AD9259 datasheet(21/52 Pages) AD

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AD9259

Rev. 0 | Page 21 of 52

Clock Jitter Considerations

High speed, high resolution ADCs are sensitive to the quality of the

clock input. The degradation in SNR at a given input frequency

(fA) due only to aperture jitter (tJ) can be calculated by

SNR degradation = 20 × log 10 [1/2 × π × fA × tJ]

In this equation, the rms aperture jitter represents the root mean

square of all jitter sources, including the clock input, analog input

signal, and ADC aperture jitter specifications. IF undersampling

applications are particularly sensitive to jitter (see Figure 47).

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

where aperture jitter may affect the dynamic range of the AD9259.

Power supplies for clock drivers should be separated from the

ADC output driver supplies to avoid modulating the clock signal

with digital noise. Low jitter, crystal-controlled oscillators make

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

type of source (by gating, dividing, or other methods), it should

be retimed by the original clock at the last step.

Refer to the AN-501 Application Note and the AN-756

Application Note for more in-depth information about jitter

performance as it relates to ADCs (visit www.analog.com).

100

1000

16 BITS

14 BITS

12 BITS

100

110

120

130

0.125ps

0.25ps

0.5ps

1.0ps

2.0ps

ANALOG INPUT FREQUENCY (MHz)

10 BITS

RMS CLOCK JITTER REQUIREMENT

Figure 47. Ideal SNR vs. Input Frequency and Jitter

Power Dissipation and Power-Down Mode

As shown in Figure 48, the power dissipated by the AD9259 is

proportional to its sample rate. The digital power dissipation

does not vary much because it is determined primarily by the

DRVDD supply and bias current of the LVDS output drivers.

ENCODE (MSPS)

250

300

350

450

500

400

100

140

120

200

180

160

DRVDD CURRENT

TOTAL POWER

AVDD CURRENT

Figure 48. Supply Current vs. fSAMPLE for fIN = 10.3 MHz, fSAMPLE = 50 MSPS

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