ADN2813 Analog Devices, Inc., ADN2813 Datasheet - Page 19

• Manufacturer Part Number: ADN2813
• Description: Continuous Rate 10 Mb/s To 1.25 Gb/s Clock And Data Recovery Ic With Integrated Limiting Amp
• Manufacturer: Analog Devices, Inc.

The two uses of the reference clock are mutually exclusive. The
reference clock can be used either as an acquisition aid for the
ADN2813 to lock onto data or to measure the frequency of the
incoming data to within 0.01%. (There is the capability to
measure the data rate to approximately ±10% without the use of
a reference clock.) The modes are mutually exclusive because, in
the first use, the user knows exactly what the data rate is and
wants to force the part to lock onto only that data rate; in the
second use, the user does not know what the data rate is and
wants to measure it.
Lock-to-reference mode is enabled by writing a 1 to I
Bit CTRLA[0]. Fine data rate readback mode is enabled by
writing a 1 to I
of these bits at the same time causes an indeterminate state and
is not supported.
VCC
OSC
CLK
NC
VCC
Figure 22. Single-Ended REFCLK Configuration
Figure 21. Differential REFCLK Configuration
2
OUT
REFCLKN
C Register Bit CTRLA[1]. Writing a 1 to both
REFCLKP
REFCLKP
REFCLKN
Figure 23. No REFCLK Configuration
REFCLKP
REFCLKN
10
11
10
11
ADN2813
ADN2813
100kΩ
ADN2813
100kΩ
100kΩ
100kΩ
100kΩ
BUFFER
BUFFER
100kΩ
BUFFER
VCC/2
VCC/2
VCC/2
2
C Register
Rev. 0 | Page 19 of 28
Using the Reference Clock to Lock onto Data
In this mode, the ADN2813 locks onto a frequency derived
from the reference clock according to
The user must know exactly what the data rate is and provide a
reference clock that is a function of this rate. The ADN2813 can
still be used as a continuous rate device in this configuration,
provided that the user has the ability to provide a reference clock
that has a variable frequency (see Application Note AN-632).
The reference clock can be anywhere between 10 MHz and
160 MHz. By default, the ADN2813 expects a reference clock of
between 10 MHz and 20 MHz. If it is between 20 MHz and
40 MHz, 40 MHz and 80 MHz, or 80 MHz and 160 MHz, the
user needs to configure the ADN2813 to use the correct
reference frequency range by setting two bits of the CTRLA
register, CTRLA[7:6].
Table 11. CTRLA Settings
CTRLA[7:6]
00
01
10
11
The user can specify a fixed integer multiple of the reference
clock to lock onto using CTRLA[5:2], where CTRLA should be
set to the data rate/DIV_F
divided-down reference referred to the 10 MHz to 20 MHz
band. For example, if the reference clock frequency is
38.88 MHz and the input data rate is 622.08 Mb/s, CTRLA[7:6]
is set to [01] to give a divided-down reference clock of
19.44 MHz. CTRLA[5:2] is set to [0101], that is, 5, because
In this mode, if the ADN2813 loses lock for any reason, it relocks
onto the reference clock and continues to output a stable clock.
While the ADN2813 is operating in lock-to-reference mode, if
the user ever changes the reference frequency, the F
(CTRLA[7:6]) or the F
followed by writing a 0 to 1 transition into the CTRLA[0] bit to
initiate a new lock-to-reference command.
Using the Reference Clock to Measure Data Frequency
The user can also provide a reference clock to measure the
recovered data frequency. In this case, the user provides a
reference clock, and the ADN2813 compares the frequency of
the incoming data to the incoming reference clock and returns a
ratio of the two frequencies to 0.01% (100 ppm). The accuracy
error of the reference clock is added to the accuracy of the
ADN2813 data rate measurement. For example, if a 100 ppm
accuracy reference clock is used, the total accuracy of the
measurement is within 200 ppm.
Data Rate/2
622.08 Mb/s/19.44 MHz = 2
CTRLA[5:2]
Range (MHz)
10 to 20
20 to 40
40 to 80
80 to 160
REF
= REFCLK/2
ratio (CTRLA[5:2]), this must be
REF
, where DIV_F
5
CTRLA[5:2]
0000
0001
n
1000
CTRLA[7:6]
REF
represents the
ADN2813
REF
Ratio
1
2
2
256
range
n