AD7763 datasheet(19/32 Pages) AD

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AD7763

Rev. 0 | Page 19 of 32

CLOCKING THE AD7763

The AD7763 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 AD7763. 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)

This option is pin selectable (Pin 58). On power-up, the default

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

mum 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 determined by

)

(

)

(

SNR

rms

OSR

Where:

OSR

= oversampling ratio =

ODR

ICLK

fIN

= maximum input frequency.

SNR

(dB) = target SNR.

EXAMPLE 1

This example is taken from Table 6, where:

ODR

= 625 kHz.

fICLK

= 20 MHz.

fIN

(maximum) = 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

Following is a second example from Table 6, where:

ODR

= 48 kHz.

fICLK

= 12.288 MHz.

fIN

(maximum) = 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 is 3 dB below full scale, the allowable

jitter is 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 29 and Figure 30

illustrate this point, showing the maximum slew rate of a sine

wave of the same frequency but with different amplitudes.

1.0

–1.0

0.5

–0.5

Figure 29. Maximum Slew Rate of Sine Wave with Amplitude of 2 V p-p

1.0

–1.0

0.5

–0.5

Figure 30. Maximum Slew Rate of Same Frequency Sine Wave

with Amplitude of 1 V p-p

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