AD9484 Analog Devices, AD9484 Datasheet - Page 16

• Manufacturer Part Number: AD9484
• Description: 8-Bit, 500 MSPS, 1.8 V Analog-to-Digital Converter
• Manufacturer: Analog Devices

Specifications of AD9484

• Resolution (bits): 8bit
• # Chan: 1
• Sample Rate: 500MSPS
• Interface: Par
• Analog Input Type: Diff-Bip
• Ain Range: 1.5 V p-p,Bip 0.75V
• Adc Architecture: Pipelined
• Pkg Type: CSP

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 clock duty cycle. A 5% tolerance is commonly
required on the clock duty cycle to maintain dynamic performance
characteristics. The AD9484 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 AD9484. When the DCS is on, noise and distortion perfor-
mance are nearly flat for a wide range of duty cycles.
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 15 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
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 35).
Treat the clock input as an analog signal in cases where aperture
jitter may affect the dynamic range of the AD9484. Separate the
power supplies for clock drivers 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
Application Note for more in-depth information about jitter
performance as it relates to ADCs (visit www.analog.com).
AD9484
CLOCK
1
50Ω RESISTOR IS OPTIONAL.
INPUT
A
) due only to aperture jitter (t
SNR Degradation = 20 × log
Figure 34. Single-Ended 1.8 V CMOS Input Clock (Up to 200 MHz)
50Ω
AN-501
0.1µF
1
V
CC
1kΩ
1kΩ
Application Note and the
AD951x
CMOS DRIVER
J
) can be calculated by
10
(1/2 × π × f
OPTIONAL
0.1µF
100Ω
A
0.1µF
AN-756
× t
J
)
CLK+
CLK–
AD9484
ADC
Rev. A | Page 16 of 24
POWER DISSIPATION AND POWER-DOWN MODE
As shown in Figure 18, the power dissipated by the AD9484 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.
By asserting PDWN (Pin 29) high, the AD9484 is placed in
standby mode or full power-down mode, as determined by the
contents of Serial Port Register 08. Reasserting the PDWN pin
low returns the AD9484 to its normal operational mode.
An additional standby mode is supported by means of varying
the clock input. When the clock rate falls below 50 MHz, the
AD9484 assumes a standby state. In this case, the biasing network
and internal reference remain on, but digital circuitry is powered
down. Upon reactivating the clock, the AD9484 resumes normal
operation after allowing for the pipeline latency.
DIGITAL OUTPUTS
Digital Outputs and Timing
The AD9484 differential outputs conform to the ANSI-644
LVDS standard on default power-up. This can be changed to a
low power, reduced signal option similar to the IEEE 1596.3
standard using the SPI. This LVDS standard can further reduce
the overall power dissipation of the device, which reduces the
power by ~39 mW. See the Memory Map section for more infor-
mation. The LVDS driver current is derived on chip and sets
the output current at each output equal to a nominal 3.5 mA.
A 100 Ω differential termination resistor placed at the LVDS
receiver inputs results in a nominal 350 mV swing at the receiver.
The AD9484 LVDS outputs facilitate interfacing with LVDS
receivers in custom ASICs and FPGAs that have LVDS capability
for superior switching performance in noisy environments.
Single point-to-point net topologies are recommended with a
100 Ω termination resistor placed as close to the receiver as
possible. No far-end receiver termination or poor differential
trace routing may result in timing errors. It is recommended
that the trace length be no longer than 24 inches and that the
130
120
110
100
90
80
70
60
50
40
30
1
10 BITS
8 BITS
RMS CLOCK JITTER REQUIREMENT
Figure 35. Ideal SNR vs. Input Frequency and Jitter
ANALOG INPUT FREQUENCY (MHz)
10
0.125ps
0.25ps
0.5ps
1.0ps
2.0ps
100
16 BITS
14 BITS
12 BITS
1000