Data Sheet
ADN2814
Rev. C | Page 19 of 28
100k
Ω
VCC/2
100k
Ω
ADN2814
REFCLKP
10
11
REFCLKN
BUFFER
04949-
021
Figure 21. Differential REFCLK Configuration
100k
Ω
VCC/2
100k
Ω
ADN2814
REFCLKP
OUT
REFCLKN
BUFFER
VCC
CLK
OSC
04949-
022
Figure 22. Single-Ended REFCLK Configuration
100k
Ω
VCC/2
100k
Ω
ADN2814
REFCLKP
10
11
NC
REFCLKN
BUFFER
VCC
04949-
023
Figure 23. No REFCLK Configuration
The two uses of the reference clock are mutually exclusive. The
reference clock can be used either as an acquisition aid for the
ADN2814 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 I2C Register
Bit CTRLA[0]. Fine data rate readback mode is enabled by
writing a 1 to I2C Register Bit CTRLA[1]. Writing a 1 to both of
these bits at the same time causes an indeterminate state and is
not supported.
Using the Reference Clock to Lock onto Data
In this mode, the ADN2814 locks onto a frequency derived
from the reference clock according to
Data Rate/2CTRLA[5:2] = REFCLK/2CTRLA[7:6]
The user must know exactly what the data rate is and provide a
reference clock that is a function of this rate. The ADN2814 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 ADN2814 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 ADN2814 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]
Range (MHz)
CTRLA[5:2]
Ratio
00
10 to 20
0000
1
01
20 to 40
0001
2
10
40 to 80
n
2n
11
80 to 160
1000
256
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_FREF, where DIV_FREF represents the
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] would be set to [0101], that is, 5, because
622.08 Mb/s/19.44 MHz = 25
In this mode, if the ADN2814 loses lock for any reason, it relocks
onto the reference clock and continues to output a stable clock.
While the ADN2814 is operating in lock-to-reference mode, if
the user ever changes the reference frequency, the FREF range
(CTRLA[7:6]), or the FREF ratio (CTRLA[5:2]), this must be
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 ADN2814 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
ADN2814 data rate measurement. For example, if a 100 ppm
accuracy reference clock is used, the total accuracy of the
measurement is within 200 ppm.
The reference clock can range from 10 MHz and 160 MHz.
The ADN2814 expects a reference clock between 10 MHz
and 20 MHz by default. If it is between 20 MHz and 40 MHz,
40 MHz and 80 MHz, or 80 MHz and 160 MHz, the user needs