AD9609 Analog Devices, AD9609 Datasheet - Page 21

• Manufacturer Part Number: AD9609
• Description: 10-Bit, 20 MSPS/40 MSPS/65 MSPS/80 MSPS, 1.8 V Analog-to-Digital Converter
• Manufacturer: Analog Devices

Specifications of AD9609

• Resolution (bits): 10bit
• # Chan: 1
• Sample Rate: 80MSPS
• Interface: Par
• Analog Input Type: Diff-Uni
• Ain Range: 2 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

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 AD9609 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 AD9609. Noise and distortion
performance are nearly flat for a wide range of duty cycles with
the DCS on, as shown in Figure 51.
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.
80
75
70
65
60
55
50
45
40
10
20
Figure 51. SNR vs. DCS On/Off
30
DCS ON
POSITIVE DUTY CYCLE (%)
DCS OFF
40
50
60
70
80
Rev. 0 | Page 21 of 32
Jitter Considerations
High speed, high resolution ADCs are sensitive to the quality
of the clock input. The degradation in SNR from the low fre-
quency SNR (SNR
jitter (t
In the previous equation, the rms aperture jitter represents the
clock input jitter specification. IF undersampling applications
are particularly sensitive to jitter, as illustrated in Figure 52.
The clock input should be treated as an analog signal when
aperture jitter may affect the dynamic range of the AD9609. To
avoid modulating the clock signal with digital noise, keep power
supplies for clock drivers separate from the ADC output driver
supplies. Low jitter, crystal-controlled oscillators make the best
clock sources. If the clock is generated from another type of source
(by gating, dividing, or another method), it should be retimed by
the original clock at the last step.
For more information, see the AN-501 Application Note and
the AN-756 Application Note available at www.analog.com.
SNR
80
75
70
65
60
55
50
45
JRMS
1
HF
) can be calculated by
= −10 log[(2π × f
Figure 52. SNR vs. Input Frequency and Jitter
LF
) at a given input frequency (f
10
FREQUENCY (MHz)
INPUT
× t
JRMS
)
100
2
+ 10
3.0ps
(
−
SNR
0.05ps
0.2ps
0.5ps
1.0ps
1.5ps
2.0ps
2.5ps
INPUT
LF
AD9609
/
10
) due to
)
1k
]