User`s manual

4200-DR Series LaserSource User’s Manual · Page 21
Grounding Considerations
A key feature of the 4200-DR is the optical isolation of both the photodiode and
modulation inputs. By isolating there inputs, earth grounding of the photodiode
anode or cathode, or earth grounding the modulation input, cannot cause a
ground loop through the instrument. Likewise, the laser anode and cathode
connections are also isolated from earth ground.
However, if you use the earth ground pin of the OUTPUT connector, it is
possible to create a ground loop if the instrument’s earth ground is connected to
a fixture or optical table that is also earth grounded. Make sure that from your
laser diode package there is only a single path to earth ground.
Analog Modulation
The analog modulation input BNC allows for external control of the current set
point using a 0 to 10V analog signal. 10V equals the maximum operating range,
which is dependent on the model and Range setting. For example, a 4205-DR
operating in the 500mA range 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 4205-
DR 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
point or two point calibration and apply the calibration to your voltage input. The
instrument will display actual current flowing when the display is in large font
mode (press the knob button to change modes), so turn the output on with the
Io set point at 0mA, apply a modulation input signal, and the instrument will
indicate the actual current, which you can use to calibrate your voltage set point.
When modulating at higher frequencies (>1 kHz), it is critical that high quality,
shielded twisted pair cabling be used, and wire size suitable for the applied
current (or the equivalent if using a multi-conductor cable). See the chart below
for suggested sizes. Failure to do so may result in current oscillations that may
damage your laser. When operating in these modes, always use shielded cable
and never use individual wires to connect the instrument to your laser. If in
doubt, use a current probe attached to an oscilloscope to monitor actual
performance.
The 1220B or 1221B cables recommended by Arroyo Instruments are shielded
twisted pair, and will provide good modulation performance.