Datasheet

AD9653 Data Sheet
Rev. 0 | Page 26 of 40
Figure 65. Differential LVDS Sample Clock (Up to 1 GHz)
Figure 66. Single-Ended 1.8 V CMOS Input Clock (Up to 200 MHz)
Input Clock Divider
The AD9653 contains an input clock divider with the ability
to divide the input clock by integer values between 1 and 8.
The AD9653 clock divider can be synchronized using the
external SYNC input. Bit 0 and Bit 1 of Register 0x109 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 synchronization 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 vari-
ety 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 AD9653 contains a duty cycle stabilizer (DCS) that retimes
the nonsampling (falling) edge, providing an internal clock
signal with a nominal 50% duty cycle. This feature minimizes
performance degradation in cases where the clock input duty
cycle deviates from 50% greater than the specified ±5%. Noise and
distortion performance are nearly flat for a wider range of duty
cycles with the DCS on, as shown in Figure 67 and Figure 68.
Figure 67. SNR vs. DCS On/Off, V
REF
= 1.0 V
Figure 68. SNR vs. DCS On/Off, V
REF
= 1.3 V
Jitter in the rising edge of the input is still of 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 must be considered in applications in which the
clock rate can change dynamically. A wait time of 1.5 µs to 5 µs
is required after a dynamic clock frequency increase or decrease
before the DCS loop is relocked to the input signal.
Jitter Considerations
High speed, high resolution ADCs are sensitive to the quality of the
clock input. The degradation in SNR at a given input frequency
(f
A
) due only to aperture jitter (t
J
) can be calculated by
SNR Degradation = 20 log
10








××
J
A
tf π 2
1
In this equation, the rms aperture jitter represents the root mean
square of all jitter sources, including the clock input, analog input
signal, and ADC aperture jitter specifications. IF undersampling
applications are particularly sensitive to jitter (see Figure 69).
The clock input should be treated as an analog signal in cases
where aperture jitter may affect the dynamic range of the AD9653.
Power supplies for clock drivers should be separated from the
100Ω
0.1µF
0.1µF
0.1µF
0.1µF
50kΩ 50kΩ
CLK–
CLK+
ADC
CLOCK
INPUT
CLOCK
INPUT
AD951x
LVDS DRIVER
10538-065
OPTIONAL
100Ω
0.1µF
0.1µF
0.1µF
50Ω
1
1
50Ω RESISTOR IS OPTIONAL.
CLK–
CLK+
ADC
V
CC
1kΩ
1kΩ
CLOCK
INPUT
AD951x
CMOS DRIVER
10538-066
84
70
40 60555045
SNR (dBFS)
DUTY CYCLE (%)
10538-076
72
74
76
78
80
82
SNRFS (DCS OFF)
SNRFS (DCS ON)
84
70
40 60555045
SNR (dBFS)
DUTY CYCLE (%)
10538-077
72
74
76
78
80
82
SNRFS (DCS OFF)
SNRFS (DCS ON)