685 Series
ComboPak
User’s Manua
l · Page 37
value becomes “compensated” based on these values. The compensation
formula (a linear equation) is very simple:
Compensated = Slope * Uncompe Offset
Depending on how you wish to calibrate, a simple offset calibration may be
sufficient, where only the offset value is adjusted and the slope is left at 1. This
level of calibration can be done using a single measurement, and if absolute
accuracy is only critical at a single point, this may be sufficient. For a slope and
offset calibration, you will need at least two points to compute the slope and
offset, and greater than two points can be used in a least-squares-fit algorithm.
Single Point Offset Calibration
Let’s say that you want to precisely calibrate the
laser drive current at 300mA.
You set the instrument at 300mA, observe that the actual current is 298.6mA and
the current measured by the
ComboPak
is 301.2mA.
To correct the set point, you need to adjust the offset value for the high range Io
set point, which is index 2. Considering that you need to
raise
the set point by
1.4mA, then index should be adjusted to +1.4mA. The command would look
like:
LASER:USERCAL:PUT 2,1,1.4
This will compensate the set point by
increasing
it 1.4mA and bring it closer to
300.0mA.
The measurement adjustment is similar, but in this case, you need to adjust
index 7
to −1.2mA to bring
down
the measurement to 300mA:
LASER:USERCAL:PUT 7,1,-1.2
In both cases, the slope was left at 1, and only the offset was adjusted.
Multi-Point Calibration
Multiple point calibration is mathematically more complex but improves
calibration to operate across a range of values rather than at a single point. This
addresses a downside of single point offset calibration where it can cause the
instrument to no longer be in calibration at points away from the single
calibration point. To keep the math simple in our example, we will only do a two
point calibration, but a higher number of points are possible, and the slope can
be calculated using a least-squares-fit algorithm (offset calculation is the same in
either case).
