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Data Sheet

AD7762

Rev. A | Page 15 of 28

CLOCKING THE AD7762

The AD7762 requires an external low jitter clock source. This

signal is applied to the MCLK pin, and the MCLKGND pin is

used to sense the ground from the clock source. An internal

clock signal (ICLK) is derived from the MCLK input signal.

The ICLK controls the internal operations of the AD7762. The

maximum ICLK frequency is 20 MHz, but due to an internal

clock divider, a range of MCLK frequencies can be used. There

are two ways to generate the ICLK:

ICLK = MCLK (CDIV = 1)

ICLK = MCLK/2 (CDIV = 0)

These options are selected from the control register (see the

AD7762 Registers section for more details). On power-up, the

default is ICLK = MCLK/2 to ensure that the part can handle

the maximum MCLK frequency of 40 MHz. For output data

rates equal to those used in audio systems, a 12.288 MHz ICLK

frequency can be used. As shown in Table 6, output data rates

of 192 kHz, 96 kHz, and 48 kHz are achievable with this ICLK

frequency. As mentioned previously, this ICLK frequency can

be derived from different MCLK frequencies.

The MCLK jitter requirements depend on a number of factors

and are given by

)

(

)

(

SNR

rms

OSR

where:

OSR = Over-sampling ratio =

ODR

fICLK

fIN = Maximum input frequency

SNR(dB) = Target SNR

EXAMPLE 1

This example can be taken from Table 6, where:

ODR = 625 kHz

fICLK = 20 MHz

fIN (max) = 250 kHz

SNR = 108 dB

250

)

(

rms

This is the maximum allowable clock jitter for a full-scale,

250 kHz input tone with the given ICLK and output data rate.

EXAMPLE 2

Take a second example from Table 6, where:

ODR = 48 kHz

fICLK = 12.288 MHz

fIN (max) = 19.2 kHz

SNR = 120 dB

133

256

)

(

rms

The input amplitude also has an effect on these jitter figures.

If, for example, the input level was 3 dB below full scale, the

allowable jitter would be increased by a factor of √2, increasing

the first example to 2.53 ps rms. This happens when the

maximum slew rate is decreased by a reduction in amplitude.

Figure 23 and Figure 24 illustrate this point, showing the

maximum slew rate of a sine wave of the same frequency but

with different amplitudes.

–1.0

0.5

–0.5

Figure 23. Maximum Slew Rate of Sine Wave with Amplitude of 2 V p-p

–1.0

0.5

–0.5

Figure 24. Maximum Slew Rate of Same Frequency Sine Wave with

Amplitude of 1 V p-p

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