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CHAPTER 5
5.
Tips and Techniques
5.1 Introduction
This chapter is intended to further explain specific operational details of the
Model 5600, as well as provide application examples.
5.2 Laser Limits
The laser driver has several limits to protect the laser diode from damage.
The limits include current, compliance, photodiode current, and photodiode
power.
Io Limit and Vcomp Limit
Two of these limits, current and compliance, are implemented in hardware.
By implementing these limits in hardware, these limits are continuously
monitored without any additional load on the main processor. The current
limit is actually two limit circuits. The first circuit maintains the current limit
by not allowing the drive current to exceed the limit’s set point. The form of
limiting is called soft limiting, and does not affect the operation of the laser
except to limit the drive current. The second circuit monitors the actual
current drive, and if that exceeds the limit set point, will shutdown the
output. This form of limit is called the hard limit. The laser will always be
shutdown on a hard limit.
Operating at or Near Io and Vcomp Limits
Because of the sensitivity of the limit circuits, operating at or near the limit
unless necessary is not recommended because AC line transients, RF
interference, or static can be enough to trigger these limits. The causes for
hard limits and voltage compliance limits can vary, but can include the
following:
•
Static discharges, which may cause enough noise to trigger the circuit.
•
Turning on the laser when its set point is at or near the limit. When
turning on the output with the set point at the limit can cause a small
overshoot in the drive current, which the limit circuitry may pick up as a
hard limit and shutdown the laser.
•
The same is true of Im set points that are much higher than what the
laser is capable. When turning the laser on with an excessive Im set
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