ACS101A Semtech Corporation, ACS101A Datasheet - Page 5

• Manufacturer Part Number: ACS101A
• Description: Acapella Optical Modem ic
• Manufacturer: Semtech Corporation

Figure 4. shows the recommended power supply decoupling. The
LED should be sited very close to the LDN and LDP pins. A
generous ground plane should be provided, especially around the
sensitive LDN and LDP pins. The modem should be protected
from EMI/RFI sources in the standard ways.
The link budget is the difference between the power coupled to the
fiber via the transmit LED and the power required to realise a
current of 500 nA (minimum amplifier sensitivity) via the receive
LED. The link budget is normally specified in dB or dBm, and
represents the maximum attenuation allowed between
communicating LEDs. The budget is utilised in terms of the cable
length, cable connectors and splices.
operating margin to allow for degradation in LED performance.
The power coupled to the cable is a function of the efficiency of the
LED, the current applied to the LED and the diameter of the fiber
optic cable. The larger the cable diameter the greater the power
coupled. The conversion current produced by the receive diode is
a function of the LED efficiency and the cable diameter. The
conversion efficiency is measured in terms of its ability to convert
the available power to current, known as the responsivity, given by
(A/W). Some examples of link budgets are given in Note 7: Link
Budget Examples. High quality LEDs can offer > 12dB link budget
on 50µm fiber.
The internal timing chain within the ACS101A supports a
maximum link length of 5km, with 2 modes giving provision for the
link length to 2.5km ('standard' mode) and 5km ('double' mode)
with a crystal frequency of 9.216MHz. However, the maximum link
length as determined by the ACS101A timing chain is inversely
proportional to the crystal frequency.
crystal the range is decreased to 1.25km. The maximum link
length of 5km in 'double' mode is subject to the optical link budget
that is available.
Link Budgets
Maximum Link Length
Hence with a 18.4MHz
It usually includes an
5
There is a choice of pins for TxD, pins 8 and 27. Only one input
should be used. The other input will pull-up to VDD via an internal
resistor. Pin 8 is the recommended choice since it is further away
from the sensitive analogue pins. However, pin 27 is available for
designers wishing to maintain compatibility with the ACS100.
The ACS101A may be used as a controller and data buffer, with
external analogue circuitry for data transmission and reception.
This external circuitry could be used to create a higher-
performance fiber link, or allow the ACS101A to be used in
non-fiber applications such as wire or RF interfaces.
The digital interface mode is selected by connecting pins 5
(DP2) and pin 6 (DP3) to ground and pin 2 (DP1) to VD+. In
this mode the data to be sent over the link is produced on pin
17 and the received data from the link is input on pin 18,
instead of using the LDN and LDP pins.
connection is also possible with pin 17 of the first I.C. driving
into pin 18 of the second I.C. and pin 17 of the second I.C.
driving into pin 18 of the first I.C.
In addition, in digital mode, to aid data reception with external
amplifiers or AGCs, pin 26 produces a signal which indicates
the data reception time of the data burst over the link. After
the data burst is sent, the data output on pin 17 goes high.
Figure 5 shows the inter IC wiring connections for a simple
digital communications link using the digital interface mode.
This example shows a single wire connection between the two
ICs (two including ground) for data to and from each IC. This
only demonstrates how a simple single medium interface can
be used, because each IC transmits it's data at different
times, automatically interleaving it's transmitted data with the
data from the other IC. Any other medium for connecting the
ICs could be used, such as RF, twisted pair or mains power
carrier for example.
TxD Inputs
Digital Mode
ACS101A Issue 1.2 October 1999.
A simple digital
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