AD7760BCP datasheet(14/22 Pages) AD

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AD7760

Preliminary Technical Data

Rev. PrN | Page 14 of 22

CLOCKING THE AD7760

The AD7760 requires an external low jitter clock source. This

signal is applied to the MCLK and MCLK pins. An internal

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

This ICLK controls all the internal operation of the AD7760.

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

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

are three possibilities available to generate the ICLK:

ICLK = MCLK (CDIV[1:0] = 10)

ICLK = MCLK / 2 (CDIV[1:0] = 00)

ICLK = MCLK / 4 (CDIV[1:0] = 01)

These options are selected from the control register (See

ICLK = MCLK / 4 to ensure that the part can handle the

maximum MCLK frequency of 80MHz. If the user wishes to get

output data rates equal to those used in audio systems, a 12.288

MHz ICLK frequency can be used. As shown in Table 5, output

data rates of 192, 96 and 48kHz 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 the following equation:

)

(

)

(

SNR

RMS

OSR

OSR = Over-sampling ratio =

ODR

ICLK

fIN

= Maximum Input Frequency

SNR(dB) = Target SNR.

Taking an example from Table 5:

ODR = 2.5MHz, fICLK = 20MHz, fIN (max) = 1MHz, SNR =

108dB

RMS

)

(

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

1MHz input tone with the given ICLK and Output Data Rate.

Taking a second example from Table 5:

ODR = 48kHz, fICLK = 12.288MHz, fIN (max) = 19.2kHz, SNR =

120dB

RMS

133

256

)

(

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

for example, the input level was 3dB down from full-scale, the

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

the first example to 2.53ps RMS. This is due to the fact that the

maximum slew rate is reduced by a reduction in amplitude.

Figure 14 and Figure 15 illustrate this point showing the

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

with different amplitudes.

Figure 14. Maximum Slew Rate of Sine Wave with Amplitude of 2V Pk-Pk

Figure 15. Maximum Slew Rate of Same Frequency Sine Wave with

Amplitude of 1V Pk-Pk

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