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Chapter 3
3.2.2
Limit DAC
The microprocessor loads the current limit value into the 12-bit DAC. The Limit
DAC converts a digital limit signal from the microprocessor to a voltage which
becomes the Limit Set Point voltage for the Output Stage. The current limit value is
updated at power-up, at a "bin" recall, and whenever a LIM I value is changed.
3.2.3
Set Point DAC
The microprocessor loads the current set point value into the 16-bit DAC. The Set
Point DAC converts a set point value from the microprocessor to a voltage which
becomes the current or I
set point value is updated at power-up, at a "bin" recall, and whenever a laser set
point value is changed.
3.2.4
A/D Converter
The 16-bit A/D converter measures the limit current, actual current, and photodiode
current.
3.2.5
Current Source Voltage
The current source voltage is formed by taking the unregulated DC voltage from the
power supply and passing it through a regulator and the associated circuitry.
3.2.6
Output Shorting
A relay shorts the LD ANODE and LD CATHODE terminals whenever the laser
output is turned off. At the same time a FET is switched on to shunt any current
which may appear at the output. When the laser output is turned on, the shunt circuit
and short are removed gradually, and in two stages. This ensures transient protection
of the laser output.
3.2.7
Modulation/Voltage Control Input Port
The rear panel MOD input connector drives a precision wide-band instrumentation
amplifier allowing the differential control signal applied to this port to use a different
ground than the laser output terminals. However, due to the input common-mode
voltage restrictions the MOD input should be within ±10 volts of the laser output
terminals.
Each 100 mV change in the modulation input is equal to 1% of the maximum drive
current of the module. For example, 100 mV input on a 6560A module (6 A driver)
would equate to 60 mA of drive current. However, regardless of the input voltage,
the current cannot exceed the current limit.
Principles of Operation
set point input to the laser output stage. The laser current
PD
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