Arroyo Instruments ComboSource 6300 Series - Page 49

6300 Series ComboSource User’s Manual · Page 49
The short form for Slope is typically ‘m’, and the short form for offset is typically
‘b’, which is why you see these letters used in the instrument menu (you will
often see a similar version of the equation above written as y=mx+b).
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,
and you are using a 6305 in high range. You set the instrument at 300mA,
observe that the actual current is
298.6mA and the current measured by the 6305
is 301.2mA.
To correct the set point, you need to adjust the offset value for the high range Io
set point, which is the I Set HiB menu entry. Considering that you need to raise
the set point by 1.4mA, then the I Set HiB menu entry should be adjusted to
+1.4mA (the I Set HiM menu entry should be set to 1.000). This will
compensate the set point by increasing it 1.4mA and bring it close to 300.0mA.
The measurement adjustment is similar, but in this case, you need to adjust the I
HiB menu entry to −1.2mA to bring down the measurement to 300mA.
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).
Consider the following two data points, where ‘actual’ is what you are measuring
with your meter, and ‘measured’ is the measured current as displayed by the
6305:
Set point Actual Measured
100 101.5 100.6
300 298.6 301.2