Current sensing amplifiers refer to a type of amplifier that creates a voltage that corresponds to the flowing current in the rail of power. Moreover, current sensing amplifiers also refer to current shunt resistor amplifiers as it uses a type of shunt resistor in a power plane. When the current passes through the shunt resistor, it drops a small amount of voltage. However, the current sensing amplifier detects and senses this drop of voltage, amplifies it, and takes it out in an output form.
These current-sensing amplifiers are mainly responsible for amplifying even the smallest amount of voltage through a shunt resistor. The range of voltage typically ranges between 100 mV to 10 mV. Current sensing amplifiers are used for DC accuracy and high CMRR. It measures the flow of current in one direction, or it also measures the flow of current in two directions while using a sense resistor. However, if an amplifier can sense current in two directions, then it is referred to as a bidirectional current-sensing amplifier.
This article gives a detailed insight into the usage, working principle, and type of current sensor amplifier.
Difference Between Current Sense Amplifiers & Normal Amplifiers
Current sense and Normal amplifiers come with distinct types of specifications. Both use different types of building materials. Whereas, Op-amp also has various types. Normal amplifiers cannot amplify the low voltage amount. It also contains low CMMR. Whereas current sensing amplifiers identify and amplify even the smallest voltage. It also contains high CMRR.
In standard operational and normal differential amplifiers, both connect the power source between two rails of power supply, Vee and Vcc. Also, amplifiers only work over the signals that are present behind the rails of power or contain common paths of the ground plane. Any external voltage of the utilized power rail can trigger inner protection diodes of ESD; however, if the outer voltage applies to the normal amplifier input pin. Thus causing a large flow of current.
Whereas current sensing amplifiers, regardless of power rails with low voltage, can bear greater pin voltage as compared to the applied voltage. This amplifier utilizes a special power path method for its function. When the Vcc starts increasing, and the input voltage decreases, the amplifier starts getting a power supply from an input voltage.
Mode Voltage And CMMR of Current Sense Amplifiers
Common-mode voltage also refers to CM voltage or CMV. It plays an essential role in both current sensing amplifiers and simple amplifiers. The voltage of common mode refers to an average type of voltage that applies over two inputs of an amplifier. This type of voltage plays a significant role since the op-amp contains limited abilities to identify and create outputs based on CM voltage. However, a simple op-amp allows a very small amount of CMV, which does not fall appropriate for the accuracy of the current sensing function. But in current sensing amplifiers, CMV ranges higher than the real voltage supply of an amplifier. For instance, in current sensing amplifiers, INA240 supports CMV ranges between +80 volts to -4 volts while giving a low supply of 2.7 volts.
Whereas the common mode rejection ratio also refers to CMRR. It refers to a type of ratio between the CM gain and the differential gain. In real op-amp, the value of CMRR ranges infinitely. But in simple circuits, CMRR ranges between 100 to 80 dB. The high value of CMRR indicates the quantity of CM signal that will show in the measurement. However, in a current sensing amplifier, CMMR plays a significant role as it reflects low CM signals throughout the output. This enables the sensor to look into a wide variety of capabilities to sense the current. Moreover, CMMR also reduces the noise in the lines of current sensing.
Types Of Current Sense Amplifiers
Low Side Current Sense
It uses a type of shunt resistor that senses or identifies the current. The technique of current sensing varies based on the placement of the shunt resistor. The measurement of low-side current uses this. It, thus, creates an active load for the ground plane in a manner that when the load comes back to the ground plane, it always passes through a shunt resistor.
· Advantages
The measurement of low-side current offers various benefits. The implementation of the process is simple. This happens because the voltage supply across the shunt resistor stays in the ground plane reference. In this situation, it uses a low current voltage sense amplifier because of the small drop of voltage in a shunt resistor, because of the low voltage sense. It ignores the rejection of the common mode.
· Disadvantages
The main drawback of measurement of low-side current is that this load does not set according to the ground reference. It occurs because of the shunt resistor placement in a series format in a ground plane. The damage ground reference thus causes issues in the circuit, like short circuits between the ground and the load, because the current shunt resistor cannot identify the high flow of current in a load circuit.
High Side Current Sense
The measurement of high-side current uses the same shunt resistor as the measurement of low-side current. However, the placement of this shunt resistor happens between the active load and the power source.
· Advantages
The measurement of high-side current comes with two basic benefits to the measurement of low-side current. Firstly, it overcomes the issue of low-side current of not being able to identify the short circuit of load with a ground. It needs a current shunt resistor in a plain of power; then, it easily identifies any type of short circuits in-ground and load.
Secondly, in this type of circuit, the load uses a complete ground reference to place. Thus a shunt resistor differential input can easily identify the exact amount of load current. And it does not need any extra effort; it uses ADC conversion.
· Disadvantages
The measurement of the high-side current method needs a high rejection common mode. This happens due to the low voltage that develops across the shunt resistor of the current beneath the supply voltage load.
