AD9739-R2-EBZ datasheet(41/50 Pages) AD

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Data Sheet

AD9739

Rev. E | Page 41 of 50

CLOCK INPUT CONSIDERATIONS

The quality of the clock source and its drive strength are

important considerations in maintaining the specified ac

performance. The phase noise and spur characteristics of the

clock source should be selected to meet the target application

requirements. Phase noise and spurs at a given frequency offset

on the clock source are directly translated to the output signal. It

can be shown that the phase noise characteristics of a reconstructed

output sine wave are related to the clock source by 20 × log10

(fOUT/fCLK) when the DAC clock path contribution, along with

thermal and quantization effects, are negligible.

The AD9739 clock receiver provides optimum jitter performance

when driven by a fast slew rate originating from the LVPECL or

CML output drivers. For optimal ac performance out of the

AD9739, the recommended minimum differential peak-to-peak

voltage is approximately 1.4 V p-p. For a low jitter sinusoidal

clock source, the ADCLK914 can be used to square-up the

signal and provide a CML input signal for the AD9739 clock

receiver. Note that all specifications and characterization

presented in the data sheet are with the ADCLK914 driven by a

high quality RF signal generator with the clock receiver biased

at an 800 mV level. A dc blocking capacitor is used between the

clock driver output and clock receiver input to allow for

different dc bias levels. To minimize signal loss for high clock

rates, a high quality, dc blocking RF capacitor is recommended.

ESD

DACCLK_P

DACCLK_N

VDDC

VSSC

CLKx_OFFSET

DIR_x = 0

CLKx_OFFSET

DIR_x = 0

4-BIT PMOS

IOUT ARRAY

4-BIT NMOS

IOUT ARRAY

Figure 58. Clock Input and Common-Mode Control

The AD9739 clock receiver features the ability to independently

adjust the common-mode level of its inputs over a span of

±100 mV centered about its midsupply point (that is, VDDC/2)

as well as an offset for hysteresis purposes. Figure 58 shows the

equivalent input circuit of one of the inputs. ESD diodes are not

shown for clarity purposes. It has been found through

characterization that the optimum setting is for both inputs to

be biased at approximately 0.8 V. This can be achieved by writing a

0x0F (corresponding to a −15) setting to both cross controller

registers (that is, Register 0x22 and Register 0x23).

VCC

VEE

VREF

Ω

Ω 50Ω

Ω

DACCLK_P

DACCLK_N

100

Ω

1nF

ADCLK914

AD9739

10nF

Ω

Figure 59. ADCLK914 Interface to the AD9739 CLK Input

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