Datasheet

Data Sheet AD9222
Rev. F | Page 25 of 60
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 specifications. IF undersampling
applications are particularly sensitive to jitter (see Figure 68).
The clock input should be treated as an analog signal in cases
where aperture jitter may affect the dynamic range of the AD9222.
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. If the clock is generated from another
type of source (by gating, dividing, or other methods), it should
be retimed by the original clock at the last step.
Refer to the AN-501 Application Note and the AN-756
Application Note for more in-depth information about jitter
performance as it relates to ADCs.
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)
05967-055
Figure 68. Ideal SNR vs. Input Frequency and Jitter
Power Dissipation and Power-Down Mode
As shown in Figure 69, the power dissipated by the AD9222 is
proportional to its sample rate. The digital power dissipation
does not vary much because it is determined primarily by the
DRVDD supply and bias current of the LVDS output drivers.
05967-057
ENCODE (MSPS)
CURRENT (A)
10 50
0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
15 20 25 30 35 40 45
0.500
0.550
0.600
0.650
0.700
0.750
0.800
POWER (W)
TOTAL POWER
AVDD CURRENT
DRVDD CURRENT
Figure 69. Supply Current vs. f
SAMPLE
for f
IN
= 10.3 MHz, AD9222- 50
ENCODE (MSPS)
CURRENT (mA)
10 605020 30 40
700
750
800
850
900
950
POWER (mW)
TOTAL POWER
AVDD CURRENT
DRVDD CURRENT
0
50
100
150
200
250
300
350
400
450
500
05967-103
Figure 70. Supply Current vs. f
SAMPLE
for f
IN
= 10.3 MHz, AD9222- 65