AD9257 Analog Devices, AD9257 Datasheet - Page 21

• Manufacturer Part Number: AD9257
• Description: Octal, 14-Bit, 40/65 MSPS Serial LVDS 1.8 V A/D Converter
• Manufacturer: Analog Devices

Data Sheet
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 52. The AD9510/AD9511/AD9512/
AD9513/AD9514/AD9515/AD9516/AD9517
excellent jitter performance.
A third option is to ac couple a differential LVDS signal to the
sample clock input pins, as shown in Figure 53. 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 54).
Input Clock Divider
The
to divide the input clock by integer values between 1 and 8.
The
external SYNC input. Bit 0 and Bit 1 of Register 0x109 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.
AD9257
AD9257
contains an input clock divider with the ability
clock divider can be synchronized using the
CLOCK
CLOCK
CLOCK
CLOCK
CLOCK
INPUT
INPUT
INPUT
INPUT
INPUT
Figure 54. Single-Ended 1.8 V CMOS Input Clock (Up to 200 MHz)
50kΩ
50kΩ
Figure 53. Differential LVDS Sample Clock (Up to 520 MHz)
Figure 52. Differential PECL Sample Clock (Up to 520 MHz)
clock drivers offer
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
Rev. 0 | Page 21 of 40
AD951x
240Ω
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 vari-
ety 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 per-
formance of the AD9257. Noise and distortion performance are
nearly flat for a wide range of duty cycles with the DCS on.
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.
OPTIONAL
240Ω
100Ω
0.1µF
AD9257
100Ω
100Ω
0.1µF
0.1µF
0.1µF
0.1µF
0.1µF
contains a duty cycle stabilizer (DCS) that retimes
CLK+
CLK–
CLK+
CLK–
CLK+
CLK–
ADC
ADC
ADC
AD9257