ADN2811 Ver la hoja de datos (PDF) - Analog Devices
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ADN2811 Datasheet PDF : 20 Pages
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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).
ADN2811
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.
V1
CIN V2
PIN
ADN2811
+
50Ω
TIA
VREF LIMAMP
V1b CIN V2b
50Ω
CDR
NIN
COUT
DATAOUTP
COUT
DATAOUTN
1
2
V1
V1b
V2
V2b
VDIFF
VDIFF = V2–V2b
VTH = ADN2811 QUANTIZER THRESHOLD
3
4
VREF
VTH
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 VREF LEVEL,
WHICH EFFECTIVELY INTRODUCES A DIFFERENTIAL DC OFFSET ACROSS THE AC COUPLING CAPACITORS.
3. WHEN THE BURST OF DATA STARTS AGAIN,THE DIFFERENTIAL DC OFFSET ACROSS THE AC COUPLING CAPACITORS IS APPLIED TO THE INPUT LEVELS,
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.
4. THE DC OFFSET SLOWLY DISCHARGES UNTIL THE DIFFERENTIAL INPUT VOLTAGE EXCEEDS THE SENSITIVITY OF THE ADN2811. THE QUANTIZER WILL BE
ABLE TO RECOGNIZE BOTH HIGH AND LOW STATES AT THIS POINT.
Figure 22. Example of Baseline Wander
Rev. B | Page 17 of 20
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