Datasheet

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MAX3945
1.0625Gbps to 11.3Gbps,
SFP+ Dual-Path Limiting Amplifier
LOS Circuitry
The LOS circuitry has two operational modes controlled
by the LOS1_EN and LOS2_EN bits (see Table 8). In the
first mode, the LOS block detects the differential ampli-
tude of the input signal and compares it against a preset
threshold controlled by the 6-bit SET_LOS register. In
the second mode, the LOS block compares the voltage
at the RPMIN pin to a preset threshold also controlled by
the 6-bit SET_LOS register. The second mode enables
low-power LOS detection based on average photodiode
current.
The LOS assert threshold is approximately 1.5mV
P-P
x
SET_LOS[5:0]. The LOS deassert level is approximately
1.6 times the assert level to avoid LOS chatter. LOS
polarity, squelch, and LOS masking time are unaffected
by the selection of LOS1_EN or LOS2_EN.
Programmable LOS Output Masking Time
This feature masks false input signals that can occur
after a loss-of-light event in a fiber optic link. These false
input signals, caused by some transimpedance amplifier
implementations, can corrupt the LOS output and cause
system-level link diagnostic errors.
The LOS output masking time can be programmed from 0
to 4500Fs in 35Fs steps using the 7-bit SET_LOSTIMER[6:0]
register. The output mask timer is initiated on the first
transition of the LOS signal and prevents any further
changes in the LOS output signal until the end of the
programmed LOS timing period. The LOS output mask-
ing time should be carefully chosen to extend beyond
any expected input glitch. Figure 3 shows the LOS signal
changing after approximately 800Fs to a change in the
input signal where the LOS output masking time function
is not used. Figure 4 shows masking of the LOS signal
by the LOS output masking time function to a change in
the input signal.
Table 8. LOS Control
Figure 3. LOS Response to a Short Burst of Input Signal
Figure 4. LOS Response to a Short Burst of Input Signal (Any
changes in LOS are masked until the end of the LOS masking
period.)
400µs/div
50mV/div
2mV/div
400µs/div
50mV/div
2mV/div
LOS2_EN LOS1_EN OPERATION MODE DESCRIPTION
0 0 LOS circuitry is disabled and powered down
X 1 LOS circuitry is enabled and Rx input amplitude is detected
1 0 LOS circuitry is enabled and RPMIN input amplitude is detected