ADN2807 Analog Devices, Inc., ADN2807 Datasheet - Page 17

• Manufacturer Part Number: ADN2807
• Description: 155/622 Mb/s Clock And Data Recovery Ic With Integrated Limiting Amp
• Manufacturer: Analog Devices, Inc.

CHOOSING AC COUPLING CAPACITORS
The ac coupling capacitors at the input (PIN, NIN) and output
(DATAOUTP, DATAOUTN) of the ADN2807 must be chosen
so that the device works properly at both OC-3 and OC-12 data
rates. When choosing the capacitors, the time constant formed
with the two 50 Ω resistors in the signal path must be
considered. When a large number of consecutive identical digits
(CIDs) are applied, the capacitor voltage can drop due to
baseline wander (Figure 23), causing pattern dependent jitter
(PDJ). For the ADN2807 to work robustly at both OC-3 and
OC-12, a minimum capacitor of 0.1 µF to PIN/NIN and 0.1 µF
on DATAOUTP/DATAOUTN should be used. This is based on
the assumption that 1000 CIDs must be tolerated, and that the
PDJ should be limited to 0.01 UI p-p.
DC-COUPLED APPLICATION
The inputs to the ADN2807 can also be dc-coupled. This may
be necessary in burst mode applications where there are long
periods of CIDs, and where baseline wander cannot be
tolerated. If the inputs to the ADN2807 are dc-coupled, care
must be taken not to violate the input range and common-mode
level requirements of the ADN2807 (Figure 24 to Figure 26). If
dc coupling is required and the output levels of the TIA do not
adhere to the levels shown in Figure 25 and Figure 26, there
must be level shifting and/or an attenuator between the TIA
outputs and the ADN2807 inputs.
NOTES
1. DURING DATA PATTERNS WITH HIGH TRANSITION DENSITY, DIFFERENTIAL DC VOLTAGE AT V1 AND V2 IS 0.
2. WHEN THE OUTPUT OF THE TIA GOES TO CID, V1 AND V1b ARE DRIVEN TO DIFFERENT DC LEVELS. V2 AND V2b
3. WHEN THE BURST OF DATA STARTS AGAIN,THE DIFFERENTIAL DC OFFSET ACROSS THE AC COUPLING CAPACITORS IS APPLIED TO THE INPUT LEVELS,
4. THE DC OFFSET SLOWLY DISCHARGES UNTIL THE DIFFERENTIAL INPUT VOLTAGE EXCEEDS THE SENSITIVITY OF THE ADN2807. THE QUANTIZER WILL BE
DISCHARGE TO THE V
CAUSING A DC SHIFT IN THE DIFFERENTIAL INPUT. THIS SHIFT IS LARGE ENOUGH SUCH THAT ONE OF THE STATES, EITHER HIGH OR LOW DEPENDING ON
THE LEVELS OF V1 AND V1b WHEN THE TIA WENT TO CID, IS CANCELLED OUT. THE QUANTIZER WILL NOT RECOGNIZE THIS AS A VALID STATE.
ABLE TO RECOGNIZE BOTH HIGH AND LOW STATES AT THIS POINT.
V
V1b
V2b
DIFF
V1
V2
VTH = ADN2807 QUANTIZER THRESHOLD
V
DIFF
= V2–V2b
1
TIA
REF
LEVEL, WHICH EFFECTIVELY INTRODUCES A DIFFERENTIAL DC OFFSET ACROSS THE AC COUPLING CAPACITORS.
V1b
V1
C
C
IN
IN
2
V2b
V2
NIN
PIN
Figure 23. Example of Baseline Wander
50Ω
50Ω
V
REF
Rev. A | Page 17 of 20
+
LIMAMP
ADN2807
3
LOL TOGGLING DURING LOSS OF INPUT DATA
If the input data stream is lost due to a break in the optical link
(or for any reason), the clock output from the ADN2807 stays
within 1000 ppm of the VCO center frequency as long as there
is a valid reference clock. The LOL pin will toggle at a rate of
several kHz. This is because the LOL pin will toggle between a
Logic 1 and Logic 0 while the frequency loop and phase loop
swap control of the VCO. The chain of events is as follows:
•
•
•
•
•
The ADN2807 is locked to the input data stream; LOL = 0.
The input data stream is lost due to a break in the link. The
VCO frequency drifts until the frequency error is greater
than 1000 ppm. LOL is asserted to a Logic 1 as control of
the VCO is passed back to the frequency loop.
The frequency loop pulls the VCO to within 500 ppm of its
center frequency. Control of the VCO is passed back to the
phase loop and LOL is deasserted to Logic 0.
The phase loop tries to acquire, but there is no input data
present so the VCO frequency drifts.
The VCO frequency drifts until the frequency error is
greater than 1000 ppm. LOL is asserted to a Logic 1 as
control of the VCO is passed back to the frequency loop.
This process is repeated until a valid input data stream is
re-established.
CDR
4
C
C
OUT
OUT
DATAOUTP
DATAOUTN
VTH
V
REF
ADN2807