Datasheet

AD9287 Data Sheet
Rev. E | Page 22 of 52
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 by
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 47).
The clock input should be treated as an analog signal in cases
where aperture jitter may affect the dynamic range of the AD9287.
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 are 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 during the last step.
Refer to the AN-501 Application Note and to the AN-756
Application Note for more in-depth information about jitter
performance as it relates to ADCs at 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
0.25ps
0.5ps
1.0ps
2.0ps
ANALOG INPUT FREQUENCY (MHz)
10 BITS
8 BITS
RMS CLOCK JITTER REQUIREMENT
SNR (dB)
05966-038
Figure 47. Ideal SNR vs. Input Frequency and Jitter
Power Dissipation and Power-Down Mode
As shown in Figure 48, the power dissipated by the AD9287 is
proportional to its sample rate. The digital power dissipation
does not vary significantly because it is determined primarily by
the DRVDD supply and bias current of the LVDS output drivers.
0
0 120
ENCODE (MSPS)
CURRENT (mA)
POWER (mW)
300
250
200
150
100
50
300
600
550
500
450
400
350
20 40 60 80
100
AVDD CURRENT
TOTAL POWER
DRVDD CURRENT
05966-069
Figure 48. Supply Current vs. f
SAMPLE
for f
IN
= 10.3 MHz, f
SAMPLE
= 100 MSPS