AD9279 Datasheet, PDF (28/44 Pages) Analog Devices – Octal LNA/VGA/AAF/ADC and CW I/Q Demodulator

AD9279
3.3V
VFAC3
OUT
0.1µF
50Ω*
AD951x FAMILY
CLK
OPTIONAL
100Ω
0.1µF
CMOS DRIVER
CLK+
0.1µF
CLK
AD9279
0.1µF
CLK–
*50Ω RESISTOR IS OPTIONAL.
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 (fA)
due only to aperture jitter (tJ) can be calculated as follows:
SNR Degradation = 20 × log 10(1/2 × π × fA × tJ)
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).
130
RMS CLOCK JITTER REQUIREMENT
120
110
100
16 BITS
90
14 BITS
80
70
10 BITS
60
8 BITS
50
40
30
1
0.125ps
0.25ps
0.5ps
1.0ps
2.0ps
10
100
ANALOG INPUT FREQUENCY (MHz)
12 BITS
1000
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
MODE III, f_SAMPLE = 80MSPS
250
MODE II, f_SAMPLE = 65MSPS
200
MODE I, f_SAMPLE = 40MSPS
150
100
50
I_DRVDD
0
0
10 20
30 40 50
60 70 80
SAMPLING FREQUENCY (MSPS)
Figure 59. Supply Current vs. fSAMPLE for fIN = 5 MHz
180
170
MODE III, f_SAMPLE = 80MSPS
160
150
MODE II, f_SAMPLE = 65MSPS
140
MODE I, f_SAMPLE = 40MSPS
130
120
110
0
10 20 30 40 50 60 70 80
SAMPLING FREQUENCY (MSPS)
Figure 60. Power per Channel vs. fSAMPLE for fIN = 5 MHz
The AD9279 features scalable LNA bias currents (see Table 19,
Register 0x12). The default LNA bias current settings are high.
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