AD9228 datasheet(21/52 Pages) AD

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AD9228

Rev. 0 | Page 21 of 52

CLOCK INPUT CONSIDERATIONS

For optimum performance, the AD9228 sample clock inputs

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

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

via a transformer or capacitors. These pins are biased internally

and require no additional bias.

Figure 50 shows one preferred method for clocking the AD9228.

The low jitter clock source is converted from single-ended to

differential using an RF transformer. The back-to-back Schottky

diodes across the secondary transformer limit clock excursions

into the AD9228 to approximately 0.8 V p-p differential. This

helps prevent the large voltage swings of the clock from feeding

through to other portions of the AD9228 and preserves the fast

rise and fall times of the signal, which are critical to low jitter

performance.

0.1µF

SCHOTTKY

DIODES:

HSM2812

CLOCK

INPUT

50Ω

100Ω

CLK–

CLK+

ADC

AD9228

MIN-CIRCUITS

ADT1–1WT, 1:1Z

XFMR

Figure 50. Transformer Coupled Differential Clock

If a low jitter clock is available, another option is to ac-couple a

differential PECL signal to the sample clock input pins as shown

in Figure 51. The AD9510/AD9511/AD9512/AD9513/AD9514/

AD9515 family of clock drivers offers excellent jitter performance.

CLOCK

INPUT

100Ω

0.1µF

240Ω

CLOCK

INPUT

AD9510/1/2/3/4/5

PECL DRIVER

50Ω*

CLK

*50Ω RESISTORS ARE OPTIONAL

CLK–

CLK+

ADC

AD9228

Figure 51. Differential PECL Sample Clock

CLOCK

INPUT

100Ω

0.1µF

50Ω*

CLOCK

INPUT

AD9510/1/2/3/4/5

LVDS DRIVER

50Ω*

CLK

*50Ω RESISTORS ARE OPTIONAL

CLK–

CLK+

ADC

AD9228

Figure 52. Differential LVDS Sample Clock

In some applications, it is acceptable to drive the sample clock

inputs with a single-ended CMOS signal. In such applications,

CLK+ should be directly driven from a CMOS gate, and the

CLK− pin should be bypassed to ground with a 0.1 μF capacitor

in parallel with a 39 kΩ resistor (see Figure 53). Although the

CLK+ input circuit supply is AVDD (1.8 V), this input is

designed to withstand input voltages up to 3.3 V, making the

selection of the drive logic voltage very flexible.

CLOCK

INPUT

0.1µF

39kΩ

AD9510/1/2/3/4/5

CMOS DRIVER

50Ω*

OPTIONAL

100Ω

0.1µF

CLK

*50Ω RESISTOR IS OPTIONAL

CLK–

CLK+

ADC

AD9228

Figure 53. Single-Ended 1.8 V CMOS Sample Clock

CLOCK

INPUT

0.1µF

AD9510/1/2/3/4/5

CMOS DRIVER

50Ω*

OPTIONAL

100Ω

CLK

*50Ω RESISTOR IS OPTIONAL

0.1µF

CLK–

CLK+

ADC

AD9228

Figure 54. Single-Ended 3.3 V CMOS Sample Clock

Clock Duty Cycle Considerations

Typical high speed ADCs use both clock edges to generate a

variety of internal timing signals. As a result, these ADCs may

be sensitive to clock duty cycle. Commonly, a 5% tolerance is

required on the clock duty cycle to maintain dynamic performance

characteristics. The AD9228 contains a duty cycle stabilizer (DCS)

that retimes the nonsampling edge, providing an internal clock

signal with a nominal 50% duty cycle. This allows a wide range

of clock input duty cycles without affecting the performance of

the AD9228. When the DCS is on, noise and distortion perfor-

mance are nearly flat for a wide range of duty cycles. The DCS

function cannot be turned off.

The duty cycle stabilizer uses a delay-locked loop (DLL) to

create the nonsampling edge. As a result, any changes to the

sampling frequency require approximately 10 clock cycles

to allow the DLL to acquire and lock to the new rate.

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