Theory Of Operation - THORLABS EK2000 User Manual

OEM Laser Diode Driver Evaluation Kit Chapter 4: LD2000R Overview
The photodiode transimpedance amplifier has an internal gain of 20 kΩ which
yields a 50 µA/V output. Since the maximum voltage of the feedback loop is 2.5
V, this limits the maximum photodiode current to 125 µA. This upper limit can be
easily increased by adding an external resistor (see page 15).
The setpoint voltage used by the feedback loop is the difference between the PD
CURRENT SETPOINT voltage and the Analog Modulation Voltage as follows:
�� = �� − ��
���������������� ���� �������������� ���������������� ������������
The control loop integrator has a time constant of approximately 16.5 µs set by a
0.033 µF integrating capacitor. The loop time constant can be extended by
adding an external capacitor across CX1 and CX2.
Note all control signals are based on the photodiode current. The user must refer
to the manufactures spec sheets of the particular diode that will be used to
correlate this to the laser output power.
4.4.

Theory of Operation

The LD2000R uses the internal monitor photodiode provided on most low power
diode lasers for feedback when operating in the Constant Power Mode. The
following figure is a block diagram of the LD2000R laser driver.
Figure 3 LD2000R Simplified Block Diagram
Page 11 Rev F, May 21, 2018