AN-019 - Modulating Output with a Fixed Voltage
Page 3
at 5V, your voltage source will need to drive up to 12.5mA. Most sources should be able to deliver
12.5m, but might be something to confirm.
One Resistor Method
Alternatively, you could omit R2 altogether and use R3 as the fixed lower resistor. In this design, wire R1
as series resistor to the modulation input, as shown in the diagram below:
To calculate R1, use the equation below:
𝑅1 = (
𝑅3 ∗ 𝑉𝑠𝑜𝑢𝑟𝑐𝑒
𝑉𝑚𝑜𝑑
) − 𝑅3
In our 4308 example, this would calculate out to R1=3.33k
Ω.
When considering the single resistor design, for noise reasons it is recommended to only use it for larger
modulation ranges. This is because higher resistances increase the risk of noise injection into the
modulation input due to It is recommended to place R1 as close to the input BNC as possible to
minimize noise injection into the circuit.
The single resistor method works well when modulating near the maximum modulation range, as the
values for R1 are small. Larger values of R1 will increase the likelihood of noise injection due to high
impedance, so for smaller modulation ranges, it is recommended you use the dual resistor approach
above.
Using a Potentiometer
Replacing R1 with a potentiometer is a good solution if you need to be able to fine adjust the
modulation range. First calculate R1 using your typical operating parameters, then select a
potentiometer that works well around that range.
In the dual resistor example above, a 1k
Ω
potentiometer would allow for an adjustment range of 1.15V
to 5V, or 0.92A to 4A. A 10k
Ω
potentiometer widens the range to 0.145V to 5V, or 116mA to 5A, but at a
loss of adjustment sensitivity.
In the single resistor example, a 10k
Ω
potentiometer would allow for an adjustment range of 2.5V to 5V,
or 2A to 4A.
Another consideration in selecting a potentiometer is to use a 10-
turn version… the additional turns
will
allow for finer adjustments.
Modulation Input BNC
10K
MOD
Tip
Shell
R1
INPUT +
INPUT -
R3
