ACS104A Semtech Corporation, ACS104A Datasheet - Page 5

• Manufacturer Part Number: ACS104A
• Description: Acs104a Fiber Modem
• Manufacturer: Semtech Corporation

however is very much lower. The DC balanced nature of data
encoding means the LED consumes current for approximately
50 % of the 'transmit' window time. The average current delivered
to the LED is therefore a function of both the peak current and the
duration of the 'transmit' window.
Handshake signals frequency
Handshake data which is interleaved with the main data channel is
generated and written to the time compress FIFO each time a
change is detected on either RTS or DTR. The power consumption
is lower when the signals change at low frequency or are held at a
DC level. It is possible to limit the power consumption dedicated to
the handshake signals by limiting the frequency of operation using
HD(1:2) input pins.
Signals.
XTAL frequency
The ACS104A uses CMOS technology and therefore the power
consumption is proportional to the frequency of switching.
Consequently, the effect of reducing the value of the XTAL alone
will result in lower power consumption.
component delivered to the LED and sourced from outputs such as
RxD and HBT are static and as such are independent of the XTAL
frequency.
It is worth noting that a modem pair configured with an XTAL of
10MHz and a sample-clock of XTAL/40 will yield the same
performance as a modem pair configured with an XTAL of 5MHz
and a sample-clock of XTAL/20. However, the modem pair with the
lower value XTAL is likely to consume the higher power with a
higher data delay (see section headed Data delay and skew). This
is because, although the dynamic power has reduced, the higher
sample-clock leads to a much longer active time, a factor which
dominates the overall power calculation.
Supply voltage
The ACS104A has been designed to operate over the voltage
range 3.3 Volts to 5.25 Volts. For the purpose of this specification
the power consumption figures presume a worst case supply of
5.25 Volts.
reduction in power consumption where the device is operated at
lower voltages.
Current and Power Consumption
The average current consumption may be split into two
components; the dynamic component and the static component.
The dynamic component is dependent on the XTAL frequency
while the static component is dependent on static current loads.
(See Calculating average current and power consumption for
details).
Since the peak current can be very much greater than the
average current, it is important to use a substantial smoothing
capacitor on VA+ and VD+. The recommended values are at
least 47µF
seen in Figure 1. (* Capacitor tolerance +/- 20 %)
The Full Duplex Delay (FDD) through the system, which applies to
TxD à RxD, RTS à CTS and DTR à DSR, is shown in Table 5.
Table 5. FDD with XTAL = 10MHz
The FDD is inversely proportional to the XTAL frequency and may
be calculated for other XTALs using the formula below:
FDD
The skew between the main TxD data channel and handshake
signals is 1 - 2 data bits as long as the maximum handshake data-
rate of 2kbps is respected. For handshake frequencies above
Data delay and skew
DR3
0
1
1
1
1
XTAL
Advanced Communications
= (10
DR2
*
1
0
0
1
1
for VD+ and 100µF
It is anticipated that there will be a significant
7
/ XTAL) * FDD
DR1
1
0
1
0
1
See section headed RS-232 Handshake
10MHz
*
FDD
6.5ms
3.8ms
2.8ms
2.3ms
2.0ms
for VA+. The configuration can be
However, the current
5
2kbps, the skew will be proportional to the handshake signal
frequency.
Since LEDs from different suppliers may emit different
wavelengths, it is recommended that the LEDs in a
communicating pair of modems are obtained from the same
supplier. The ACS104A can support any wavelength LED or
LASER. Furthermore, the emission spectrum is a function of
temperature, so a temperature difference between the ends of a
link reduces the responsivity of the receiving LED, resulting in a
reduction in the link budget. Information is given in the suppliers’
data sheets. The following manufacturers have components that
will be tested with the ACS104A and Acapella will be glad to
assist with contact names and addresses on request:
HP
Mitsubishi
AMP/Lytel
OKI
UTP
Honeywell
The ACS104A contains a highly sensitive amplifier, capable of
responding to extremely low current levels.
sensitivity it is important to reduce external noise to a low level
compared to the input signal from the LED or PIN. The modem
should have an independent power trace to the point where
power enters the board.
Figures 3 to 4 show the recommended power supply decoupling.
The LED and PIN should be sited very close to the PINP, PINN,
LAN and LAP pins.
provided, especially around the sensitive PINP, PINN, LAN and
LAP pins.
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
the minimum input-amplifier current via the receive LED/PIN.
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 type of the fiber
optic cable employed. The conversion current produced by the
reverse biased LED is a function of the LED efficiency and the
fiber type.
The power supply requirements of the ACS104A have been
designed to be low enough so that no external power supply is
required. Instead the current required to supply the device can be
extracted from the RS232 data lines. Figure 5 illustrates a possible
application circuit that may be used to extracted enough power
from the RS232 data input signals to power the Acapella IC and
the optics and to drive back the RS232 output signals, in order to
provide a full duplex RS232 link. In the system described, three
input data lines are used for power extraction : TxD, RTS and DTR.
In the application circuit illustrated, only the RxD and CTS interface
signals are driven back again to the port, with the DCD (data
carrier detect) and DSR (data set ready) signals permanently
activated by being tied to the positive supply. Data communication
is then possible using all communication protocols, including
hardware handshaking, with communication under the control of
RTS and CTS. If required the DCD and DSR signals could be
actively driven from the Acapella I.C. via another switching device
or dedicated line driver. Alternatively these could be disconnected
to reduce the current extraction requirements.
Power Supply Decoupling
LED considerations & Suppliers
Link Budgets
Power extraction from RS232 Data lines
The modem should be protected from EMI/RFI
Optek Technology
Rohm
MITEL
A generous ground plane should be
ACS104A Data Sheet
It usually includes an
To exploit this
2