User`s manual

Page 28 · 4200-DR Series LaserSource User’s Manual

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. The

downside of single point offset calibration is that it can cause the instrument to

no longer be in calibration at points away from the 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

4200-DR:

Set point Actual Measured

100 101.5 100.6

300 298.6 301.2

As stated above, correction is done using a linear equation, and we need to

calculate the slope and offset. Slope must be calculated first, and is calculated

using the following formula:

Slope = (y

– y

) / (x

– x

)

For the set point calibration, ‘x’ is the actual current and ‘y’ is the set point. For

the measurement calibration, ‘x’ is the measurement and ‘y’ is the actual current.

Make sure to note how the actual current changes position depending on what

you are calibrating.

The offset is equally simple to calculate:

Offset = y

– Slope * x

So for the set point calibration, you would have the following two calculations:

Slope = (300 – 100) (298.6 – 101.5) = 200 / 197.1= 1.0147 (slightly rounded)

Offset = 100 – 1.0147 * 101.5 = −2.9934