AD9629 datasheet(21/32 Pages) AD

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AD9629

Rev. 0 | Page 21 of 32

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. Commonly, a 50% duty cycle clock

with ±5% tolerance is required to maintain optimum dynamic

performance as shown in Figure 51.

Jitter on the rising edge of the clock input can also impact dynamic

performance and should be minimized as discussed in the Jitter

Considerations section.

POSITIVE DUTY CYCLE (%)

Figure 51. SNR vs. Clock Duty Cycle

Jitter Considerations

High speed, high resolution ADCs are sensitive to the quality

of the clock input. The degradation in SNR from the low fre-

quency SNR (SNRLF) at a given input frequency (fINPUT) due to

jitter (tJRMS) can be calculated by

SNRHF = −10 log[(2π × fINPUT × tJRMS)2 + 10

]

)

(

SNR

−

In the previous equation, the rms aperture jitter represents the

clock input jitter specification. IF undersampling applications

are particularly sensitive to jitter, as illustrated in Figure 52.

100

FREQUENCY (MHz)

0.5ps

0.2ps

0.05ps

1.0ps

1.5ps

2.0ps

2.5ps

3.0ps

Figure 52. SNR vs. Input Frequency and Jitter

The clock input should be treated as an analog signal in cases in

which aperture jitter may affect the dynamic range of the AD9629.

To avoid modulating the clock signal with digital noise, keep

power supplies for clock drivers separate from the ADC output

driver supplies. Low jitter, crystal-controlled oscillators make

the best clock sources. If the clock is generated from another type

of source (by gating, dividing, or another method), it should be

retimed by the original clock at the last step.

For more information, see the AN-501 Application Note and the

AN-756 Application Note available on www.analog.com.

POWER DISSIPATION AND STANDBY MODE

As shown in Figure 53, the analog core power dissipated by

the AD9629 is proportional to its sample rate. The digital

power dissipation of the CMOS outputs are determined

primarily by the strength of the digital drivers and the load

on each output bit.

The maximum DRVDD current (IDRVDD) can be calculated as

IDRVDD = VDRVDD × CLOAD × fCLK × N

where N is the number of output bits (13, in the case of the

AD9629).

This maximum current occurs when every output bit switches

on every clock cycle, that is, a full-scale square wave at the Nyquist

frequency of fCLK/2. In practice, the DRVDD current is estab-

lished by the average number of output bits switching, which

is determined by the sample rate and the characteristics of the

analog input signal.

Reducing the capacitive load presented to the output drivers

can minimize digital power consumption. The data in Figure 53

was taken using the same operating conditions as those used for

the Typical Performance Characteristics, with a 5 pF load on

each output driver.

CLOCK RATE (MSPS)

AD9231-20

AD9231-40

AD9231-65

AD9231-80

Figure 53. Analog Core Power vs. Clock Rate

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