CALIBRATION AND TROUBLESHOOTING
Calibration Overview
120 LDC-3700 Series
CHAPTER 5
LED flashing.
10 Change the Laser Mode to I
HBW
(high bandwidth constant current mode). Repeat the
calibration procedure as described above.
I
PD
Current Calibration
The following procedure is for calibrating the LASER I
PD
(photodiode monitor) ammeter. This
procedure calibrates the feedback circuits for constant I
PD
and constant P
PD
modes. When these
values are reached and are stable, the user enters the actual value of the monitor current, measured
with a DMM. The LDC−3700B Series Laser Diode Controller then automatically calibrates the
LASER feedback circuits.
Configure the Controller in the following manner for the Laser Controller calibration:
1 With the LASER output off, connect a calibrated ammeter to the PD Anode output of the
Laser Controller output connector (9 pin on rear panel), and connect the circuit of Figure
5.1 on page 114 to the LASER and PD outputs with the correct load resistor for the
controller model you are calibrating (See Table 5.2 on page 115).
Note: If a precision ammeter is not available, use a zero−Ohm jumper in place of the
ammeter. Then, place a calibrated DMM (with 0.1 mV resolution) to measure the voltage
across the resistor, R1, as shown in Figure 6.1. Calculate the current in the following steps
by using Ohm’s Law:
I = E / R
where E is the accurately measured voltage across the resistor, and R is the accurately
measured load resistance. I is the actual pd current to be measured by the controller. (A 4−
point probe resistance measurement is recommended.)
2 Press the (LASER MODE) ON switch to turn the LASER output on. Verify proper
operation, i.e., the I
PD
set point should be close, and the unit should not be in current limit.
3 Press the (GPIB) LOCAL and (LASER DISPLAY) I
PD
switches at the same time to place
the Controller in its I
PD
Current Calibration mode.
After a few seconds the LASER display will show the I
PD
set point value.
4 After the value on the LASER display is stable (has not changed by more than one digit
for several seconds) the LDC−3700B Series Laser Diode Controller is ready for the actual
I
PD
value to be entered.
Press and hold in the (PARAMETER) SET switch and turn the ADJUST knob until the
LASER display shows the actual monitor current, as shown on the calibrated ammeter (or
the calculated I
PD
value from Step 1).
5 Release the (PARAMETER) SET switch to store the first calibration value into non−
volatile memory. The controller will then set the second calibration current, approximately
Function State
Laser Mode P (constant power)
Laser Range High Range
CAL PD set equal to zero
LASER LIMIT 90% of FS (full scale of range)
Intitial I
PD
Set POINT 4000 µA
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