AN-019 - Modulating Output with a Fixed Voltage
Page 2
Modulating at Half Range of the Driver
L
et’s assume your laser requires 5A operating current.
With 5V trigger input, your maximum modulation
range is limited to 4A. Therefore, to achieve a maximum of 5A, connect your trigger source directly to
the modulation input BNC and change the
Io Setpoint
to 1000mA: your output current range would be
between 1A and 5A. With the laser output is turned on, whenever the trigger was at +5V, the 4308 will
drive 5A (1A from the setpoint plus 4A from the modulation input); whenever it was at 0V, the 4308 will
drive 1A.
By increasing the set point, you can increase the maximum
current… for example, with a
n
Io Setpoint
of
4000mA, you could modulate from 4A to 8A
… use the
Io Setpoint
to establish a base current and
modulate above that.
Modulating at Less Than Half Range of the Driver
If your modulation range is less than the maximum modulation range, you will need to scale your signal
by using resistors to create a divider network that outputs the required voltage.
Two Resistor Method
The 4308 transfer function for the modulation input is 800mA/V. Therefore, 3A would require a
modulation input voltage of 3.75V. To produce 3.75V from 5V, wire in two resistors as shown below to
create the voltage divider you’ll need:
The presence of R3 makes the selection of R2 a bit trickier, as R2 and R3 act as parallel resistors, so R3
must be considered when calculating the resistance for R2. So long as your source voltage can source
the necessary current, using lower values for R1 and R2 can improve the noise performance of the
circuit by lowering the total input impedance and also minimize the influence of R3. The parallel
resistance required for R2||R3 can be calculated as follows:
𝑅2||𝑅3 =
(𝑅1 ∗ 𝑉𝑠𝑜𝑢𝑟𝑐𝑒)
𝑉𝑚𝑜𝑑
1 − 𝑉𝑚𝑜𝑑/𝑉𝑠𝑜𝑢𝑟𝑐𝑒
To calculate R2:
𝑅2 =
1
(
1
𝑅2||𝑅3) −
1
𝑅3)
For example, if you used 100
Ω for R1, R2/R3 need
s
to be 300Ω. With R3 at 10kΩ, R2 needs to be 309Ω
to effectively create 300Ω.
With these resistors, the effective input impedance is lowered to 400Ω, and
Modulation Input BNC
10K
MOD
Tip
Shell
R1
INPUT +
INPUT -
R2
R3