VITESSE
SEMICONDUCTOR CORPORATION
3.125Gb/s Integrated Transimpedance
and Limiting Amplifier with Signal Detect
Advance Product Information
VSC7969
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
driver with a differential pair connected to V
CC
via 50
Ω
internal pull-up resistors. The overall effective differ-
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
applied to only V
CCS
. For +5V operation, the supply voltage should be applied to only V
CCD
.
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.
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VITESSE
SEMICONDUCTOR CORPORATION
• 741 Calle Plano • Camarillo, CA 93012
Tel: (800) VITESSE • FAX: (805) 987-5896 • Email: prodinfo@vitesse.com
Internet: www.vitesse.com
G52355-0, Rev 2.0
02/09/01
VITESSE [ VITESSE SEMICONDUCTOR CORPORATION ]
