ADN2811
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.
VCC
TIA
50Ω
50Ω
0.1µ F
ADN2811
PIN
NIN
50Ω 50Ω
VREF
INPUT (V)
Figure 23. ADN2811 with DC-Coupled Inputs
V p-p = PIN – NIN = 2 × VSE = 10mV AT SENSITIVITY
PIN
VSE = 5mV MIN
VCM = 0.4V MIN
(DC-COUPLED)
NIN
Figure 24. Minimum Allowed DC-Coupled Input Levels
INPUT (V)
PIN
V p-p = PIN – NIN = 2 × VSE = 2.4V MAX
VSE = 1.2V MAX
VCM = 0.6V
(DC-COUPLED)
NIN
Figure 25. Maximum Allowed DC-Coupled Input Levels
Rev. B | Page 18 of 20