Datasheet

Data Sheet AD9212
Rev. E | Page 23 of 56
CLOCK INPUT CONSIDERATIONS
For optimum performance, the AD9212 sample clock inputs
(CLK+ and CLK−) should be clocked with a dierential signal.
This signal is typically ac-coupled into the CLK+ and CLK− pins
via a transformer or capacitors. These pins are biased internally
and require no additional biasing.
Figure 51 shows the preferred method for clocking the AD9212.
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 AD9212 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 AD9212, and it preserves the
fast rise and fall times of the signal, which are critical to low
jitter performance.
0.1µF
0.1µF
0.1µF0.1µF
SCHOTTKY
DIODES:
HSM2812
CLK+
50
100
CLK–
CLK+
ADC
AD9212
Mini-Circuits
®
ADT1–1WT, 1:1Z
XFMR
05968-022
Figure 51. Transformer-Coupled Differential Clock
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 family of clock
drivers offers excellent jitter performance.
100
0.1µF
0.1µF
0.1µF
0.1µF
240240
AD9510/AD9511/
AD9512/AD9513/
AD9514/AD9515
50
1
50
1
CLK
CLK
1
50 RESISTORS ARE OPTIONAL.
CLK–
CLK+
ADC
AD9212
PECL DRIVER
05968-023
CLK+
CLK–
Figure 52. Differential PECL Sample Clock
100
0.1µF
0.1µF
0.1µF
0.1µF
50
1
LVDS DRIVER
50
1
CLK
CLK
1
50 RESISTORS ARE OPTIONAL.
CLK–
CLK+
ADC
AD9212
AD9510/AD9511/
AD9512/AD9513/
AD9514/AD9515
05968-024
CLK+
CLK–
Figure 53. Differential LVDS Sample Clock
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 F capacitor
in parallel with a 39 kΩ resistor (see Figure 54). 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, making the
selection of the drive logic voltage very flexible.
0.1µF
0.1µF
0.1µF
39k
CMOS DRIVER
50
1
OPTIONAL
100
0.1µF
CLK
CLK
1
50 RESISTOR IS OPTIONAL.
CLK–
CLK+
ADC
AD9212
AD9510/AD9511/
AD9512/AD9513/
AD9514/AD9515
05968-025
CLK+
Figure 54. Single-Ended 1.8 V CMOS Sample Clock
0.1µF
0.1µF
0.1µF
CMOS DRIVER
50
1
OPTIONAL
100
CLK
CLK
1
50 RESISTOR IS OPTIONAL.
0.1µF
CLK–
CLK+
ADC
AD9212
AD9510/AD9511/
AD9512/AD9513/
AD9514/AD9515
05968-026
C
LK+
Figure 55. Single-Ended 3.3 V CMOS Sample Clock
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 AD9212 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 AD9212. 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.
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.