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Current Sense Amplifier Circuit and Application Overview

Author: Apogeeweb
Date: 12 Jan 2021
 1515
current shunt amplifier

Introduction

In electronics, current sense amplifiers are special-purpose amplifiers that output a voltage proportional to the current flowing in a power rail. They are often referred to as current shunt amplifiers because they use a shunt resistor in the power rail that provides a slight voltage drop when the current flows through the resistor. These devices are designed to handle common mode voltages in excess of their own supply voltage. The working principle of a current sensing amplifier is based on Ohm's law. And the voltage drop is converted and amplified to a small output voltage by the current sensor amplifier.

Catalog

Introduction

Ⅰ Current Sense Amplifier Overview

1.1 What is Current Sense Amplifier?

1.2 Common Mode Voltage and CMRR

1.3 Main Types of Current Sense Amplifiers

Ⅱ Current Sense Amplifiers vs Common Amplifiers

Ⅲ How to Design a Circuit Using Current Sense Amplifiers.

Ⅳ Common Applications of Current Sensing ICs

4.1 Low-side Current Sense IC

4.2 High-side Current Sense IC

4.3 Bidirectional Current Sense IC

4.4 Isolated Current Sense IC

Ⅴ Conclusion


Ⅰ Current Sense Amplifier Overview

1.1 What is Current Sense Amplifier?

Sense amplifiers are designed for a special reason, such that the amplifier can amplify a very limited amount of sensed voltage across the shunt resistor, usually within a range of 10 to 100 mV. Present Sense amplifiers are designed for DC precision (e.g. low input offset voltage) and high common-mode rejection ratio (CMRR). Current sensing amplifiers can measure the current flowing in a single direction or they can measure the current flow in both directions through the sensing resistor. In such a case, if the amplifier is capable of detecting current flow in both directions, it is called bi-directional current sensing amplifiers.

1.2 Common Mode Voltage and CMRR

Popular mode voltage is critical for regular amplifiers and current sense amplifiers. The common-mode voltage refers to the average voltage applied to the inputs of the amplifier. This is so critical voltage because the op-amp has a restricted ability to distinguish and differentiate depending on the common-mode voltage. An ordinary op-input amp's range is not sufficient for the precision level current sensing operations. In current sensors, the common-mode voltage range includes higher than the real output voltage of the amplifier. For eg, this current sense amplifier can also attain supply operating voltage range of -4 V to +80 V.

Op-Amp CMRR (Common Mode Rejection Ratio) Explained

The CMRR (common-mode rejection ratio) is the ratio of differential gain and the common-mode gain. For an ideal op-amp, the CMRR is infinite, but when it comes to real circuits, we see it ranges from 80 to 100 decibels. A high CMRR means that a large amount of the common-mode signal would influence the measurement. For a current sense amplifier, one significant parameter is its resistance since it will represent the common-mode signal across the output. This in turn will open up a wide range of sensing capabilities. The CMRR or common-mode rejection ratio is very high in current sense amplifiers and so it can sense tiny common-mode signal. The CRC also eliminates noise on the existing sense lines.

1.3 Main Types of Current Sense Amplifiers

  • High-side Amplifiers
    The current is measured between the supply rail and the load. The DC voltage applied on the input pins can be much higher than the power supply.
  • Low-side Amplifiers
    The current is measured between the load and the ground. The voltage applied on the input pins is close to the ground.

high-side current sensing amplifier

Figure 1. High-side Current Sensing Amplifier

Ⅱ Current Sense Amplifiers vs Common Amplifiers

Current sense amplifiers and common amplifiers have different requirements and are equipped for particular purposes. There are several types of Op-Amps, you can read this tech note to learn about the common op-amps IC and their applications. Normal amplifiers do not amplify a very small voltage and have a low CMRR. However, precise current sense amplifiers could detect and amplify a very small amount of voltage, just as the CMRR is relatively high.
For normal operational amplifiers, the power source has to be connected between the power supply rails (Vcc and Vee) and the amplifiers can only function on the input signals that are independent of the rails. In a standard amplifier, an outside voltage of the power rail will activate the internal ESD safety diodes if an external voltage is applied into the input pin of the standard amplifier, allowing a large current to flow.
However, current sense amplifiers are built such a way that despite the lower input voltage (such as 3.3V), they can handle the pin voltage of the supplied voltage . The amplifiers operate on an outstanding power path protocol. When the input voltage is lower than VCC, the amplifier gets driven from the input voltages.

 

Ⅲ How to Design a Circuit Using Current Sense Amplifiers

Suppose a design with a 12V, 1A line where a high sensing current is required. One can use current sense amplifiers as sense current paths. However, one needs to choose current sense amplifier to make this application possible.
In this case, a 12V sensing amplifier with more than 1A rating should be chosen. One should pick something between the low level and high level as discussed earlier. A controller circuit having a high side current sensing may be used for detecting a fault or short circuit condition. Low side current sensing can also work too. We will compare these two later in this post.
The advantage will be better shielding of sensitive electronics and better protection against inrush current. For this use, it can use LT6108. The gain of this amplifier is also important, up to 125dB. This amplifier will operate both in 12V supply as well as in 0V supply.

Figure 2. LT6108 Circuit Fault Protection with very Fast Latching Load Disconnect


The above sense amplifier circuit is rendered with the used of LT6108. Using the 1-ohm resistor as the sense resistor that causes a voltage drop when current is flowing through it. The IRF9640 is the MOSFET switching configuration and the 2N2700 is the MOSFET monitoring arrangement. The Amplifier (switching MOSFET) might shut off for a pre-set value. In this circuit, an end point is created through 250mA. Thus, the relay will open the circuit to overload current flow if it reaches more than 250mA. However for over 1A, the setting can be operated with the voltage divider across the current sense amplifier pin. The Vout can be used to evaluate the current flow through the current sense resistor. Such a circuit can also be built using other current sense amplifier designs that have a broader input range than 12V.

