ACS401 Semtech Corporation, ACS401 Datasheet - Page 2

• Manufacturer Part Number: ACS401
• Description: Optical Modem ic
• Manufacturer: Semtech Corporation

A c a p e l l a O p t i c a l M o d e m IC
Inter-IC Encoding Technique
The 3B4B encoding method is used for communication between
ACS401s, thus ensuring that there is no DC component in the
signal. The encoding and decoding is transparent to the user.
Transmitter and Receiver Functions
Signals TxD and RxD in this specification refer to the set of signals
TxD(1:4) and RxD(1:4) respectively.
The TxD input data of the transmitting modem is time compressed
and encoded in 3B4B format. In the receiving modem, 3B4B
encoding ensures easy extraction of the bit-clock. The received
data is filtered, decoded, then stored in an output memory. The
memory provides time expansion, de-jittering and frequency
compensation. The data is finally directed to the RxD output pin.
Operational Modes
The ACS401 has four operational modes controlled by DP(1:4).
There are modes of operation to support PPLED and LASER
duplex devices on single fiber. In addition, LED/PIN and
LASER/PIN are supported on dual fiber.
Operational
Mode
1. PPLED
2. 4-pin Duplex
3. LED & PIN
4. LASER & PIN Dual
N.B. All LASERs must be 4-pin LASERS. DP(1:4) combinations
not listed above are factory IC test modes, and should not be
selected by the user. Damage to the PPLED/Duplex & LED/PIN
components may result if these illegal modes are selected.
N.B. for all Operational modes 1, 2, 3, 4 SETB (Pin2) MUST be
tied High.
Mode 1 - LED single fiber mode
This is the operational mode for single fiber LED transmission, i.e.
the LED is a ‘ping-pong’ type (PPLED) used for both transmit and
receive. Connect PINP of ACS401 to GND and leave PINN floating.
Mode 2 4-pin Laser duplex single fiber mode
This is a single fiber mode where the Laser duplex device is
employed. This duplex device comprises a 4-pin Laser for
transmission and a PIN diode for reception in a single housing.
The Duplex devices are driven by the ACS401 in a half-duplex
manner, so potential cross-talk between the transmitter and
receiver is of no consequence.
It is necessary to use a small number of external components
(shown in the diagram) to support the power regulation technique.
The method for setting the Laser power is described in section
headed Control of LASER current .
VA+ is the +5V supply used to power the analogue of the ACS401.
AGND is analogue ground (pin 49 of the ACS401).The loop is
stable when the average current from the monitor pin equals the
current through variable resistor VR2. The voltage VC is stable at
this point and sets the laser drive current.
128 kbps Optical Modem for Long Haul Transmission for Single/Dual Fiber applications
LAP
LAN
No. of
Fibers
Single
Single
Dual
LED single fiber mode
LED
DP4
1
1
1
1
DP3
0
0
0
0
Single
Fiber
DP2
0
0
0
0
DP1
1
0
0
0
Mode 3 - LED/PIN dual fiber mode
This is a dual fiber mode where the LED is used for transmission
and a separate PIN diode is used for reception. This allows the
use of low cost standard LEDs.
Mode 4 4-Pin LASER/PIN dual fiber mode
This is a dual fiber mode where the 4-pin laser is used for
transmission and a separate PIN diode is used for reception.
It is necessary to use a small number of external components
(shown in the diagram) to support the power regulation technique.
The method for setting the Laser power is described in section
headed Control of LASER current .
VA+ is the +5V supply used to power the analogue of the ACS401.
AGND is analogue ground (pin 49 of the ACS401).The loop is
stable when the average current from the monitor pin equals the
current through variable resistor VR2. The voltage VC is stable at
this point and sets the laser drive current.
VA+
VA+
100 nF
ZVP4424A
ZVP4424A
100
LAN
LAN
PMOS
PMOS
10
nF
nF
nF
4-pin Laser duplex single fiber mode
with external Laser drive circuitry
4-pin Laser dual fiber mode with
external Laser drive circuitry
VR2
50k
VR2
50k
LED/PIN dual fiber mode
PINP
LAP
LAN
PINN
IN4001
IN4001
VC
VC
470 nF
470
nF
PIN diode
AGND
AGND
LED
PINN
PINN
A C S 4 0 1
PIN Diode
PIN Diode
Laser
Laser
Fiber
Fiber
Fiber
Fiber
Single
Fiber