Datasheet

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MAX3748H
Compact, Low-Power, 155Mbps to 4.25Gbps
Limiting Amplifier
Select the Offset-Correction Capacitor
The capacitor between CAZ1 and CAZ2 determines the
time constant of the signal path DC offset cancellation
loop. To maintain stability, it is important to keep a onede-
cade separation between f
IN
and the low-frequency
cutoff (f
OC
) associated with the DC offset cancellation
circuit. For ATM/SONET or other applications using
scrambled NRZ data, f
IN
< 32kHz, so f
OCMAX
< 3.2kHz.
Therefore, C
AZ
= 0.1μF (f
OC
= 2kHz). For Fibre Channel
or Gigabit Ethernet applications, leave pins CAZ1 and
CAZ2 open.
RSSI Implementation
The SFF-8472 Digital Diagnostic specification requires
monitoring of input receive power. The MAX3748H and
MAX3744 receiver chipset allows for the monitoring of
the average receive power by measuring the average DC
current of the photodiode.
The MAX3744 preamp measures the average photodi-
ode current and provides the information to the output
common mode. The MAX3748H RSSI detect block
senses the common-mode DC level of input signals
IN+ and IN- and provides a ground-referenced output
signal (RSSI) proportional to the photodiode current. The
advantage of this implementation is that it allows the TIA
to be packaged in a low-cost conventional 4-pin TO-46
header.
The MAX3748H RSSI output is connected to an analog
input channel of the DS1858/DS1859 SFP controller to
convert the analog information into a 16-bit word. The
DS1858/DS1859 provide the receive-power information
to the host board of the optical receiver through a 2-wire
interface. The DS1859 allows for internal calibration of
the receive-power monitor.
The MAX3744 and the MAX3748H have been optimized
to achieve RSSI stability of 2.5dB within the 6μA to
500μA range of average input photodiode current. To
achieve the best accuracy, Maxim recommends receive
power calibration at the low end (6μA) and the high end
(500μA) of the required range; see the RSSI Current
Gain vs. Input TIA Current graph in the Typical Operating
Characteristics.
Connecting to the DS1858/DS1859
For best use of the RSSI monitor, capacitor C1 and resis-
tor R1 shown in the Typical Operating Circuits (see first
graphic) need to be placed as close as possible to the
Maxim diagnostic monitor with the ground of C1 and R1
the same as the DS1858/DS1859 ground. Capacitor C1
suppresses system noise on the RSSI signal. R1 = 3kΩ
and C1 = 0.1μF is recommended.