AD9262EBZ datasheet(21/33 Pages) AD

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AD9262

Rev. A | Page 20 of 32

Table 11. Common Modulator Clock Multiplication Factors

CLK±

(MHz)

0x0A[5:0]

(PLLMULT)

fVCO

(MHz)

fMOD

(MHz)

30.72

1290.24

645.12

10.08

39.3216

1258.29

629.15

9.83

52.00

1300.00

650.00

10.16

61.44

1290.24

645.12

10.08

76.80

1305.60

652.80

10.20

78.00

1326.00

663.00

10.36

78.6432

1258.29

629.15

9.83

89.60

1344.00

672.00

10.50

92.16

1290.24

645.12

10.08

122.88

1228.80

614.40

9.60

134.40

1344.00

672.00

10.50

153.60

1228.80

614.40

9.60

157.2864

1258.29

629.15

9.83

Jitter Considerations

The aperture jitter requirements for continuous time Σ-Δ conver-

ters may be more forgiving than Nyquist rate converters. The

continuous time Σ-Δ architecture is an oversampled system

and to accurately represent the analog input signal to the ADC,

a large number of output samples must be averaged together. As

a result, the jitter contribution from each sample is root sum

squared, resulting in a more subtle impact on noise perfor-

mance as compared to Nyquist converters where aperture

jitter has a direct impact on each sampled output.

In the block diagram of the continuous time Σ-Δ modulator

(see Figure 37), the two building blocks most susceptible to

jitter are the quantizer and the DAC. The error introduced

through the sampling process is reduced by the loop gain and

shaped in the same way as the quantization noise and, therefore,

its effect can be neglected. On the contrary, the jitter error

associated with the DAC directly adds to the input signal, thus

increasing the in-band noise power and degrading the modulator

performance. The SNR degradation due to jitter can be

represented by the following equation.

SNR = −20 log (2πfanalogtjitter_rms) dB

where fanalog is the analog input frequency and tjitter_rms is the jitter.

The SNR performance of the AD9262 remains constant within

the input bandwidth of the converter, from DC to 10 MHz.

Therefore, the minimal jitter specification is determined at the

highest input frequency. From the calculation, the aperture

jitter of the input clock must be no greater than 1 ps to achieve

optimal SNR performance.

POWER DISSIPATION AND STANDBY MODE

The AD9262 power consumption can be further reduced by

configuring the chip in channel power-down, standby, or sleep

mode. The low power modes turn off internal blocks of the chip,

including the reference. As a result, the wake-up time is depen-

dent on the amount of circuitry that is turned off. Fewer internal

circuits that are powered down result in proportionally shorter

wake-up time. The low power modes are shown in Table 12.

In the standby mode, all clock related activity and the output

channels are disabled. Only the references and CMOS outputs

remain powered up to ensure a short recovery and link integr-

ity. During sleep mode, all internal circuits are powered down,

putting the device into its lowest power mode, and the CMOS

outputs are disabled.

Each ADC channel can be independently powered down or

both channels can be set simultaneously by writing to the

channel index, Register 0x05[1:0].

Table 12. Low Power Modes

Mode

0x08[1:0]

Analog Circuitry

Clock

Ref

Normal

0x0

Power-Down

0x1

Off

Standby

0x2

Off

Sleep

0x3

Off

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