MAX3665(2000) View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX3665
(Rev.:2000)

622Mbps, Ultra-Low-Power, 3.3V Transimpedance Preamplifier for SDH/SONET

Maxim Integrated 

622Mbps, Ultra-Low-Power, 3.3V

Transimpedance Preamplifier for SDH/SONET

Paraphase Amplifier

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 100Ω

load between OUT+ and OUT-. They can also drive

higher output impedances, resulting in increased gain

and output voltage swing.

DC Cancellation Loop

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 (CFILT) between

FILT and case ground for TO header, die-mounted oper-

ation. Increasing CFILT improves PSRR. The DC cancel-

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

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 3.6mVp-p, which means that

3.6mVp-p is the minimum signal amplitude required to

produce a fully limited output. Therefore, when used

with the MAX3665, which has an 8kΩ transimpedance,

the minimum detectable photodetector current is

450nAp-p.

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 (ρ), with units amperes per

watt (A/W). The photodetector peak-to-peak current is

related to the peak-to-peak optical power as follows:

Ip-p = (Pp-p)(ρ)

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):

Average Optical Power = PAVG = (P0 + P1) / 2

Extinction Ratio = re = P1 / P0

Peak-to-Peak Signal Amplitude = Pp-p = P1 - P0

POWER

P1

PAVG

P0

TIME

Figure 2. Optical Power Definitions

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