ADN2806 Analog Devices, Inc., ADN2806 Datasheet - Page 13

• Manufacturer Part Number: ADN2806
• Description: 622 Mbps Clock And Data Recovery Ic
• Manufacturer: Analog Devices, Inc.

FUNCTIONAL DESCRIPTION
FREQUENCY ACQUISITION
The ADN2806 acquires frequency from the data. The lock
detector circuit compares the frequency of the VCO and the
frequency of the incoming data. When these frequencies differ
by more than 1000 ppm, LOL is asserted. This initiates a frequency
acquisition cycle. When the VCO frequency is within 250 ppm
of the data frequency, LOL is deasserted.
Once LOL is deasserted, the frequency-locked loop is turned
off. The PLL/DLL pulls the VCO frequency in the rest of the
way until the VCO frequency equals the data frequency.
The frequency loop requires a single external capacitor between
CF1 and CF2, Pin 14 and Pin 15. A 0.47 μF ± 20%, X7R ceramic
chip capacitor with <10 nA leakage current is recommended.
Leakage current of the capacitor can be calculated by dividing
the maximum voltage across the 0.47 μF capacitor, ~3 V, by the
insulation resistance of the capacitor. The insulation resistance
of the 0.47 μF capacitor should be greater than 300 MΩ.
INPUT BUFFER AMPLIFIER
The input buffer has differential inputs (PIN/NIN), which are
internally terminated with 50 Ω to an on-chip voltage reference
(VREF = 2.5 V typically). The minimum differential input level
required to achieve a BER of 10
LOCK DETECTOR OPERATION
The lock detector on the ADN2806 has three modes of
operation: normal mode, REFCLK mode, and static LOL mode.
Normal Mode
In normal mode, the ADN2806 is a CDR that locks onto a
622 Mbps data rate without the use of a reference clock as an
acquisition aid. In this mode, the lock detector monitors the
frequency difference between the VCO and the input data
frequency and deasserts the loss of lock signal, which appears
on Pin 16, LOL, when the VCO is within 250 ppm of the data
frequency. This enables the D/PLL, which pulls the VCO
frequency in the remaining amount and acquires phase lock.
Once locked, if the input frequency error exceeds 1000 ppm
(0.1%), the loss-of-lock signal is reasserted and control returns
to the frequency loop, which begins a new frequency
acquisition. The LOL pin remains asserted until the VCO locks
onto a valid input data stream to within 250 ppm frequency
error. This hysteresis is shown in Figure 15.
–1000
Figure 15. Transfer Function of LOL
–250
1
0
LOL
−10
is 200 mV p-p.
250
1000
f
(ppm)
VCO
ERROR
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LOL Detector Operation Using a Reference Clock
In REFCLK mode, a reference clock is used as an acquisition aid
to lock the ADN2806 VCO. Lock-to-reference mode is enabled
by setting CTRLA[0] to 1. The user also needs to write to the
CTRLA[7, 6] and CTRLA[5:2] bits to set the reference
frequency range and the divide ratio of the data rate with
respect to the reference frequency. For more details, see the
Reference Clock (Optional) section. In this mode, the lock
detector monitors the difference in frequency between the
divided down VCO and the divided down reference clock. The
loss-of-lock signal, which appears on Pin 16, LOL, is deasserted
when the VCO is within 250 ppm of the desired frequency. This
enables the D/PLL, which pulls the VCO frequency in the
remaining amount with respect to the input data and acquires
phase lock. Once locked, if the input frequency error exceeds
1000 ppm (0.1%), the loss-of-lock signal is reasserted and
control returns to the frequency loop, which reacquires with
respect to the reference clock. The LOL pin remains asserted
until the VCO frequency is within 250 ppm of the desired
frequency. This hysteresis is shown in Figure 15.
Static LOL Mode
The ADN2806 implements a static LOL feature that indicates if
a loss-of-lock condition has ever occurred. This feature remains
asserted, even if the ADN2806 regains lock, until the static LOL
bit is manually reset. The I
LOL bit. If there is ever an occurrence of a loss-of-lock condition,
this bit is internally asserted to logic high. The MISC[4] bit remains
high even after the ADN2806 has reacquired lock to a new data
rate. This bit can be reset by writing a 1 followed by 0 to I
Register Bit CTRLB[6]. Once reset, the MISC[4] bit remains
deasserted until another loss-of-lock condition occurs.
Writing a 1 to I
Pin 16, to become a static LOL indicator. In this mode, the LOL
pin mirrors the contents of the MISC[4] bit and has the
functionality described in the previous paragraph. The CTRLB[7]
bit defaults to 0. In this mode, the LOL pin operates in the
normal operating mode, that is, it is asserted only when the
ADN2806 is in acquisition mode and deasserts when the
ADN2806 has reacquired lock.
SQUELCH MODES
Two modes for the SQUELCH pin are available with the
ADN2806: squelch data outputs and clock outputs mode and
squelch data outputs or clock outputs mode. Squelch data outputs
and clock outputs mode is selected when CTRLC[1] is 0 (default
mode). In this mode, when the SQUELCH input, Pin 27, is driven
to a TTL high state, both the data outputs (DATAOUTN and
DATAOUTP) and the clock outputs (CLKOUTN and CLKOUTP)
are set to the zero state to suppress downstream processing. If
the squelch function is not required, Pin 27 should be tied to VEE.
2
C Register Bit CTRLB[7] causes the LOL pin,
2
C register bit, MISC[4], is the static
ADN2806
2
C