 

Ⅳ Common Applications of Current Sensing ICs

4.1 Low-side Current Sense IC

The current sensing dependent on shunt resistor location varies. A low side measure is carried out by using. Due to this, the GND path of the active load still rises along with the increased current. In the charts below, low side measurement circuits are demonstrated.
These are real-life representations of the Low side current sensing techniques. In this way, the data from INA181 current sense amplifier can be interpreted by the ADAC. Another way to go is to use the AD8202 current sensing amplifier from Analog Devices. Present is detecting an inductive load in this way.
1) Advantages
Low side current measurements are a convenient way to measure low level voltage. In this setup, an integral circuit can be used because of the slight voltage drop around the shunt resistor. Because of low threshold voltage, common-mode rejection is useless.
2) Disadvantages
One of the main problems of using the low side current calculation is that the measurement is reversible. In this way, this occurs because of series positioning of shunt resistor in the ground plane. Since the ground reference is broken, the whole thing will not work well.

4.2 High-side Current Sense IC

Not like low side current sense, in high side current sense, a high side shunt resistor is mounted between the load and the positive power supply. In the above photos, you can see high current sense amplifier circuits.
Here are realistic examples of the "high side current sensing techniques". Input data is provided by TI INA240 pin that will provide data in I2C protocol. With a LT6100, it is a high side current sensing circuit along with a fuse load.

TI INA240 in circuit

Figure 3. TI INA240 in Circuit


1) Advantages
The high side current measurement has two benefits over the low side current measurement. Second, it serves as an interference for any short circuit with the field. In order to detect short circuit in the power circuit, a shunt resistor is mounted at the neutral stage.
Secondly, this circuit measures voltages accurately to ensure the exact burden load present.
2) Disadvantages
However, since the small voltage that is produced across the current shunt resistor is below the load supply voltage, the high side current measuring technique requires high common-mode rejection.

4.3 Bidirectional Current Sense IC

A simple shunt resistor is used for a bidirectional current sense circuit, but the amplifier requires the ability to sense a wide range of common-mode voltages. The current bidirectional sense methodology is a little more complicated than the sensing of the low and high sides. There are various ways for bidirectional currents to be detected. In certain situations, where two current INA300 sense amplifiers are linked in a way that each of the current sense amplifiers can detect the current flow in both directions. The circuit needs reverse polarity protection along with a switching technique which, depending on the polarity of the circuit, will switch the output of the amplifier.
Using an input voltage for a reference is another way to detect the bidirectional current, then the amplifier detects the drop voltage via the sense resistor and compares it with the reference voltage. If the drop voltage is above the known reference voltage, it is in one direction, while the drop voltage is in the opposite direction, below the known reference voltage.

4.4 Isolated Current Sense IC

The technique of isolated current sensing uses proper induction of the CT transformer or hall effect that generates a proportional voltage in a transformer tap when current flows through another transformer tap. Another practical example of the existing sense amplifier using the isolated current transformer is the above picture.

low-side current sensing ic

Figure 4. Low-side Current Sensing Circuit

Ⅴ Conclusion

As an important component of modern CMOS IC, current sense amplifiers provide a high-flexibility and high-precision solution in a wide array of automotive and power management applications. For example, INA280 high-precision current sense amplifier is a current sense amplifier that can measure voltage drops across shunt resistors. With different circuit requirements, high voltage, high resolution, bidirectional current sense amplifiers are available in the market.

 

Frequently Asked Questions about Current Sense Amplifier Circuit

1. What is current sense amplifier?
Current sense amplifiers (also called current shunt amplifiers) are special-purpose amplifiers that output a voltage proportional to the current flowing in a power rail. They utilize a "current-sense resistor" to convert the load current in the power rail to a small voltage, which is then amplified by the current-sense amplifiers. The currents in the power rail can be in the range of 1 A to 20 A, requiring the current-sense resistor to be a resistor typically in the range of 1 to 100 mΩ.

 

2. How does a current sense amplifier work?
Normal differential amplifiers and operational amplifiers are often as current sense amplifiers. They powered between two power supply rails (say VCC and VEE) can only handle signals that lie between these two power rails. If a voltage outside the power supply rails is applied to the input, internal ESD protection diodes turn on, causing large currents to flow and damage these parts.

 

3. What are the main types of current sense amplifiers?
Types of current sense amplifiers include Low Side Current Sense type, High Side Current Sense type, Bidirectional Current Sense type, and Isolated Current Sense type.

 

4. What is the main purpose of current sense amplifier?
Current Sense Amplifiers are special-purpose amplifiers that output a voltage that is proportional to the current flowing in the power rail. It could amplify a very small amount of sensed voltage across the shunt resistor, typically in 10 to 100 mV range. Current Sense Amplifiers can measure the current flowing in a single direction or measure current flow in both directions through the sense resistor.

 

5. What is the difference between voltage amplifier and current amplifier?
Normal amplifiers and current sense amplifiers have different specifications and they are made for specific things. Normal amplifiers could not amplify a very small amount of voltage and have low CMRR. On the other hand, precision current-sense amplifiers could detect and amplify a very small amount of voltages as well as the CMRR is relatively high.
For the normal differential amplifiers or standard operational amplifiers, the power source is connected between two power supply rails (Vcc and Vee) and the amplifiers can only operate on the signals that lie behind the power rails or have common ground paths. While current sense amplifiers are designed in a way that despite the low-voltage power rail (such as Vcc = 3.3 V and V = 0V), the amplifier can withstand a much higher pin voltage than the supplied Vcc.

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