AD9228 Analog Devices, AD9228 Datasheet - Page 23

• Manufacturer Part Number: AD9228
• Description: Quad, 12-Bit, 40/65 MSPS Serial LVDS 1.8 V A/D Converter
• Manufacturer: Analog Devices

Specifications of AD9228

• Resolution (bits): 12bit
• # Chan: 4
• Sample Rate: 65MSPS
• Interface: LVDS,Ser
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: 2 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

Data Sheet
CLOCK INPUT CONSIDERATIONS
For optimum performance, the AD9228 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
and require no additional biasing.
Figure 52 shows a preferred method for clocking the AD9228. 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 AD9228 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 AD9228,
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 53. The AD9510/
AD9511/AD9512/AD9513/AD9514/AD9515
drivers offers excellent jitter performance.
CLK+
CLK–
CLK+
CLK–
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
CLK+
50Ω*
*50Ω RESISTORS ARE OPTIONAL
50Ω*
*50Ω RESISTORS ARE OPTIONAL
50Ω
Figure 52. Transformer-Coupled Differential Clock
0.1µF
0.1µF
0.1µF
50Ω*
0.1µF
0.1µF
50Ω*
Figure 54. Differential LVDS Sample Clock
Figure 53. Differential PECL Sample Clock
100Ω
CLK
CLK
CLK
CLK
PECL DRIVER
LVDS DRIVER
ADT1-1WT, 1:1Z
Mini-Circuits
XFMR
0.1µF
AD9510/AD9511/
AD9512/AD9513/
AD9514/AD9515
AD9514/AD9515
AD9510/AD9511/
AD9512/AD9513/
240Ω
®
0.1µF
0.1µF
SCHOTTKY
240Ω
HSM2812
DIODES:
100Ω
100Ω
0.1µF
0.1µF
0.1µF
0.1µF
family of clock
CLK+
CLK–
CLK+
CLK–
CLK+
CLK–
AD9228
AD9228
AD9228
ADC
ADC
ADC
Rev. E | Page 23 of 56
in parallel with a 39 kΩ resistor (see Figure 55). Although the
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.
CLK+
CLK+
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 performance
characteristics. The AD9228 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 AD9228. When the DCS is on, noise and distortion perfor-
mance 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 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
20 MHz nominal. The loop has a time constant associated with
it that must be considered in applications where the clock rate
can change dynamically. This requires a wait time of 1.5 μs to
5 μs after a dynamic clock frequency increase (or decrease)
before the DCS loop is relocked to the input signal. During the
period that the loop is not locked, the DCS loop is bypassed and
the internal device timing is dependent on the duty cycle of the
input clock signal. In such applications, it may be appropriate to
disable the duty cycle stabilizer. In all other applications,
enabling the DCS circuit is recommended to maximize ac
performance.
*50Ω RESISTOR IS OPTIONAL
50Ω*
50Ω*
*50Ω RESISTOR IS OPTIONAL
0.1µF
0.1µF
Figure 55. Single-Ended 1.8 V CMOS Sample Clock
Figure 56. 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
AD9510/AD9511/
AD9512/AD9513/
AD9514/AD9515
0.1µF
OPTIONAL
OPTIONAL
100Ω
100Ω
39kΩ
0.1µF
0.1µF
0.1µF
CLK+
CLK–
CLK+
CLK–
AD9228
AD9228
AD9228
ADC
ADC