AD9639 Analog Devices, Inc., AD9639 Datasheet - Page 19

• Manufacturer Part Number: AD9639
• Description: Quad, 12-bit, 170 Msps/210 Msps Serial Output 1.8 V Adc
• Manufacturer: Analog Devices, Inc.

CLOCK INPUT CONSIDERATIONS
For optimum performance, the AD9639 sample clock inputs
(CLK+ and CLK−) should be clocked with a differential signal.
This signal is typically ac-coupled to the CLK+ and CLK− pins
via a transformer or capacitors. These pins are biased internally
to 1.2 V and require no additional biasing.
Figure 43 shows a preferred method for clocking the AD9639. The
low jitter clock source is converted from a single-ended signal
to a differential signal using an RF transformer. The back-to-
back Schottky diodes across the secondary transformer limit
clock excursions into the AD9639 to approximately 0.8 V p-p
differential. This helps to prevent the large voltage swings of the
clock from feeding through to other portions of the AD9639,
and it preserves the fast rise and fall times of the signal, which
are critical to low jitter performance.
Another option is to ac-couple a differential PECL signal to the
sample clock input pins as shown in Figure 44. The AD9510/
AD9511/AD9512/AD9513/AD9514/AD9515/AD9516/AD9518
family of clock drivers offers excellent jitter performance.
In some applications, it is acceptable to drive the sample clock
inputs with a single-ended CMOS signal. In such applications,
CLK+ should be driven directly from a CMOS gate, and the
CLK− pin should be bypassed to ground with a 0.1 μF capacitor
in parallel with a 39 kΩ resistor (see Figure 46). Although the
CLK+
CLK+
CLK–
CLK–
CLK+
*50Ω RESISTORS ARE OPTIONAL.
50Ω*
*50Ω RESISTORS ARE OPTIONAL.
50Ω*
Figure 43. Transformer-Coupled Differential Clock
50Ω
0.1µF
0.1µF
50Ω*
0.1µF
0.1µF
50Ω*
0.1µF
Figure 45. Differential LVDS Sample Clock
Figure 44. Differential PECL Sample Clock
ADT1-1WT, 1:1Z
CLK
CLK
CLK
CLK
PECL DRIVER
LVDS DRIVER
0.1µF
XFMR
AD9510/AD9511/
AD9512/AD9513/
AD9514/AD9515/
AD9516/AD9518
AD9510/AD9511/
AD9512/AD9513/
AD9514/AD9515/
AD9516/AD9518
240Ω
0.1µF
0.1µF
SCHOTTKY
HSMS-2812
DIODES:
240Ω
100Ω
100Ω
0.1µF
0.1µF
0.1µF
0.1µF
CLK+
CLK–
AD9639
CLK+
CLK–
CLK+
CLK–
ADC
AD9639
AD9639
ADC
ADC
Rev. 0 | Page 19 of 36
CLK+ input circuit supply is AVDD (1.8 V), this input is
designed to withstand input voltages of up to 3.3 V and,
therefore, offers several selections for the drive logic voltage.
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 the clock duty cycle. Commonly, a 5% tolerance
is required on the clock duty cycle to maintain dynamic perfor-
mance characteristics.
The AD9639 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 AD9639.
When the DCS is on (default), noise and distortion performance
are nearly flat for a wide range of duty cycles. However, some
applications may require the DCS function to be off. If so, keep
in mind that the dynamic range performance may be affected
when operated in this mode. See the Memory Map section for
more details on using this feature.
Jitter in the rising edge of the input is an important concern,
and it is not reduced by the internal stabilization circuit. The
duty cycle control loop does not function for clock rates of less
than 50 MHz nominal. It is not recommended that this ADC
clock be dynamic in nature. Moving the clock around dynami-
cally requires long wait times for the back end serial capture to
retime and resynchronize to the receiving logic. This long time
constant far exceeds the time that it takes for the DCS and the
PLL to lock and stabilize. Only in rare applications would it be
necessary to disable the DCS circuitry in the clock register (see
Address 0x09 in Table 15). Keeping the DCS circuit enabled is
recommended to maximize ac performance.
CLK+
CLK+
*50Ω RESISTOR IS OPTIONAL.
50Ω*
*50Ω RESISTOR IS OPTIONAL.
50Ω*
0.1µF
0.1µF
Figure 46. Single-Ended 1.8 V CMOS Sample Clock
Figure 47. Single-Ended 3.3 V CMOS Sample Clock
0.1µF
0.1µF
CLK
CLK
CLK
CLK
CMOS DRIVER
CMOS DRIVER
AD9510/AD9511/
AD9512/AD9513/
AD9514/AD9515/
AD9516/AD9518
AD9510/AD9511/
AD9512/AD9513/
AD9514/AD9515/
AD9516/AD9518
0.1µF
OPTIONAL
OPTIONAL
100Ω
100Ω
39kΩ
0.1µF
0.1µF
0.1µF
CLK+
CLK–
CLK+
CLK–
AD9639
AD9639
AD9639
ADC
ADC