AD9257 datasheet(21/40 Pages) AD

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Data Sheet

AD9257

Rev. 0 | Page 21 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 52. The AD9510/AD9511/AD9512/

AD9513/AD9514/AD9515/AD9516/AD9517 clock drivers offer

excellent jitter performance.

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

sample clock input pins, as shown in Figure 53. The AD9510/

AD9511/AD9512/AD9513/AD9514/AD9515/AD9516/AD9517

clock drivers offer excellent jitter performance.

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 54).

Input Clock Divider

The AD9257 contains an input clock divider with the ability

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

The AD9257 clock divider can be synchronized using the

external SYNC input. Bit 0 and Bit 1 of Register 0x109 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 vari-

ety of internal timing signals and, as a result, may be sensitive to

clock duty cycle. Commonly, a ±5% tolerance is required on the

clock duty cycle to maintain dynamic performance characteristics.

The AD9257 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 per-

formance of the AD9257. Noise and distortion performance are

nearly flat for a wide range of duty cycles with the DCS on.

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.

100Ω

0.1µF

240Ω

50kΩ

CLK–

CLK+

CLOCK

INPUT

CLOCK

INPUT

ADC

AD951x

PECL DRIVER

Figure 52. Differential PECL Sample Clock (Up to 520 MHz)

100Ω

0.1µF

50kΩ

CLK–

CLK+

ADC

CLOCK

INPUT

CLOCK

INPUT

AD951x

LVDS DRIVER

Figure 53. Differential LVDS Sample Clock (Up to 520 MHz)

OPTIONAL

100Ω

0.1µF

50Ω1

150Ω RESISTOR IS OPTIONAL.

CLK–

CLK+

ADC

VCC

1kΩ

CLOCK

INPUT

AD951x

CMOS DRIVER

Figure 54. Single-Ended 1.8 V CMOS Input Clock (Up to 200 MHz)

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