AD9648 Analog Devices, AD9648 Datasheet - Page 29

• Manufacturer Part Number: AD9648
• Description: 14-Bit, 125 MSPS/105 MSPS, 1.8 V Dual Analog-to-Digital Converter
• Manufacturer: Analog Devices

Specifications of AD9648

• Resolution (bits): 14bit
• # Chan: 2
• Sample Rate: 125MSPS
• Interface: Par
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p,2 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

CLOCK
If a low jitter clock source is not available, another option is to
ac couple a differential PECL signal to the sample clock input
pins, as shown in Figure 55. The AD9510/AD9511/AD9512/
AD9513/AD9514/AD9515/AD9516/AD9517
excellent jitter performance.
CLOCK
A third option is to ac couple a differential LVDS signal to the
sample clock input pins, as shown in Figure 56. The AD9510/
AD9511/AD9512/AD9513/AD9514/AD9515/AD9516/AD9517
clock drivers offer excellent jitter performance.
In some applications, it may be acceptable to drive the sample
clock inputs with a single-ended 1.8 V CMOS signal. In such
applications, drive the CLK+ pin directly from a CMOS gate, and
bypass the CLK− pin to ground with a 0.1 μF capacitor (see
Figure 57).
CLOCK
CLOCK
CLOCK
INPUT
INPUT
INPUT
INPUT
INPUT
Figure 57. Single-Ended 1.8 V CMOS Input Clock (Up to 200 MHz)
50kΩ
50kΩ
Figure 56. Differential LVDS Sample Clock (Up to 1 GHz)
Figure 55. Differential PECL Sample Clock (Up to 1 GHz)
50Ω
1
50Ω RESISTOR IS OPTIONAL.
0.1µF
1
0.1µF
0.1µF
50kΩ
0.1µF
0.1µF
50kΩ
V
CC
1kΩ
1kΩ
LVDS DRIVER
PECL DRIVER
CMOS DRIVER
AD951x
AD951x
AD951x
240Ω
OPTIONAL
240Ω
100Ω
0.1µF
100Ω
100Ω
0.1µF
0.1µF
0.1µF
0.1µF
0.1µF
clock drivers offer
CLK+
CLK–
CLK+
CLK–
CLK+
CLK–
ADC
ADC
ADC
Rev. 0 | Page 29 of 44
Input Clock Divider
The
to divide the input clock by integer values between 1 and 8.
The
external SYNC input. Bit 1 and Bit 2 of Register 0x3A allow the
clock divider to be resynchronized on every SYNC signal or
only on the first SYNC signal after the register is written. A
valid SYNC causes the clock divider to reset to its initial state.
This synchronization feature allows multiple parts to have their
clock dividers aligned to guarantee simultaneous input sampling.
Clock Duty Cycle
Typical high speed ADCs use both clock edges to generate
a variety of internal timing signals and, as a result, may be
sensitive to clock duty cycle. Commonly, a ±5% tolerance is
required on the clock duty cycle to maintain dynamic
performance characteristics.
The
the nonsampling (falling) edge, providing an internal clock
signal with a nominal 50% duty cycle. This allows the user to
provide a wide range of clock input duty cycles without affecting
the performance of the AD9648. Noise and distortion perform-
ance are nearly flat for a wide range of duty cycles with the DCS
on, as shown in Figure 58.
Jitter in the rising edge of the input is still of concern and is not
easily reduced by the internal stabilization circuit. The duty
cycle control loop does not function for clock rates less than
20 MHz, nominally. The loop has a time constant associated
with it that must be considered in applications in which the
clock rate can change dynamically. A wait time of 1.5 µs to 5 µs
is required after a dynamic clock frequency increase or decrease
before the DCS loop is relocked to the input signal.
80
75
70
65
60
55
50
45
40
AD9648
AD9648
AD9648
35
contains an input clock divider with the ability
clock divider can be synchronized using the
contains a duty cycle stabilizer (DCS) that retimes
40
Figure 58. SNR vs. DCS On/Off
SNR (DCS OFF)
POSITIVE DUTY CYCLE (%)
45
50
SNR (DCS ON)
55
60
AD9648
65