MAX3665EUA Maxim Integrated Products, MAX3665EUA Datasheet - Page 5

• Manufacturer Part Number: MAX3665EUA
• Description: IC PREAMP 622MBPS TRANS 8-UMAX
• Manufacturer: Maxim Integrated Products
• Type: Transimpedance Preamplifierr

Specifications of MAX3665EUA

• Applications: Optical Networks
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

The paraphase amplifier converts single-ended inputs to
differential outputs, and introduces a voltage gain. This
signal drives a differential pair of transistors, Q2 and Q3,
which form the output stage. Resistors R1 and R2 provide
back-termination at the output, absorbing reflections
between the MAX3665 and its load.
The differential outputs are designed to drive a 100
load between OUT+ and OUT-. They can also drive
higher output impedances, resulting in increased gain
and output voltage swing.
The DC cancellation loop removes the DC component
of the input signal by using low-frequency feedback.
This feature centers the signal within the MAX3665’s
dynamic range, reducing pulse-width distortion on
large input signals.
The output of the transimpedance amplifier is sensed
through resistors R3 and R4 and then filtered, amplified,
and fed back to the base of transistor Q4. The transistor
draws the DC component of the input signal away from
the transimpedance amplifier’s summing node.
Connect a 400pF or larger capacitor (C
FILT and case ground for TO header, die-mounted oper-
ation. Increasing C
lation loop can sink up to 300µA of current at the input.
The MAX3665 minimizes pulse-width distortion for data
sequences that exhibit a 50% mark density. A mark
density other than 50% causes the device to generate
pulse-width distortion.
DC cancellation current is drawn from the input and
adds noise. For low-level signals with little or no DC
component, this is not a problem. Preamplifier noise will
increase for signals with a significant DC component.
___________Applications Information
The MAX3665 is a low-noise, wide-bandwidth transim-
pedance amplifier that is ideal for 622Mbps SDH/
SONET receivers. Its features allow easy design into a
fiber optic module, in three simple steps.
Step 1: Selecting a Preamplifier for a 622Mbps
Receiver
Fiber optic systems place requirements on the band-
width, gain, and noise of the transimpedance preampli-
fier. The MAX3665 optimizes these characteristics for
SDH/SONET receiver applications that operate at
622Mbps.
In general, the bandwidth of a fiber optic preamplifier
should be 0.6 to 1 times the data rate. Therefore, in a
622Mbps system, the bandwidth should be between
Transimpedance Preamplifier for SDH/SONET
_______________________________________________________________________________________
FILT
improves PSRR. The DC cancel-
DC Cancellation Loop
Paraphase Amplifier
622Mbps, Ultra-Low-Power, 3.3V
FILT
) between
375MHz and 622MHz. Lower bandwidth causes pat-
tern-dependent jitter and a lower signal-to-noise ratio,
while higher bandwidth increases thermal noise. The
MAX3665 typical bandwidth is 470MHz, making it ideal
for 622Mbps applications.
The preamplifier’s transimpedance must be high
enough to ensure that expected input signals generate
output levels exceeding the sensitivity of the limiting
amplifier (quantizer) in the following stage. The
MAX3676 clock recovery and limiting amplifier IC has an
input sensitivity of 3.6mV
3.6mV
produce a fully limited output. Therefore, when used
with the MAX3665, which has an 8k
the minimum detectable photodetector current is
450nA
It is common to relate peak-to-peak input signals to
average optical power. The relationship between opti-
cal input power and output current for a photodetector
is called the responsivity ( ), with units amperes per
watt (A/W). The photodetector peak-to-peak current is
related to the peak-to-peak optical power as follows:
Based on the assumption that SDH/SONET signals
maintain a 50% mark density, the following equations
relate peak-to-peak optical power to average optical
power and extinction ratio (Figure 2):
Figure 2. Optical Power Definitions
Average Optical Power = P
Extinction Ratio = r
Peak-to-Peak Signal Amplitude = P
P-P
P-P
POWER
.
P
is the minimum signal amplitude required to
AVG
P0
P1
I
P-P
e
= P1 / P0
= (P
P-P
P-P
AVG
)( )
, which means that
= (P0 + P1) / 2
P-P
transimpedance,
= P1 - P0
TIME
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