AD9626 Analog Devices, AD9626 Datasheet - Page 19

• Manufacturer Part Number: AD9626
• Description: 12-Bit, 170 MSPS/210 MSPS/250 MSPS, 1.8 V Analog-to-Digital Converter
• Manufacturer: Analog Devices

Specifications of AD9626

• Resolution (bits): 12bit
• # Chan: 1
• Sample Rate: 250MSPS
• Interface: Par
• Analog Input Type: Diff-Uni
• Ain Range: 1 V p-p,1.25 V p-p,1.5 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

CLOCK INPUT CONSIDERATIONS
For optimum performance, the AD9626 sample clock inputs
(CLK+ and CLK−) should be clocked with a differential signal.
This signal is typically ac-coupled into the CLK+ pin and the
CLK− pin via a transformer or capacitors. These pins are biased
internally and require no additional bias.
Figure 45 shows one preferred method for clocking the AD9626.
The low jitter clock source is converted from single-ended to
differential using an RF transformer. The back-to-back Schottky
diodes across the secondary transformer limit clock excursions
into the AD9626 to approximately 0.8 V p-p differential. This
helps prevent the large voltage swings of the clock from feeding
through to other portions of the AD9626 and preserves the fast
rise and fall times of the signal, which are critical to low jitter
performance.
If a low jitter clock is available, another option is to ac couple a
differential PECL signal to the sample clock input pins, as
shown in Figure 46. The AD9510/AD9511/AD9512/AD9513/
AD9514/AD9515
performance.
CLOCK
CLOCK
*50Ω RESISTORS ARE OPTIONAL.
INPUT
INPUT
CLOCK
CLOCK
INPUT
INPUT
CLOCK
INPUT
50Ω*
*50Ω RESISTORS ARE OPTIONAL.
50Ω*
Figure 45. Transformer-Coupled Differential Clock
50Ω
0.1µF
Figure 47. Differential LVDS Sample Clock
Figure 46. Differential PECL Sample Clock
50Ω*
0.1µF
0.1µF
0.1µF
0.1µF
50Ω*
family of clock drivers offers excellent jitter
100Ω
CLK
CLK
LVDS DRIVER
CLK
CLK
PECL DRIVER
ADT1–1WT, 1:1Z
MINI-CIRCUITS
AD9510/AD9511/
AD9512/AD9513/
AD9514/AD9515
XFMR
0.1µF
AD9510/AD9511/
AD9512/AD9513/
AD9514/AD9515
240Ω
0.1µF
0.1µF
SCHOTTKY
HSM2812
DIODES:
240Ω
100Ω
100Ω
0.1µF
0.1µF
0.1µF
0.1µF
CLK+
CLK–
AD9626
ADC
CLK+
CLK–
CLK+
CLK–
AD9626
AD9626
ADC
ADC
Rev. 0 | Page 19 of 36
In some applications, it is acceptable to drive the sample clock
inputs with a single-ended CMOS signal. In such applications,
CLK+ should be directly driven 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 48). Although the
CLK+ input circuit supply is AVDD (1.8 V), this input is
designed to withstand input voltages up to 3.3 V, making the
selection of the drive logic voltage very flexible.
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 per-
formance characteristics. The AD9626 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 AD9626. When the DCS is on, 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 can be affected when operated in this mode. See the
AD9626 Configuration Using the SPI section for more details
on using this feature.
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 eight clock cycles
to allow the DLL to acquire and lock to the new rate.
CLOCK
*50Ω RESISTOR IS OPTIONAL.
*50Ω RESISTOR IS OPTIONAL.
INPUT
CLOCK
INPUT
0.1µF
50Ω*
Figure 48. Single-Ended 1.8 V CMOS Sample Clock
Figure 49. Single-Ended 3.3 V CMOS Sample Clock
0.1µF
50Ω*
0.1µF
0.1µF
CLK
CLK
CLK
CLK
CMOS DRIVER
CMOS DRIVER
AD9510/AD9511/
AD9512/AD9513/
AD9514/AD9515
AD9510/AD9511/
AD9512/AD9513/
AD9514/AD9515
0.1µF
OPTIONAL
OPTIONAL
100Ω
100Ω
39kΩ
0.1µF
0.1µF
0.1µF
CLK+
CLK–
CLK+
CLK–
AD9626
AD9626
AD9626
ADC
ADC