AD6649EBZ datasheet(21/41 Pages) AD

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AD6649

Data Sheet

Rev. C | Page 20 of 40

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 ±5% tolerance is required on the

clock duty cycle to maintain dynamic performance characteristics.

The AD6649 contains a duty cycle stabilizer (DCS) that retimes

the nonsampling (falling) edge, providing an internal clock

signal with a nominal 50% duty cycle. This allows the user to

provide a wide range of clock input duty cycles without affecting

the performance of the AD6649.

Jitter on the rising edge of the input clock is still of paramount

concern and is not reduced by the duty cycle stabilizer. The duty

cycle control loop does not function for clock rates less than

40 MHz nominally. The loop has a time constant associated

with it that must be considered when the clock rate can change

dynamically. A wait time of 1.5 µs to 5 µs is required after a

dynamic clock frequency increase or decrease before the DCS

loop is relocked to the input signal. During the time period that

the loop is not locked, the DCS loop is bypassed, and internal

device timing is dependent on the duty cycle of the input clock

signal. In such applications, it may be appropriate to disable the

duty cycle stabilizer. In all other applications, enabling the DCS

circuit is recommended to maximize ac performance.

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 (fIN) due to jitter (tJ) can be calculated by

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

)

(

SNR

]

In the equation, the rms aperture jitter represents the root-

mean-square of all jitter sources, which include the clock input,

the analog input signal, and the ADC aperture jitter specification.

IF undersampling applications are particularly sensitive to jitter,

as shown in Figure 36.

100

INPUT FREQUENCY (MHz)

1000

0.05ps

0.20ps

0.50ps

1.00ps

1.50ps

MEASURED

Figure 36. SNR (95 MHz BW) vs. Input Frequency and Jitter

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

where aperture jitter may affect the dynamic range of the AD6649.

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 another method), it should be

retimed by the original clock at the last step.

Refer to the AN-501 Application Note, Aperture Uncertainty

and ADC System Performance, and the AN-756 Application

Note, Sampled Systems and the Effects of Clock Phase Noise and

Jitter, for more information about jitter performance as it relates

to ADCs.

POWER DISSIPATION AND STANDBY MODE

As shown in Figure 37, the power dissipated by the AD6649 is

proportional to its sample rate. The data in Figure 37 was taken

using the same operating conditions as those used for the Typical

Performance Characteristics.

0.1

0.2

0.3

0.4

0.5

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

100

120

140

160

180

200

220

250

ENCODE FREQUENCY (MSPS)

IAVDD

IDRVDD

TOTAL POWER

Figure 37. AD6649 Power and Current vs. Sample Rate

By asserting PDWN (either through the SPI port or by asserting

the PDWN pin high), the AD6649 is placed in power-down

mode. In this state, the ADC typically dissipates 10 mW. During

power-down, the output drivers are placed in a high impedance

state. Asserting the PDWN pin low returns the AD6649 to its

normal operating mode. Note that PDWN is referenced to the

digital output driver supply (DRVDD) and should not exceed

that supply voltage.

Low power dissipation in power-down mode is achieved by

shutting down the reference, reference buffer, biasing networks,

and clock. Internal capacitors are discharged when entering

power-down mode and then must be recharged when returning

to normal operation. As a result, wake-up time is related to the

time spent in power-down mode, and shorter power-down

cycles result in proportionally shorter wake-up times.

When using the SPI port interface, the user can place the ADC

in power-down mode or standby mode. Standby mode allows

the user to keep the internal reference circuitry powered when

faster wake-up times are required. See the Memory Map Register

Description section and the AN-877 Application Note, Interfacing

to High Speed ADCs via SPI, for additional details.

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