
Page 24 · 685 Series
ComboPak
User’s Manua
l
Using the Laser Limits
Both the laser current and voltage limits are implemented in hardware, providing
for fast response to changes in laser diode operation. When a voltage limit is
detected, the output is immediately shutdown. Because of the sensitivity of the
voltage limit, operating near the limit (within one to two hundred millivolts) is not
recommended. In general, you should set the voltage limit to 0.3V to 0.5V higher
than any anticipated operating point. The voltage limit is tested against
the
voltage at the connector
, unless in remote voltage sense mode. Any
Laser
Cable R
value is ignored, as
Laser
Cable R
is a software only calculation, and
the voltage limit is implemented in hardware. See
Compensating for Voltage
Loss
for more information on the
Laser
Cable R
setting.
Unlike the voltage limit, the current limit simply prevents the
ComboPak
from
delivering more current than the limit is set to. When the current limit engages,
the output will remain on.
The photodiode current and photodiode power limits are implemented in
software and may take up to one second to trigger when these conditions occur,
and therefore should not be relied on to provide fast protection of the laser
diode.
Analog Modulation
The instrument supports external analog modulation using the modulation pins
on the analog interface connector. Modulation rates vary by model, so see your
model’s specification for the maximum modulation rates. Only
Io HiBW
mode
supports high speed modulation. All other modes of operation have a
modulation bandwidth of 10Hz or less. See
Using Modulation
below for more
details, especially with regards to wiring considerations.
The analog modulation input on pins 5 and 6 of the Control I/O interface allows
for external control of the current set point using a 0 to 10V analog signal. 10V
equals the maximum operating range. For example, a 500mA model of the
ComboPak
will attempt to drive 500mA when 10V is applied to the modulation
input (so the “transfer function” is 50mA/V).
In addition, the set point
adds
to the modulation input, so using the same
ComboPak
example, if you were to apply 5V and the set point was 100mA, then
the resulting drive current would be 350mA (5V = 250mA plus the 100mA set
point).
You must also consider that the modulation input is not a precision input, so for
accurate current control, you should calibrate the modulation input using a one