Datasheet

AD9279
Rev. 0 | Page 28 of 44
0.1µF
0.1µF
0.1µF
CMOS DRIVER
OPTIONAL
100Ω
0.1µF
CLK
CLK
*50Ω RESISTOR IS OPTIONAL.
AD951x FAMILY
3.3
V
OUT
VFAC3
09423-059
CLK–
CLK+
AD9279
50Ω*
Figure 57. Single-Ended 3.3 V CMOS Sample Clock
Clock Duty Cycle Considerations
Typical high speed ADCs use both clock edges to generate a
variety of internal timing signals. As a result, these ADCs may
be sensitive to the clock duty cycle. Commonly, a 5% tolerance is
required on the clock duty cycle to maintain dynamic performance
characteristics. The AD9279 contains a duty cycle stabilizer (DCS)
that retimes the nonsampling edge, providing an internal clock
signal with a nominal 50% duty cycle. This allows a wide range
of clock input duty cycles without affecting the performance of
the AD9279. When the DCS is on, noise and distortion perfor-
mance are nearly flat for a wide range of duty cycles. However,
some applications may require the DCS function to be off. If so,
keep in mind that the dynamic range performance can be affected
when operated in this mode. See Table 19 for more details on
using this feature.
The duty cycle stabilizer uses a delay-locked loop (DLL) to
create the nonsampling edge. As a result, any changes to the
sampling frequency require approximately eight clock cycles
to allow the DLL to acquire and lock to the new rate.
Clock 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 as follows:
SNR Degradation = 20 × log 10(1/2 × π × f
A
× t
J
)
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. IF undersampling applications
are particularly sensitive to jitter (see Figure 58).
The clock input should be treated as an analog signal in cases
where aperture jitter may affect the dynamic range of the AD9279.
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, such as the Valpey Fisher VFAC3 series.
If the clock is generated from another type of source (by gating,
dividing, or other methods), it should be retimed by the original
clock during the last step.
Refer to the AN-501 Application Note and the AN-756
Application Note for more in-depth information about how
jitter performance relates to ADCs (visit www.analog.com).
1 10 100 1000
16 BITS
14 BITS
12 BITS
30
40
50
60
70
80
90
100
110
120
130
0.125ps
0.5ps
1.0ps
2.0ps
ANALOG INPUT FREQUENCY (MHz)
10 BITS
8 BITS
RMS CLOCK JITTER REQUIREMENT
SNR (dB)
09423-060
0.25ps
Figure 58. Ideal SNR vs. Input Frequency and Jitter
Power Dissipation and Power-Down Mode
As shown in Figure 59 and Figure 60, the power dissipated by
the AD9279 is proportional to its sample rate. The digital power
dissipation does not vary significantly because it is determined
primarily by the DRVDD supply and the bias current of the
LVDS output drivers.
350
300
250
200
150
100
50
0
0 1020304050607080
SAMPLING FREQUENCY (MSPS)
CURRENTS (mA)
09423-061
MODE III, f
SAMPLE
= 80MSPS
I
DRVDD
MODE II, f
SAMPLE
= 65MSPS
MODE I, f
SAMPLE
= 40MSPS
Figure 59. Supply Current vs. f
SAMPLE
for f
IN
= 5 MHz
180
170
160
150
140
130
120
110
0 1020304050607080
SAMPLING FREQUENCY (MSPS)
POWER/CHANNEL (mW/CH)
09423-062
MODE III, f
SAMPLE
= 80MSPS
MODE II, f
SAMPLE
= 65MSPS
MODE I, f
SAMPLE
= 40MSPS
Figure 60. Power per Channel vs. f
SAMPLE
for f
IN
= 5 MHz
The AD9279 features scalable LNA bias currents (see Table 19,
Register 0x12). The default LNA bias current settings are high.