Datasheet

AD9640
Rev. B | Page 29 of 52
CLK+ can be directly driven from a CMOS gate. Although the
CLK+ input circuit supply is AVDD (1.8 V), this input is designed
to withstand input voltages up to 3.6 V, making the selection of
the drive logic voltage very flexible.
OPTIONAL
100Ω
0.1µF
0.1µF
0.1µF
39kΩ
50Ω
1
1
50Ω RESISTOR IS OPTIONAL
CLK–
CLK+
ADC
AD9640
V
CC
1kΩ
1kΩ
CLOCK
INPUT
06547-038
AD951x
CMOS DRIVER
Figure 60. Single-Ended 1.8 V CMOS Sample Clock (Up to 150 MSPS)
1
50Ω RESISTOR IS OPTIONAL
OPTIONAL
100Ω
0.1µF
0.1µF
0.1µF
V
CC
50Ω
1
CLK–
CLK+
ADC
AD9640
1kΩ
1kΩ
CLOCK
INPUT
06547-039
AD951x
CMOS DRIVER
Figure 61. Single-Ended 3.3 V CMOS Sample Clock (Up to 150 MSPS)
Input Clock Divider
The AD9640 contains an input clock divider with the ability to
divide the input clock by integer values between 1 and 8. If a
divide ratio other than 1 is selected, the duty cycle stabilizer is
automatically enabled.
The AD9640 clock divider can be synchronized using the external
SYNC input. Bit 1 and Bit 2 of Register 0x100 allow the clock
divider to be resynchronized on every SYNC signal or only on
the first SYNC signal after the register is written. A valid SYNC
causes the clock divider to reset to its initial state. This synchro-
nization feature allows multiple parts to have their clock dividers
aligned to guarantee simultaneous input sampling.
Clock Duty Cycle
Typical high speed ADCs use both clock edges to generate
a variety of internal timing signals and, as a result, may be
sensitive to clock duty cycle. Commonly, a ±5% tolerance is
required on the clock duty cycle to maintain dynamic
performance characteristics.
The AD9640 contains a duty cycle stabilizer (DCS) that retimes
the nonsampling (falling) edge, providing an internal clock
signal with a nominal 50% duty cycle. This allows the user to
provide a wide range of clock input duty cycles without affecting
the performance of the AD9640. Noise and distortion performance
are nearly flat for a wide range of duty cycles with the DCS on,
as shown in Figure 43.
Jitter in the rising edge of the input is still of paramount concern
and is not easily reduced by the internal stabilization circuit.
The duty cycle control loop does not function for clock rates
less than 20 MHz nominally. The loop has a time constant
associated with it that needs to be considered where the clock
rate can change dynamically. This requires a wait time of 1.5 µs
to 5 µs after a dynamic clock frequency increase or decrease before
the DCS loop is relocked to the input signal. During the time
period the loop is not locked, the DCS loop is bypassed, and
internal device timing is dependent on the duty cycle of the input
clock signal. In such applications, it may be appropriate to disable
the duty cycle stabilizer. In all other applications, enabling the DCS
circuit is recommended to maximize ac performance.
Jitter Considerations
High speed, high resolution ADCs are sensitive to the quality
of the clock input. The degradation in SNR from the low
frequency SNR (SNR
LF
) at a given input frequency (f
INPUT
) due
to jitter (t
JRMS
) can be calculated by
SNR
HF
= −10 log[(2π × f
INPUT
× t
JRMS
)
2
+ 10 ]
)10/(
LF
SNR−
In the equation, the rms aperture jitter represents the clock input
jitter specification. IF undersampling applications are particularly
sensitive to jitter, as illustrated in Figure 62.
75
70
65
60
55
50
45
40
1 10 100 1000
SNR (dBc)
INPUT FREQUENCY (MHz)
06547-041
3.00ps
0.05ps
MEASURED
PERFORMANCE
0.20ps
0.5ps
1.0ps
1.50ps
2.00ps
2.50ps
Figure 62. SNR vs. Input Frequency and Jitter
The clock input should be treated as an analog signal in cases
where aperture jitter may affect the dynamic range of the AD9640.
Power supplies for clock drivers should be separated from the
ADC output driver supplies to avoid modulating the clock
signal with digital noise. Low jitter, crystal-controlled oscillators
make the best clock sources. If the clock is generated from
another type of source (by gating, dividing, or other methods),
it should be retimed by the original clock at the last step.
See the AN-501 Application Note and AN-756 Application
Note for more information about jitter performance as it
relates to ADCs.