ACS411CS Semtech Corporation, ACS411CS Datasheet - Page 5

• Manufacturer Part Number: ACS411CS
• Description: Acapella Optical Modem ic
• Manufacturer: Semtech Corporation

Kohms. It is important that R
adjusted to its maximum resistance value of 50
Kohms prior to applying power to the ACS9020 for
the first time and prior to following the procedure
detailed in section headed, Laser Adjustment
Procedure .
Where R
TXMON is used to monitor the current delivered to
the LED or Laser. TXMON is a current source that
proportionally mirrors the current flow through the
LED or Laser. By placing an appropriate external
resistor R
voltage developed (referenced to GND), will be
proportional to the transmit current. During the Laser
setup procedure TXMON should be monitored to
ensure that the Laser manufacturer's maximum
current specification is not exceeded.
The transmit current monitor is a current source
flowing from VDD out of pin TXMON. This current is
representative of the Laser/LED drive current.
I
I
I
average modulation current is half this value.
Average drive current, I
Therefore I
TXMON may also be employed during normal
operation to continuously check the Laser current.
The voltage developed across R
within an internally generated reference voltage of
1.25V. In the event that the reference voltage is
exceeded, the TXFLAG is set High, otherwise it is
set Low. In this way, the value of resistor on TXMON
can be chosen to activate TXFLAG at any desired
transmit current
e.g. If R
exceeds 62.5mA.
If desired, TXFLAG activation can be delayed by
adding a damping capacitor between TXMON and
GND.
The output power from the Laser should be measured
with an optical power meter during the setup
procedure. In addition TXMON may be monitored to
ensure that manufacturers maximum current limits
are not exceeded during the set-up process. Select
one of the laser drive modes in accordance with the
section headed, Optical Operational Modes.
TXMON
BIAS
MOD
TXMON and TxFLG
Laser Adjustment Procedure
I
(PMN - AVERAGE)
is the Low level bias current.
is the peak Modulation level bias current. The
= I
RMODSET
TXMON
BIAS
TXMON
TXMON
/50 + I
> 1Kohm, tolerance +/- 20%
= 1KW, then TXFLAG will be set if I
(BIAS + MOD) = 1/ R
= I
between TXMON and GND, the
MOD
AVG
/100
/50
AVG
= (I
RMODSET
BIAS
TXMON
+ I
MOD
is inserted and
RMODSET
is compared
) /2
AVG
5
Start be setting the current control resistors R
and R
is recommended).
The bias current is then set to the desired level by
adjusting the variable resistor R
current sets the optical low-level for the Laser, it is
essential that the Laser driver data inputs are set at
a continuous logic low level. The resistor value
(typically a 50K potentiometer) is reduced until the
desired bias current is achieved or until the desired
low-level optical output power is achieved. It should
be understood that since the bias current is fixed
(not regulated), the low level optical output power
will vary across the temperature and voltage range.
Once the bias current is set, the modulation current
maybe set by adusting the variable resistor R
The automatic power regulation circuitry for the
modulation current maintains the average optical
output power and not the peak power. For this reason,
during the set-up process in the absence of the
appication data, it is recommended that the Laser
driver is stimulated with a square wave. Most
application data used in fiber optic transmission is
dc-balanced (equal number of ones and zeros), so a
square-wave is an accurate representation of the
real data.
The resistor value (typically a 50K potenmtiometer)
is reduced until the desired optical-high ouput power
is achieved. The modulation ouput power will then
be regulated such that the average ouput optical
output power (bias + modulation) is mainatined over
the recommended temperature and voltage range.
The ACS9020 incorporates a power meter which
generates a current source on the RXMON pin, which
is proportional to the received signal strength. A
voltage is generated on an internal 50Kohm resistor
which is continuously compared with an internally
generated reference of 1.25 volts.
The RXFLAG is set when the RXMON voltage
exceeds the 1.25 volt reference. The flag is used to
indicate that there is sufficient signal strength to give
a minimum differential output signal on the receiver
output pins DOUTP and DOUTN. If the voltage on
DOUTP/DOUTN exceeds 500 mV peak-to-peak then
the RXMON voltage will exceed 1.25 Volts and the
RXFLAG will be set.
Because of process tolerances on the internal resistor
and the internally generated reference voltage, the
RXFLAG should be considered only as a guide to
the receive signal strength. The receive threshold
can be adjusted by placing a 1Mohm external
potentiometer between the RXMON pin and Ground.
Receive Monitor RXMON and RXFLAG
RBIASSET
to their highest values (at least 50Kohm
ACS411CS PRE-RELEASE Issue 6.0 July 1999.
RBIASSET.
Since the bias
RMODSET.
RMODSET