Newport 5600 Series - A;D Converter; Current Source Voltage; Limit DAC; Modulation Input Port

Chapter 4 Principles of Operation 35
4.2.2 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.
4.2.3 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
PD
set point input to the laser output
stage. The laser current set point value is updated at power-up, at a "bin"
recall, and whenever a laser set point value is changed.
4.2.4 A/D Converter
The 16-bit A/D converter measures the forward voltage, laser current, and
photodiode current.
4.2.5 Current Source Voltage
Taking the unregulated DC voltage from the power supply forms the current
source voltage.
4.2.6 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.
4.2.7 Modulation Input Port
The rear panel modulation 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 driver. However, regardless of the input
voltage, the current cannot exceed the current limit.
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