ACS102A Semtech Corporation, ACS102A Datasheet - Page 6

• Manufacturer Part Number: ACS102A
• Description: Acs102a Fiber Modem
• Manufacturer: Semtech Corporation

The HBT pin is active High and can supply up to 16 mA at a voltage of
> VDD - 0.5 Volts. The display LED should be placed between the
HBT pin and GND with a series resistor. The resistor value is a
function of the efficiency of the display LED, and the power budget.
Example: Calculating the HBT resistor value
LED on voltage:
VDD (ACS102A):
Resistor voltage:
Current to LED:
Resistor value:
Average current:
Average power:
Note: The LED referred to in this section is of the inexpensive display
type and should not be confused with the LED that interfaces with the
fiber optic cable itself.
The power consumption of the ACS102A is a function of the
following:
i.
ii.
iii.
iv.
v.
The sample-clock
The sample-clock selected by DR(1:3), see section headed Data-
Rate Selection, determines the quantity of data transmitted over the
fiber link. The 'transmit' window opens once each frame and closes
when the time compress FIFO is empty. The 'receive' window is
aligned with the 'transmit' window of the far-end modem, and tracks
the 'transmit' window such that it closes on detection of the last data
bit. Clearly, the lower the sample-clock the smaller the active time
and the lower the power consumption.
The transmit current setting
The formula given in section headed LED current control, relates to
the peak current delivered to the LED. The average current 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. See section headed RS-232 Handshake Signals.
XTAL frequency
The ACS102A 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. However, the current 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.
Current and Power Consumption
The average current consumption may be split into two components;
the dynamic component and the static component. The dynamic
Power consumption considerations
The sample-clock DR(1:3)
The transmit current setting (TRC)
Handshake signals frequency
XTAL frequency
Supply voltage
Advanced Communications
0.32mW
2.0V
5.0V
3.0V
2mA (high efficiency LED)
3/2*10
64µA
-3
= 1500
6
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
VD+ and 100µF
(* 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
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.
ACS102A 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 ACS102A and Acapella will be glad to assist with contact
names and addresses on request:
MITEL
Acapella
GCA
Honeywell
The ACS102A contains a highly sensitive amplifier, capable of
responding to extremely low current levels. To exploit this 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 4 to 6 all show the recommended power supply decoupling.
The LED/PIN/LASER should be sited very close to the PINP, PINN,
LAN and LAP pins. A generous ground plane should be provided,
especially around the sensitive PINP, PINN, LAN and LAP 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 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.
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.
Data delay and skew
LED considerations & Suppliers
Power Supply Decoupling
Link Budgets
DR3
0
1
1
1
1
XTAL
= (10
DR2
It usually includes an operating margin to allow for
1
0
0
1
1
7
/ XTAL) * FDD
*
(e.g. 1A-212ST, 1A-212SMA)
(e.g. A-ST, A-SMA)
(e.g. 1A-212-ST-05, 1A-212-SM-02)
(e.g. HFE4214-013, HFE4404-013)
for VA+. The configuration can be seen in Figure 1.
DR1
1
0
1
0
1
10MHz
FDD
6.5ms
3.8ms
2.8ms
2.3ms
2.0ms
ACS102A Data Sheet
The
*
for