
Select CFILTER
For SFP/SFF, FDDI, 155Mbps ATM LAN, Fast Ethernet,
and ESCON receivers, Maxim recommends CFILTER =
0.01F. This capacitor value ensures chatter-free
LOS/SD and provides a typical assert/deassert time of
10s. For other applications, the value of CFILTER can
be calculated using the following equation:
CFILTER =
τ / 825Ω
where
τ is the desired time constant of the power detector.
Select CAZ and CIN
External-coupling capacitors (CIN) are required on the
data inputs for the offset correction loop to function
properly. The offset correction loop bandwidth is deter-
mined by the external capacitor (CAZ) connected
between CZP and CZN. The poles associated with CIN
and CAZ must work together to provide a flat response
at the lower -3dB corner frequency. For SFP/SFF, FDDI,
155Mbps ATM LAN, Fast Ethernet, and ESCON
receivers, Maxim recommends the following:
CIN = 4700pF
CAZ = 1F
MAX3969
200Mbps SFP Limiting Amplifier
_______________________________________________________________________________________
7
VCCO
OUT-
OUT+
ESD
STRUCTURES
Figure 2. Equivalent PECL Output Circuit
LOS/LOS
ESD
STRUCTURES
VCCO
Figure 3. Equivalent TTL Output Circuit
PIN-TIA RESPONSIVITY = G
1.8dB
2.5dB
10 log(OPTICAL POWER)
20 log(VIN)
RX_MAX (SENSITIVITY)
SD HIGH / LOS LOW
SD LOW / LOS HIGH
5dB
3.6dB
VIN_TH VIN_SEN
Figure 4. Signal Levels for Power-Detect Threshold