VSC7969
Data Sheet
Advance Product Information
Subject to Change
F
UNCTIONAL
D
ESCRIPTION
The VSC7969 data path consists of several stages: transimpedance input stage, limiting amplifier, and output 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 limiting amplifier which
provides DC restoration, eliminating the DC component of the input signal. The linear photocurrent 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 50Ω internal pull-up resistors. The overall effective differential transimpedance of the
VSC7969 is typically 27kΩ. The limited output single-ended voltage swing is typically 250mVp-p.
Power Supply
The VSC7969 is supplied by a single supply voltage. For +3.3V operation, the supply voltage should be applied to
only V
CCS
and for +5V operation, the supply voltage should be applied to only V
CCD
. Power supply decoupling
capacitors are recommended for optimal performance. A power supply filter should be used to minimize supply
noise. See
Figure 1.
10µH
0.1µF
0.01µF
Figure 1. Power Supply Decoupling Scheme
Data Outputs
The outputs of the VSC7969 need to be AC-coupled. The capacitor will determine the low frequency cutoff 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 recommended. 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.
Signal Detect
The signal detect feature of the VSC7969 provides a CMOS level output corresponding to the input current level to
the transimpedance amplifier.
Table 6. Signal Detect Function
Parameter
Assert
Deassert
Hysteresis
1
1
2.2
2.5
Min
Typ
Max
9
3
4
Units
µA
µA
dB
Photodetector Current Monitor
The IMON pin provides a linear measurement of the average input current from the photodetector to the
transimpedance amplifier. For example, if 20µA is the average input current to the transimpedance amplifier, the
current through the IMON pin will be 20µA (see the typical operating curve
IMON Characteristic, 0µA to 200µA).
To
use this feature, connect the IMON pin to V
CC
using a resistor less than 2kΩ. If this feature is not used, the IMON pin
can be left unconnected.
6 of 12
G52355, Rev 2.3
11/22/02
VITESSE [ VITESSE SEMICONDUCTOR CORPORATION ]
