685 Series
ComboPak
User’s Manua
l · Page 27
While it is possible to use a DMM to measure the resistance of the cable,
because resistance is so small, you will not typically get proper readings (most
DMMs are not designed to properly measure small resistances). A better
approach is to use the instrument to drive current through the system and then
measure the voltage loss to determine resistance. There are two ways to best
calculate the voltage loss:
Measure the Voltage at the Laser
Using an accurate voltage meter, take two voltage measurements: one across
the anode and cathode pins of the laser, and a second across pin 5 and pin 9 of
the
Laser Output
connector. The difference between the measured voltages is
the voltage loss in the cable. Use this formula to calculate resistance:
Am ps
Laser
Output
I
V
V
R
Cable
−
=
Note that the current is expressed in
amps
, not
milliamps
, so divide the set point
by 1000 before using it in this equation.
Short the Connection at the End of the Cable
A second method, which does not require a DMM and can be done with just the
ComboPak
itself, is to short the anode and cathode wires at the end of the cable
(nearest the laser diode), drive current through the cable and note the measured
laser voltage (LASER:LDV?). The resistance is found using a simpler version of
the formula above:
Am ps
I
Vf
R
Cable
=
Note that the current is expressed in
amps
, not
milliamps
. The easiest way is to
drive 1A of current. When I
Amps
equals 1A (1000mA), the measured voltage is
also the resistance of the cable.
To short the cable, disconnect the laser and short the anode and cathode
together as close to the end of the cable as possible. Ideally, the short should be
done by soldering the anode and cathode wires together to minimize the
resistance in the short itself.
How the Calculation Is Used
The
ComboPak
continuously takes the measured current, multiplies it by the
resistance (
Cable R
), subtracts the result from the actual voltage at the output
