Rainbow Electronics MAX3746 - Manual (Page 9)

Select the Coupling Capacitor
When AC coupling is desired, coupling capacitors C IN
and C OUT should be selected to minimize the receiv-
er’s deterministic jitter. Jitter is decreased as the input
low-frequency cutoff (f
IN
) is decreased.
f IN = 1 / [2π(50)(C IN )]
For ATM/SONET or other applications using scrambled
NRZ data, select (C
IN , C OUT ) ≥ 0.1µF, which provides
f IN < 32kHz. For Fibre Channel, Gigabit Ethernet, or
other applications using 8B/10B data coding, select
(C
IN , C OUT ) ≥ 0.01µF, which provides f IN < 320kHz.
Refer to Application Note HFAN-1.1, Choosing AC-
Coupling Capacitors.
RSSI Implementation
The SFF-8472 Digital Diagnostic specification requires
monitoring of input receive power. The MAX3746 and
MAX3744 receiver chipset allows for the monitoring of
the average receive power by measuring the average
DC current of the photodiode.
The MAX3744/MAX3724 preamp measures the aver-
age photodiode current and provides the information to
the output common mode. The MAX3746 RSSI detect
block senses the common-mode DC level of input sig-
nals. IN+ and IN- provide 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 MAX3746 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 calibra-
tion of the receive power monitor.
The MAX3744/MAX3724 and the MAX3746 have been
optimized to achieve RSSI stability of 2.5dB within the
6µA to 500µA range of average input photodiode cur-
rent. To achieve the best accuracy, MAXIM recom-
mends receive-power calibration at the low end (6µA)
and the high end (500µA) of the required range. See
the RSSI Current Gain graph in the Typical Operating
Characteristics.
Connecting to the Dallas DS1858/DS1859
For best use of the RSSI monitor, capacitor C1 and
resistor R1 shown in the first Typical Application Circuit
need to be placed as close as possible to the Dallas
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.
EMI Performance
The MAX3746 has been designed for better EMI perfor-
mance. To help reduce EMI, special care has been
taken to produce symmetrical signal outputs. See the
eye diagram of the single-ended output in the Typical
Operating Characteristics.
MAX3746
Low-Power, 622Mbps to 3.2Gbps
Limiting Amplifier
_______________________________________________________________________________________ 9
GND
ESD
STRUCTURE
V CC
LOS
Figure 5. LOS Output Circuit
OUTPOL DESCRIPTION
V CC N oni nver ti ng outp ut w i th ful l C M L outp ut l evel
Open
N oni nver ti ng outp ut w i th r ed uced C M L outp ut
l evel
30k
Ω
to GND
Inverting output with reduced CML output
level
GND Inverting output with full CML output level
Table 1. Logic Table for Polarity and CML
Output-Level Settings
Chip Information
TRANSISTOR COUNT: 1385
PROCESS: SiGe Bipolar