Starting from Scratch: Current Sense Amplifier Design | Basic Course for Engineers | NEXTY Electronics, an Electronic Components and Semiconductor Trading Company

This article explains "What is a current sense amplifier?" from the ground up, for those who are just starting circuit design or who want to review the basics of current sense amplifiers. It introduces knowledge that will be useful in actual circuit design, including how they work, their types, and points to note when designing.

Introduction

What is a current sense amplifier?

A current sense amplifier is an amplifier used to indirectly measure the current flowing in a circuit, mainly using a shunt resistor to amplify small voltage differences. The amplifier's input voltage, i.e., the voltage across the shunt resistor, can be determined from the current sense amplifier's output voltage and its gain. Furthermore, if the resistance value of the shunt resistor is known, Ohm's law can be used to determine the current flowing through the shunt resistor.
The current sense amplifier converts the current flowing through the shunt resistor into a voltage signal that is easy to handle with analog circuits and microcontrollers.

Why do you need to measure current?

Current measurement is essential for accurately understanding the safety and operating status of electronic circuits and systems. Excessive current can damage components and cause them to overheat, while insufficient current can prevent normal operation. Therefore, accurate current measurement enables overload protection, efficiency improvement, and energy management.
For example, in motor control, torque is estimated from current values, and in battery management, charge/discharge current is monitored to extend battery life. Current measurement is an essential technology for achieving both safety and performance.
Now you know why measuring current is important.

Basic structure of a current sense amplifier

Relationship between shunt resistor and amplifier

The basic function of a current sense amplifier is to convert current into voltage. A shunt resistor is used for this purpose.
A shunt resistor is inserted in series into the current path and generates a small voltage difference proportional to the current flowing. This voltage difference is amplified by an amplifier and converted to a level that can be easily handled by subsequent circuits and microcontrollers.
Shunt resistors are resistive elements with very small resistance values, generally ranging from a few milliohms to a few hundred milliohms. Because resistance errors are converted into current measurement errors, shunt resistors must have small resistance errors and temperature coefficients.
There are also many products available that are compatible with this.

How differential inputs and outputs work

A current sense amplifier receives the voltage difference across the shunt resistor at its "differential input," amplifies this differential voltage, and outputs a single voltage, or "single output."
Differential input is characterized by its high noise resistance and ability to accurately detect minute voltage differences.
To make it easier to connect to analog circuits and ADCs, an offset voltage is added to the "single output" to keep it within the positive voltage range, enabling stable measurement over a wide range from low to high currents.

Gain setting method (fixed gain vs. variable gain)

Gain is an important parameter that determines how much the minute voltage obtained through the shunt resistor is amplified. There are two types of gain settings: fixed gain types, where the gain is determined internally in the IC, and variable gain types, where the gain can be adjusted by setting external components or TERMINALS.
Fixed gain types are simple, highly stable, and easy to design, but their applications are limited, while variable gain types can be used over a wide current range because the gain can be adjusted.

Main features and types

High-side vs. Low-side Current Sensing

The configuration of a current sense amplifier differs depending on whether the shunt resistor is placed on the "POWER SUPPLIES side (high side)" or the "GND side (low side)."

High-side detection is a method of measuring the load current on POWER SUPPLIES line, which is safe because the load GND can be connected directly, and it can accurately determine the current of the entire system. However, if POWER SUPPLIES voltage is high, the amplifier must have a wide common-mode voltage range.

Low-side detection is a method of measuring the current on the GND side of the load, which requires a low common-mode voltage range for the amplifier and simplifies the circuit. However, because the GND of the load is floating, care must be taken in terms of noise and safety. The key to design is to choose which method to use depending on the application.

High-side and low-side detection diagram

Gain accuracy, offset voltage, common mode range

These three are the key factors that determine the performance of a current sense amplifier.

Gain accuracy indicates how accurately the amplifier amplifies the set factor. Poor accuracy will result in errors in the measured current.

Offset voltage is a voltage that appears in the output even when the input difference is zero, and it has a significant effect especially on low current measurements. If the offset voltage is large, it becomes difficult to measure very small currents.
The offset voltage error is output multiplied by the gain, so it is particularly important for amplifiers with large gains (multiplication factors).

The common-mode voltage range is the range of input voltages over which an amplifier can operate normally. High-side detection must support high common-mode voltages close to POWER SUPPLIES voltage. It is important to understand these characteristics and select an amplifier that suits your application.

Design considerations

Shunt resistor selection (resistance value, allowable power, temperature coefficient)

The shunt resistor is an important component that directly affects the accuracy of current detection and the efficiency of the circuit. The larger the resistance value, the larger the detection voltage and the better the accuracy, but the power loss and heat generation increase. Conversely, a smaller resistance value reduces loss, but the detection voltage becomes minute and the circuit becomes more susceptible to the effects of noise. There is a trade-off between the SNR and power loss.
The temperature coefficient of the resistor is also important. When the current is large, changes in resistance due to heat generation can lead to measurement errors, so it is recommended to choose a precision resistor with a low temperature coefficient.

Relationship between common mode voltage limits and POWER SUPPLIES voltage

For an amplifier to operate properly, the input common-mode voltage must be within its specified range. Because high-side detection applies a high common-mode voltage close to POWER SUPPLIES voltage, it is important to check the amplifier's compatibility range.
For example, if you are doing high-side sensing with a 12V POWER SUPPLIES, the amplifier must have a common-mode range of at least 0 to 12V, otherwise measurement errors and malfunctions will occur.
When actually selecting an amplifier, this input common-mode voltage range is one of the first points to check.
It is also important to consider in advance how much voltage may actually be applied from the system side.

Effects of Gain and Offset Errors

Gain error refers to the difference between the set gain and the actual gain, and directly affects the calculation of current values. In particular, in battery management and measurement applications that require high accuracy, even a gain error of a few percent can be a major problem.
Offset error is the voltage that appears at the output even when the input voltage difference is zero, and has a significant effect in the low current range. For example, if the input-referred offset is 1 mV and the shunt resistance is 10 mΩ, it corresponds to an error of 100 mA. It is important to take these errors into consideration and perform calibration as necessary.

Summary

Current sense amplifiers are essential circuits for ensuring system safety and efficiency through current measurement. This article explains their basic structure and features, as well as points to consider when designing them.
With a proper understanding and proper design, current sense amplifiers can demonstrate high reliability in a wide range of applications. We hope you will use the information in this article as a reference and apply it to your actual circuit designs.