MAX3724-MAX3725 Datasheet, PDF (6/10 Pages) Maxim Integrated Products – 3.2Gbps SFP Transimpedance Amplifiers with RSSI

3.2Gbps SFP Transimpedance
Amplifiers with RSSI
MAX3724/
MAX3725
BOND PAD
1, 3
2, 7
4
5
6, 10
8
9
Pin Description
NAME
V_CC
N.C.
IN
FILTER
GND
OUT-
OUT+
FUNCTION
Supply Voltage
No Connection
TIA Input. Signal current from photodiode flows into this pin.
Provides bias voltage for the photodiode through a 580Ω resistor to V_CC. When grounded, this
pin disables the DC cancellation amplifier to allow a DC path from IN to OUT+ and OUT- for
testing.
Supply Ground
Inverting Data Output. Current flowing into IN causes the voltage at OUT- to decrease. For the
MAX3724, the common mode between OUT+ and OUT- is proportional to the average input
current.
Noninverting Data Output. Current flowing into IN causes the voltage at OUT+ to increase. For
the MAX3724, the common mode between OUT+ and OUT- is proportional to the average input
current.
R_F
TRANSIMPEDANCE
AMPLIFIER
IN
V_CC
FILTER
DC CANCELLATION
CIRCUIT
MAX3724
OUT+ _IN
OUT-
R_F
TRANSIMPEDANCE
AMPLIFIER
50Ω
50Ω
V_CC
DC CANCELLATION
RSSI
CIRCUIT
FILTER
V_CC
50Ω
50Ω
OUT+
OUT-
MAX3725
Figure 1. Functional Diagram
Detailed Description
The MAX3724/MAX3725 are transimpedance amplifiers
designed for up to 3.2Gbps SFF/SFP transceiver mod-
ules. A functional diagram of the MAX3724/MAX3725 is
shown in Figure 1. The MAX3724/MAX3725 comprise a
transimpedance amplifier stage, a voltage amplifier
stage, an output buffer, and a direct-current (DC) feed-
back cancellation circuit. The MAX3724 also includes a
signal strength indicator (RSSI). To provide this signal in
a standard 4-pin TO header, the RSSI level is added to
the common mode of the differential data output pins.
Transimpedance Amplifier Stage
The signal current at the input flows into the summing
node of a high-gain amplifier. Shunt feedback through
the resistor R_F converts this current to a voltage. In par-
allel with the feedback resistor are two back-to-back
Schottky diodes that clamp the output signal for large
input currents, as shown in Figure 2.
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