ADN2819 Analog Devices, Inc., ADN2819 Datasheet - Page 15

• Manufacturer Part Number: ADN2819
• Description: Multirate To 2.7 Gb/s Clock And Data Recovery Ic With Integrated Limiting Amp
• Manufacturer: Analog Devices, Inc.

The ADN2819 can accept any of the following reference clock
frequencies: 19.44 MHz, 38.88 MHz, and 77.76 MHz at LVTTL/
LVCMOS/LVPECL/LVDS levels, or 155.52 MHz at LVPECL/
LVDS levels via the REFCLKN/P inputs, independent of data
rate (including Gigabit Ethernet and wrapper rates). The input
buffer accepts any differential signal with a peak-to-peak
differential amplitude of greater than 100 mV (e.g., LVPECL or
LVDS) or a standard single-ended low voltage TTL input,
providing maximum system flexibility. The appropriate division
ratio can be selected using the REFSEL0/1 pins, according to
Table 6. Phase noise and duty cycle of the reference clock are
not critical, and 100 ppm accuracy is sufficient.
Table 6. Reference Frequency Selection
REFSEL
1
1
1
1
0
REFSEL[1..0]
00
01
10
11
XX
19.44MHz
Figure 19. Crystal Oscillator Configuration
REFCLKN
REFCLKP
NC
VCC
XO1
XO2
Applied Reference Frequency
(MHz)
19.44
38.88
77.76
155.52
REFCLKP/N Inactive. Use 19.44 MHz
XTAL on Pins XO1, XO2 (pull REFCLKP
to VCC)
REFSEL
ADN2819
100kΩ
OSCILLATOR
CRYSTAL
100kΩ
BUFFER
VCC/2
1000
500
Figure 20. Transfer Function LOL
Rev. B | Page 15 of 24
1
0
LOL
An on-chip oscillator to be used with an external crystal is also
provided as an alternative to using the REFCLKN/P inputs.
Details of the recommended crystal are given in Table 7.
Table 7. Required Crystal Specifications
Parameter
Mode
Frequency/Overall Stability
Frequency Accuracy
Temperature Stability
Aging
ESR
REFSEL must be tied to VCC when the REFCLKN/P inputs are
active, or tied to VEE when the oscillator is used. No connection
between the XO pin and the REFCLK input is necessary (see
Figure 17, Figure 18, and Figure 19). Note that the crystal should
operate in series resonant mode, which renders it insensitive to
external parasitics. No trimming capacitors are required.
LOCK DETECTOR OPERATION
The lock detector monitors the frequency difference between
the VCO and the reference clock, and deasserts the loss of lock
signal when the VCO is within 500 ppm of center frequency.
This enables the phase loop, which then maintains phase lock,
unless the frequency error exceeds 0.1%. Should this occur, the
loss of lock signal is reasserted and control returns to the fre-
quency loop, which will reacquire and maintain a stable clock
signal at the output. The frequency loop requires a single exter-
nal capacitor between CF1 and CF2. The capacitor specification
is given in Table 8.
Table 8. Recommended C
Parameter
Temperature Range
Capacitance
Leakage
Rating
500
1000
f
(ppm)
VCO
ERROR
F
Capacitor Specification
Value
Series Resonant
19.44 MHz ± 100 ppm
±100 ppm
±100 ppm
±100 ppm
50 Ω max
Value
–40°C to +85°C
>3.0 µF
<80 nA
>6.3 V
ADN2819