THORLABS EK2000 User Manual page 14

OEM Laser Diode Driver Evaluation Kit
The laser power is regulated through an integrating feedback loop. The setpoint
of the feedback is determined by the PWR LIMIT control trimpot and the
OUTPUT ADJUST knob (in external modulation mode, the external voltage is
used in place of the OUTPUT ADJUST). An internal transimpedance amplifier
converts the laser feedback current to a voltage that is used as the error signal
for the feedback loop.
Since all analog signal levels are based on a 2.5 V internal reference, we will use
this to derive the feedback gain setting resistor value:
The LD2000R has an internal transimpedance gain of 20 k. Without a user
installed feedback resistor, the transimpedance gain is:
and V is equal to:
Error
where I is the feedback monitor photocurrent. The total transimpedance gain
Mon
should be set so that the photocurrent at the maximum laser power equals 2.5 V.
Since V has a maximum value of 2.5 V, we can derive the value of an
Error
external feedback resistor needed to set the transimpedance for any laser:
where R is the transimpedance gain needed, and I
F
photocurrent for your laser.
solving for R
F EXT
2468-D01
��
��
��
�� = ��
��
,
��
�� ������
�� = 20 ��Ω
��
( �� )
= ��
���������� �� ������
2.5 V
�� =
��
��
������
= �� || 20 ��Ω
�� ������
20 kΩ
�� ������
( �� + 20 k٠)
�� ������
20,000
=
( 8,000 ( �� ) − 1 )
����
Chapter 4: LD2000R Overview
is the monitor
Mon

Page 12