200 MHz)

Input Clock Divider

The AD9608 contains an input clock divider with the ability

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

The AD9608 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 synchronization feature allows multiple parts to have their

clock dividers aligned to guarantee simultaneous input sampling.

Clock Duty Cycle

Typical high speed ADCs use both clock edges to generate

a variety of internal timing signals and, as a result, may be

sensitive to clock duty cycle. A ±5% tolerance is commonly

required on the clock duty cycle to maintain dynamic

performance characteristics.

The AD9608 contains a duty cycle stabilizer (DCS) that retimes

the nonsampling (falling) edge, providing an internal clock

signal with a nominal 50% duty cycle. This allows the user to

provide a wide range of clock input duty cycles without affecting

the performance of the AD9608. Noise and distortion perform-

ance are nearly flat for a wide range of duty cycles with the DCS

on, as shown in Figure 57.

Jitter in the rising edge of the input is still of concern and is not

easily reduced by the internal stabilization circuit. The duty

cycle control loop does not function for clock rates less than

20 MHz, nominally. The loop has a time constant associated

with it that must be considered in applications in which the

clock rate can change dynamically. A wait time of 1.5 µs to 5 µs

is required after a dynamic clock frequency increase or decrease

before the DCS loop is relocked to the input signal.

POSITIVE DUTY CYCLE (%)

DCS ON

DCS OFF

Figure 57. SNR vs. DCS On/Off

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