VITESSE

SEMICONDUCTOR CORPORATION

Advance Product Information

VSC7969

3.125Gb/s Integrated Transimpedance

and Limiting Amplifier with Signal Detect

Page 8

G52355-0, Rev 2.0

02/09/01

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Circuit Description

The VSC7969 data path consists of several stages: transimpedance input stage, limiting amplifier, and out-

put driver. The transimpedance amplifier accepts current from a photodetector connected to the input pad ‘IN’

and converts the input current to a differential output voltage. The signal then travels to the second stage limit-

ing amplifier which provides DC restoration, eliminating the DC component of the input signal. The linear pho-

tocurrent monitor and signal detect function is also provided by this stage. The final stage consists of an output

ential transimpedance of the VSC7969 is typically 27k

Ω. The limited output single-ended voltage swing is typ-

ically 250mVp-p.

Design Guidelines

Power Supply

The VSC7969 is supplied by a single supply voltage; for +3.3V operation, the supply votlage should be

Data Outputs

The outputs of the VSC7969 need to be AC-coupled. This capacitor will determine the low frequency cut-

off for the system, which is directly related to the receiver’s deterministic jitter. For ATM/SONET or other

applications using PRBS NRZ data, select a capacitor of at least 0.1µF or greater, which provides less than

32kHz low frequency cutoff. For Fibre Channel, Gigabit Ethernet, or other applications requiring 8B/10B data

coding, select a capacitor of at least 0.01µF or greater, which provides less than 320kHz low frequency cutoff.

The outputs can be used single-ended or differential. For best performance, differential operation is recom-

mended. If single-ended operation is necessary, the unused output should be AC-coupled and terminated with

an impedance equal to the load on the pin in use.