User`s manual

4200-DR Series LaserSource User’s Manual · Page 29
You can validate your calculations by applying the formula to one or both of the
data points to confirm the compensation is correct. Consider the 300mA set
point … if we apply the values just calculated, we should get a compensation
that increases the set point by slightly over 1.4mA to bring it up to 300mA. Let’s
check:
300mA test point = 1.0147 * 300 − 2.9934 = 301.4166mA – check!
Likewise, the 100mA set point should be lowered by slightly more than 1.5mA:
100mA test point = 1.0147 * 100 − 2.9934 = 98.4766mA – check!
You can make the same tests of the measurement side. By using the multiple
point calibration, you benefit from the calibration across a much wider operating
range. When choosing your two data points, you can either calibrate specifically
for your application, where the upper point should be the maximum current you
plan to operate, and the lower data point near the minimum current you plan to
operate (but above zero). Alternatively, you can take two generalized data points
of 80% of the range of the instrument as the upper point and 20% of the range
as the lower point… this latter approach provides a more generated calibration
that will function across the full range of the instrument.
Accuracy Requirements
When calibrating the instrument, we recommend that the accuracy of your meter
is four times greater than the accuracy you are trying to achieve with the
instrument. For example, if you want to calibrate the laser current to within
0.1mA, then your meter would have an accuracy of 0.025mA or better. Using
lower accuracy meters could actually degrade the accuracy of the instrument
rather than improving it.
Considerations for Calibrating the Laser Current
For current calibration, more meters have poor current measurement
performance. A better approach is to measure the voltage across a known
resistor value. However, this introduces two measurement errors: the
measurement error of the resistance of the current sense resistor, and the
measurement error of the voltage across the current sense resistor, so both
much be considered when determining the overall accuracy of your
measurement system.