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AD9262
Rev. A | Page 18 of 32
External Reference Operation
If an external reference is desired, the internal reference can be
an application using th
e ADR130B as a stable external reference.
0.5V
ADR130B
TO CURRENT
GENERATOR
0.1F
10F
AVDD
10k
07
77
2-
0
45
Figure 49. External Reference Configuration
CLOCK INPUT CONSIDERATIONS
The AD9262 offers two modes of sourcing the ADC sample
clock (CLK+ and CLK). The first mode uses an on-chip clock
multiplier that accepts a reference clock operating at the lower
input frequency. The on-chip phase-locked loop (PLL) then
multiplies the reference clock to a higher frequency, which is
then used to generate all the internal clocks required by the ADC
The clock multiplier provides a high quality clock that meets
the performance requirements of most applications. Using the
on-chip clock multiplier removes the burden of generating and
distributing the high speed clock.
The second mode bypasses the clock multiplier circuitry and
allows the clock to be directly sourced. This mode enables the
user to source a very high quality clock directly to the Σ-Δ
modulator. Sourcing the ADC clock directly may be necessary
in demanding applications that require the lowest possible ADC
SNR performance for the various PLL settings.
In either case, when using the on-chip clock multiplier or sourcing
the high speed clock directly, it is necessary that the clock
source have low jitter to maximize the ADC noise performance.
High speed, high resolution ADCs are sensitive to the quality of
the clock input. As jitter increases, the SNR performance of the
AD9262 degrades from that specified
in Table 2. The jitter
inherent in the part due to the PLL root sum squares with any
external clock jitter, thereby degrading performance. To prevent
jitter from dominating the performance of the AD9262, the input
clock source should be no greater than 1 ps rms of jitter.
if the inputs are dc-coupled, it is important to maintain the
specified 450 mV input common-mode voltage. Each input pin
can safely swing from 200 mV p-p to 1 V p-p single-ended
about the 450 mV common-mode voltage. The recommended
clock inputs are CMOS or LVPECL.
The specified clock rate of the Σ-Δ modulator, fMOD, is 640 MHz.
The clock rate possesses a direct relationship to the available
input bandwidth of the ADC.
Bandwidth = fMOD ÷ 64
In either case, using the on-chip clock multiplier to generate the
Σ-Δ modulator clock rate or directly sourcing the clock, any
deviation from 640 MHz results in a change in input band-
width. The input range of the clock is limited to 640 MHz ± 5%.
In situations where the AD9262 loses its clock and then later
regains it, it is important that the sample rate converter be reset
and reprogrammed before the desired output data rate is
achieved.
Direct Clocking
The default configuration of the AD9262 is for direct clocking
for clocking the AD9262. A 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 side of the transformer limits clock excursions into the
AD9262 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 AD9262 while preserving the
fast rise and fall times of the signal, which are critical to
achieving low jitter.
CLOCK
INPUT
XFMR
MINI-CIRCUITS
TC1-1-13M+, 1:1
SCHOTTKY
DIODES:
HSM2812
50
CLK+
CLK–
0.1F
ADC
AD9262
0.1F
0
77
72
-0
46
Figure 50. Transformer-Coupled Differential Clock
If a differential clock is not available, the AD9262 can be driven
by a single-ended signal into the CLK+ terminal with the CLK
terminal ac-coupled to ground.
Figure 51 shows the circuit
configuration.
SCHOTTKY
DIODES:
HSM2812
50
CLK+
CLK–
0.1F
ADC
AD9262
CLOCK
INPUT
0
77
72
-0
47
Figure 51. Single-Ended Clock
Another option is to ac couple a differential LVPECL signal to
family of clock drivers is recommended because it offers excellent
jitter performance.
100
240
501
150 RESISTORS ARE OPTIONAL.
501
CLK+
CLK–
0.1F
ADC
AD9262
CLOCK
INPUT
CLOCK
INPUT
0.1F
CLK
AD951x
LVPECL
DRIVER
CLK
0
7772
-0
48
Figure 52. Differential LVPECL Sample Clock