ADN2811 Analog Devices, Inc., ADN2811 Datasheet - Page 17

• Manufacturer Part Number: ADN2811
• Description: Oc-48/oc-48 Fec Clock And Data Recovery Ic With Integrated Limiting Amp
• Manufacturer: Analog Devices, Inc.

CHOOSING AC-COUPLING CAPACITORS
The choice of ac-coupling capacitors at the input (PIN, NIN)
and output (DATAOUTP, DATAOUTN) of the ADN2811 must
be chosen carefully. 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 (see Figure 22), causing pattern
dependent jitter (PDJ).
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 DISCHARGE TO THE V
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 ADN2811. THE QUANTIZER WILL BE
WHICH EFFECTIVELY INTRODUCES A DIFFERENTIAL DC OFFSET ACROSS THE AC COUPLING CAPACITORS.
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 = ADN2811 QUANTIZER THRESHOLD
V
DIFF
= V2–V2b
1
TIA
V1b
V1
C
C
IN
IN
2
V2b
V2
NIN
PIN
Figure 22. Example of Baseline Wander
50Ω
50Ω
V
REF
Rev. B | Page 17 of 20
+
LIMAMP
ADN2811
3
For the ADN2811 to work robustly at OC-48, 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.
CDR
4
C
C
OUT
OUT
DATAOUTP
DATAOUTN
REF
LEVEL,
VTH
V
REF
ADN2811