NCS333A, NCV333A, NCS2333, NCV2333, NCS4333, NCV4333, NCS333
www.onsemi.com
11
APPLICATION CIRCUITS
Low−Side Current Sensing
Low−side current sensing is used to monitor the current
through a load. This method can be used to detect
over−current conditions and is often used in feedback
control, as shown in Figure 18. A sense resistor is placed in
series with the load to ground. Typically, the value of the
sense resistor is less than 100 mW to reduce power loss
across the resistor. The op amp amplifies the voltage drop
across the sense resistor with a gain set by external resistors
R1, R2, R3, and R4 (where R1 = R2, R3 = R4). Precision
resistors are required for high accuracy, and the gain is set
to utilize the full scale of the ADC for the highest resolution.
+
−
Load
VDD
ADC
Microcontroller
control
RSENSE
R1
R2
R3
R4
VDD
VDD
VLOAD
Figure 18. Low−Side Current Sensing
Differential Amplifier for Bridged Circuits
Sensors to measure strain, pressure, and temperature are
often configured in a Wheatstone bridge circuit as shown in
Figure 19. In the measurement, the voltage change that is
produced is relatively small and needs to be amplified before
going into an ADC. Precision amplifiers are recommended
in these types of applications due to their high gain, low
noise, and low offset voltage.
Figure 19. Bridge Circuit Amplification
+
−
VDD
VDD
EMI Susceptibility and Input Filtering
Op amps have varying amounts of EMI susceptibility.
Semiconductor junctions can pick up and rectify EMI
signals, creating an EMI−induced voltage offset at the
output, adding another component to the total error. Input
pins are the most sensitive to EMI. The NCS333 op amp
family integrates low−pass filters to decrease sensitivity to
EMI.
General Layout Guidelines
To ensure optimum device performance, it is important to
follow good PCB design practices. Place 0.1 mF decoupling
capacitors as close as possible to the supply pins. Keep traces
short, utilize a ground plane, choose surface−mount
components, and place components as close as possible to
the device pins. These techniques will reduce susceptibility
to electromagnetic interference (EMI). Thermoelectric
effects can create an additional temperature dependent
offset voltage at the input pins. To reduce these effects, use
metals with low thermoelectric−coefficients and prevent
temperature gradients from heat sources or cooling fans.