CDK8307 datasheet(27/29 Pages) CADEKA

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PRELIMINARY Data Sheet

Clock Input and Jitter Considerations

Typically high-speed ADCs use both clock edges to generate

internal timing signals. In the CDK8307 only the rising

edge of the clock is used. Hence, input clock duty cycles

between 20% and 80% is acceptable.

The input clock can be supplied in a variety of formats.

The clock pins are AC-coupled internally, and hence a wide

common mode voltage range is accepted. Differential

clock sources as LVDS, LVPECL or differential sine wave

can be connected directly to the input pins. For CMOS

inputs, the CLKN pin should be connected to ground, and

the CMOS clock signal should be connected to CLKP. For

differential sine wave clock input the amplitude must be

at least ±0.8Vpp.

The quality of the input clock is extremely important for

high-speed, high-resolution ADCs. The contribution to

SNR from clock jitter with a full scale signal at a given

frequency is shown in equation below.

SNRjitter = 20 • log (2 • π • FIN •

εt)

where FIN is the signal frequency, and

εt is the total rms

jitter measured in seconds. The rms jitter is the total of all

jitter sources including the clock generation circuitry, clock

distribution and internal ADC circuitry.

For applications where jitter may limit the obtainable per-

formance, it is of utmost importance to limit the clock

jitter. This can be obtained by using precise and stable

clock references (e.g. crystal oscillators with good jitter

specifications) and make sure the clock distribution is well

controlled. It might be advantageous to use analog power

and ground planes to ensure low noise on the supplies

to all circuitry in the clock distribution. It is of utmost im-

portance to avoid crosstalk between the ADC output bits

and the clock and between the analog input signal and

the clock since such crosstalk often results in harmonic

distortion.

The jitter performance is improved with reduced rise and

fall times of the input clock. Hence, optimum jitter per-

formance is obtained with LVDS or LVPECL clock with fast

edges. CMOS and sine wave clock inputs will result in

slightly degraded jitter performance.

If the clock is generated by other circuitry, it should be re-

timed with a low jitter master clock as the last operation

before it is applied to the ADC clock input.

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