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ADN2811

Rev. B | Page 18 of 20

DC-COUPLED APPLICATION

The inputs to the ADN2811 can also be dc-coupled. This may

be necessary in burst mode applications where there are long

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

inputs to the ADN2811 are dc-coupled, care must be taken not

to violate the input range and common-mode level

requirements of the ADN2811 (see Figure 23, Figure 24, and

Figure 25). If dc-coupling is required and the output levels of

the TIA do not adhere to the levels shown in Figure 24 and

Figure 25, there needs to be level shifting and/or an attenuator

between the TIA outputs and the ADN2811 inputs.

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 ADN2811 stays

within 1000 ppm of the VCO center frequency as long as there

is a valid reference clock. The LOL pin toggles at a rate of

several kHz because the LOL pin toggles between a Logic 1 and

a Logic 0 while the frequency loop and phase loop swap control

of the VCO. The chain of events is as follows:

•

The ADN2811 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 a 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.

Ω

ADN2811

0.1

µ F

NIN

Ω

TIA

VREF

VCC

Ω

Figure 23. ADN2811 with DC-Coupled Inputs

VCM = 0.4V MIN

(DC-COUPLED)

VSE = 5mV MIN

NIN

V p-p = PIN – NIN = 2

× V

SE = 10mV AT SENSITIVITY

INPUT (V)

Figure 24. Minimum Allowed DC-Coupled Input Levels

INPUT (V)

NIN

VCM = 0.6V

(DC-COUPLED)

VSE = 1.2V MAX

V p-p = PIN – NIN = 2

× V

SE = 2.4V MAX

Figure 25. Maximum Allowed DC-Coupled Input Levels

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