Preliminary Technical Data

ADN2814

Rev. PrA | Page 23 of 28

50

Ω

50

Ω

PIN

NIN

V1

V1b

V2

V2b

TIA

LIMAMP

CDR

+

–

VCC

DATAOUTP

DATAOUTN

1

V1

V1b

V2

V2b

23

4

VREF

VTH

ADN2814

VTH = ADN2814 QUANTIZER THRESHOLD

NOTES:

1. DURING DATA PATTERNS WITH HIGH TRANSITION DENSITY, DIFFERENTIAL DC VOLTAGE AT V1 AND V2 IS ZERO.

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 CANCELED OUT. THE QUANTIZER

DOES NOT RECOGNIZE THIS AS A VALID STATE.

4. THE DC OFFSET SLOWLY DISCHARGES UNTIL THE DIFFERENTIAL INPUT VOLTAGE EXCEEDS THE SENSITIVITY OF THE ADN2814. THE

QUANTIZER CAN RECOGNIZE BOTH HIGH AND LOW STATES AT THIS POINT.

Figure 25. Example of Baseline Wander

DC-COUPLED APPLICATION

The inputs to the ADN2814 can also be dc-coupled. This might

be necessary in burst mode applications, where there are long

periods of CIDs, and baseline wander cannot be tolerated. If the

inputs to the ADN2814 are dc-coupled, care must be taken not

to violate the input range and common-mode level require-

ments of the ADN2814 (see Figure 26 through Figure 28). If dc

coupling is required, and the output levels of the TIA do not

adhere to the levels shown in Figure 27, then level shifting

and/or an attenuator must be between the TIA outputs and the

ADN2814 inputs.

50

Ω

0.1

µF

50

Ω

3k

Ω

NIN

PIN

ADN2814

2.5V

VREF

50

Ω

50

Ω

TIA

VCC

Figure 26. DC-Coupled Application

PIN

(DC-COUPLED)

NIN

Figure 27. Minimum Allowed DC-Coupled Input Levels

PIN

(DC-COUPLED)

NIN

Figure 28. Maximum Allowed DC-Coupled Input Levels