NCS333, NCV333, NCS2333, NCV2333, NCS4333, NCV4333

www.onsemi.com

9

APPLICATIONS INFORMATION

APPLICATION CIRCUITS

Low−Side Current Sensing

The goal of low−side current sensing is to detect

over−current conditions or as a method of feedback control.

A sense resistor is placed in series with the load to ground.

Typically, the value of the sense resistor is less than 100 m

W

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

VDD

VDD

Figure 17. 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 18. 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 18. 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.