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AD9211

Rev. 0 | Page 21 of 28

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 × log10[½ × 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 AD9211.

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

8 BITS

RMS CLOCK JITTER REQUIREMENT

Figure 47. Ideal SNR vs. Input Frequency and Jitter

POWER DISSIPATION AND POWER-DOWN MODE

The power dissipated by the AD9211 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.

By asserting PWDN (Pin 29) high, the AD9211 is placed in

standby mode or full power-down mode, as determined by the

contents of Serial Port Register 08. Reasserting the PWDN pin

low returns the AD9211 to its normal operational mode.

An additional standby mode is supported by means of varying

the clock input. When the clock rate falls below 20 MHz, the

AD9211 assumes a standby state. In this case, the biasing network

and internal reference remain on, but digital circuitry is powered

down. Upon reactivating the clock, the AD9211 resumes normal

operation after allowing for the pipeline latency.

DIGITAL OUTPUTS

Digital Outputs and Timing

The AD9211 differential outputs conform to the ANSI-644

LVDS standard on default power-up. This can be changed to a

low power, reduced signal option similar to the IEEE 1596.3

standard using the SPI. This LVDS standard can further reduce

the overall power dissipation of the device, which reduces the

power by ~39 mW. See the Memory Map section for more

information. The LVDS driver current is derived on-chip and

sets the output current at each output equal to a nominal

3.5 mA. A 100 Ω differential termination resistor placed at the

LVDS receiver inputs results in a nominal 350 mV swing at the

receiver.

The AD9211 LVDS outputs facilitate interfacing with LVDS

receivers in custom ASICs and FPGAs that have LVDS capability

for superior switching performance in noisy environments.

Single point-to-point net topologies are recommended with a

100 Ω termination resistor placed as close to the receiver as

possible. No far-end receiver termination and poor differential

trace routing may result in timing errors. It is recommended

that the trace length is no longer than 24 inches and that the

differential output traces are kept close together and at equal

lengths.

An example of the LVDS output using the ANSI standard (default)

data eye and a time interval error (TIE) jitter histogram with

trace lengths less than 24 inches on regular FR-4 material is

shown in Figure 48. Figure 49 shows an example of when the

trace lengths exceed 24 inches on regular FR-4 material. Notice

that the TIE jitter histogram reflects the decrease of the data eye

opening as the edge deviates from the ideal position. It is up to

the user to determine if the waveforms meet the timing budget

of the design when the trace lengths exceed 24 inches.

500

–500

–400

–300

–200

–100

100

200

300

400

–3

–2

–1

0123

TIME (ns)

–40

–20

TIME (ps)

Figure 48. Data Eye for LVDS Outputs in ANSI Mode with Trace Lengths Less

than 24 Inches on Standard FR-4, AD9211-250

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